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Bluesound Node Icon Streaming Preamplifier Measurements

Details
Parent Category: Products
Category: Preamplifier Measurements
Created: 01 June 2025

Link: reviewed by Roger Kanno on SoundStage! Simplifi on June 1, 2025

General Information

All measurements taken using an Audio Precision APx555 B Series analyzer.

The Bluesound Node Icon was conditioned for 30 minutes at 2Vrms in/out into 200k ohms before any measurements were taken.

The Node Icon offers one RCA analog input and four digital inputs—optical S/PDIF, USB-C, HDMI ARC, Bluetooth—plus network streaming. There are two analog outputs (XLR and RCA). Also included are a line-level sub-out (with internal bass management) and a ¼″ TRS headphone output. For the purposes of these measurements, unless otherwise stated, the following inputs were evaluated using the balanced XLR outputs: digital optical S/PDIF, analog (RCA), and the headphone output. Comparisons were made between unbalanced (RCA) and balanced (XLR) outputs, and no appreciable differences were seen other than twice the gain over the balanced outputs (FFTs for different configurations can be seen in this report).

Most measurements were made with a 2Vrms line-level and 0dBFS digital input with the volume set to achieve 2Vrms at the output. The signal-to-noise ratio (SNR) measurements were made with the same input signal values and for comparison, on the line-level input, a SNR measurement was also made with the volume at maximum, but with a lower input voltage to achieve the same 2Vrms output.

Based on the repeatability of the results of the left/right volume channel matching (see table below), the Icon volume control is digitally controlled and operating in the digital domain. The icon automatically digitizes the analog input, which allows it to perform DSP functions such as bass/treble control and bass management. The Icon offers a volume range of -85dB to +3.7dB for the analog input over balanced outputs, in 100 steps. Steps range between 0.5 and 1dB in size.

Volume-control accuracy (measured at preamp outputs): left-right channel tracking

Volume position Channel deviation
2 0.133dB
10 0.131dB
20 0.130dB
30 0.130dB
40 0.129dB
50 0.129dB
60 0.129dB
70 0.128dB
80 0.128dB
90 0.128dB
100 0.128dB

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Bluesound for the Node Icon compared directly against our own. The published specifications are sourced from Bluesound’s website, either directly or from the manual available for download, or a combination thereof. With the exception of frequency response, where the Audio Precision bandwidth was set at its maximum (DC to 1MHz), unless otherwise stated, assume a 1kHz sinewave at 2Vrms or 0dBFS at the input, 2Vrms at the output into 200k ohms, and a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channels.

Parameter Manufacturer SoundStage! Lab
SnR (24/96@0dBFS, max 3.7Vrms output XLR, Awgt) 129dB 130dB
SnR (24/96@0dBFS, max 1.8Vrms output RCA, Awgt) 121dB 121dB
THD+N (1kHz, 24/96@0dBFS, 2Vrms out, XLR, Awgt) <0.0004% <0.00014%
Headphone output (@0.1% THD, 600 ohms) 23mW 23mW
Headphone output (@0.1% THD, 32 ohms) 235mW 232mW

Our primary measurements revealed the following using the balanced line-level analog input and digital optical input (unless specified, assume a 1kHz sinewave at 2Vrms or 0dBFS, 2Vrms output, 200k ohm loading, 10Hz to 22.4kHz bandwidth):

Parameter Left channel Right channel
Crosstalk, one channel driven (10kHz) -98dB -99dB
DC offset <0.5mV <-0.06mV
Gain (RCA in/out) -2.3dB -2.4dB
Gain (XLR in/out) 3.7dB 3.6dB
IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) <-96dB <-97dB
IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1 ) <-92dB <-93dB
Input impedance (line input, RCA) 10k ohms 10k ohms
Maximum output voltage (at clipping 1% THD+N) 3.8Vmrs 3.8Vrms
Maximum output voltage (at clipping 1% THD+N into 600 ohms) 2.8Vrms 2.7Vrms
Noise level (with signal, A-weighted) <7.8uVrms <7.8uVrms
Noise level (with signal, 20Hz to 20kHz) <10uVrms <10uVrms
Noise level (no signal, A-weighted, volume min)* <1.14uVrms <1.06uVrms
Noise level (no signal, 20Hz to 20kHz, volume min)* <1.44uVrms <1.32uVrms
Output impedance (RCA) 116 ohms 116 ohms
Output impedance (XLR) 230 ohms 231 ohms
Signal-to-noise ratio (2Vrms out, A-weighted, 2Vrms in) 109.4dB 109.2dB
Signal-to-noise ratio (2Vrms out, 20Hz to 20kHz, 2Vrms in) 107.2dB 107.1dB
Signal-to-noise ratio (2Vrms out, 20Hz to 20kHz, max volume) 105.8dB 105.5dB
Dynamic range (2Vrms out, A-weighted, digital 24/96)* 126.0dB 126.2dB
Dynamic range (2Vrms out, A-weighted, digital 16/44.1) 96.1dB 96.1dB
THD ratio (unweighted) <0.00047% <0.00039%
THD ratio (unweighted, digital 24/96) <0.00011% <0.00005%
THD ratio (unweighted, digital 16/44.1) <0.00036% <0.00036%
THD+N ratio (A-weighted) <0.00066% <0.00059%
THD+N ratio (A-weighted, digital 24/96) <0.00014% <0.00012%
THD+N ratio (A-weighted, digital 16/44.1) <0.0016% <0.0016%
THD+N ratio (unweighted) <0.0007% <0.0007%

* Due to very low noise of DUT, analyzer self-noise has been removed from measurement to more accurately report value

Our primary measurements revealed the following using the digital optical input (gain was measured using the analog input) at the headphone output (unless specified, assume a 1kHz sinewave, 0dBFS 24/96  input, 2Vrms output, 300 ohms loading, 10Hz to 22.4kHz bandwidth):

Parameter Left and right channels
Maximum gain (analog in) 5.9dB
Maximum output power into 600 ohms 24mW
Maximum output power into 300 ohms 49mW
Maximum output power into 32 ohms 242mW
Output impedance 1 ohm
Maximum output voltage (100k ohm load) 3.85Vrms
Noise level (with signal, A-weighted) <23uVrms
Noise level (with signal, 20Hz to 20kHz) <30uVrms
Noise level (no signal, A-weighted, volume min) <2.8uVrms
Noise level (no signal, 20Hz to 20kHz, volume min) <3.6uVrms
Dynamic range (A-weighted, 1% THD, 3.6Vrms out) 123dB
Dynamic range (20Hz - 20kHz, 1% THD, 3.6Vrms out) 120dB
THD ratio (unweighted) <0.0036%
THD+N ratio (A-weighted) <0.0035%
THD+N ratio (unweighted) <0.0043%

* Default is 24/96 0dBFS in, 2Vrms out into 300 ohms

Frequency response (line-level input)

frequency response

In our measured frequency-response (relative to 1kHz) plot above, the Node Icon is near perfectly flat within the audioband (0dB at 20Hz, -0.2dB at 20kHz). At the extremes, the Node Icon is -0.25dB at 5Hz and -5dB at roughly 23kHz. The Node Icon digitizes incoming analog signals with a 48kHz sample rate. In the graph above and most of the graphs below, only a single trace may be visible. This is because the left channel (blue or purple trace) is performing identically to the right channel (red or green trace), and so they perfectly overlap, indicating that the two channels are ideally matched.

Frequency response with crossover at 80Hz (line-level input)

frequency response xover

Above is the frequency response plot (relative to 20Hz for the sub-out, 1kHz for the line-level XLR out) for the Node Icon with bass management engaged in the BluOS app, applying an 80Hz crossover point. The crossover point is correctly applied, with attenuation at 18dB per octave.

Phase response (line-level analog input)

phase response

Above is the phase response plot from 20Hz to 20kHz for the analog line level input. The Node Icon does not invert polarity. Because of the sampling of the input by the ADC, absolute phase is extremely high (-2 million degrees at 20kHz) because it includes timing delays due to digitization. Below is a phase plot which aims to show only excess phase (excluding timing delays).

Phase response (line-level analog input, excess only)

phase response excess

There is essentially no phase shift up to just past 10kHz, then a spike to +160000 degrees just below 20kHz, then down to -100000 degrees at 20kHz.

Frequency response vs. input type

frequency response vs input type

The chart above shows the Node Icon’s frequency response (relative to 1kHz) as a function of input type. The dark green trace is the same analog input data from the previous graph. The blue/red traces are for a 16bit/44.1kHz dithered digital input signal from 5Hz to 22kHz using the optical input, the purple/green traces are for a 24/96 dithered digital input signal from 5Hz to 48kHz, and finally pink/orange at 24/192 from 5Hz to 96kHz. The behavior at low frequencies is the same for all the digital sample rates; perfectly flat (0dB) down to 5Hz, as opposed to the analog signal which is -0.25dB at 5Hz. The behavior at high frequencies for all three digital sample rates differ. We see sharp, but not quite "brick-wall" filtering around 20kHz for the 16/44.1 data, with a -3dB point at 21kHz. The -3dB point for the 24/96 sampled data is at 35kHz, with a slower high-frequency roll-off. The -3dB point for the 24/192 sampled data is at 47kHz, with the slowest high-frequency roll-off. Case-in-point, the 24/192 data shows a -0.1dB at 10kHz response, while all other sample rates are at 0dB at 10kHz. 

Digital linearity (16/44.1 and 24/96 data)

digital linearity 1644 1 2496

The chart above shows the results of a linearity test for the coaxial digital input (the optical input performed identically) for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the outputs of the Node Icon. For this test, the digital input is swept with a dithered 1kHz input signal from -120dBFS to 0dBFS, and the output is analyzed by the APx555. The ideal response would be a straight flat line at 0dB. At -120dBFS, the 16/44.1 data overshot by only 1-2dB, while the 24/96 remained perfect. To verify how well the 24/96 data would perform down to -140dBFS, we extended the sweep in the chart below.

digital linearity 1644 1 2496 extended

Here we can see that the 24/96 data only overshot the mark by +1dB (left) at -135 to -140dBFS. This is an outstanding digital-linearity test result.

Impulse response (24/48 data)

impulse response 2448

The graph above shows the impulse response for a looped 24/44.1 test file that moves from digital silence to full 0dBFS (all “1”s) for one sample period then back to digital silence, measured at the balanced outputs of the Node Icon. We can see that the Node Icon utilizes a reconstruction filter that favors no pre-ringing.

J-Test (optical input)

jtest optical 2448

The chart above shows the results of the J-Test test for the optical digital input measured at the balanced line-level output of the Node Icon. J-Test was developed by Julian Dunn the 1990s. It is a test signal—specifically, a -3dBFS undithered 12kHz squarewave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz squarewave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sinewave (the main peak). In addition, an undithered 250Hz squarewave at -144dBFS is mixed with the signal. This test file causes the 22 least-significant bits to constantly toggle, which produces strong jitter spectral components at the 250Hz rate and its odd harmonics. The test file shows how susceptible the DAC and delivery interface are to jitter, which would manifest as peaks above the noise floor at 500Hz intervals (e.g. 250Hz, 750Hz, 1250Hz, etc). Note that the alternating peaks are in the test file itself, but at levels of -144dBrA and below.  The test file can also be used in conjunction with artificially injected sinewave jitter by the Audio Precision, to show how well the DAC rejects jitter.

The optical input of the Node Icon shows a strong J-Test result, with several spurious peaks but at and below the low level of -140dBrA.

J-Test (optical, 2kHz sinewave jitter at 100ns)

jtest coaxial 2448 2khz 100ns

The chart above shows the results of the J-Ttest test for the optical digital input measured at the line-level output of the Node Icon, with an additional 100ns of 2kHz sinewave jitter injected by the APx555. The results are essentially the same as the J-Test result without additional jitter. More evidence of strong jitter rejection.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone

wideband fft noise plus 19 1khz 1644 1kHz

The chart above shows a fast Fourier transform (FFT) of the Node Icon’s balanced outputs with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at -2dBFS (0dB caused clipping) fed to the optical digital input, sampled at 16/44.1. The shallow roll-off around 20kHz in the white-noise spectrum shows that the Node Icon seems to use a reconstruction filter (for 16/44.1 content) with a high-frequency roll-off somewhere between slow and fast. There are no obvious aliased images within the audioband. The primary aliasing signal at 25kHz is barely suppressed at -20dBrA.

THD ratio (unweighted) vs. frequency vs. load (analog)

thd vs frequency vs load

The chart above shows THD ratios at the balanced line-level output into 200k ohms (blue/red) and 600 ohms (purple/green) as a function of frequency for the analog inputs. THD data at the two different loads are extremely close, except beyond 10kHz, where the 600-ohm load results are roughly 10dB higher. Because the analog input is sampled at 48kHz and bandwidth is limited beyond 20kHz, THD results above 6kHz are not reliable as the signal harmonics above this point are highly suppressed by the ADC process. THD ratios are very low and slightly lower (3-4dB) for the right channel, hovering between 0.0004% and 0.0006% from 20Hz to 5kHz.

THD ratio (unweighted) vs. frequency vs. sample rate (16/44.1 and 24/96)

thd vs frequency 16 441 24 96

The chart above shows THD ratios at the balanced line-level output into 200k ohms for a 16/44.1 (blue/red) dithered signal at the optical input and a 24/96 (purple/green) signal, as a function of frequency. The 16/44.1 THD ratios were higher than the 24/96 THD ratios due to the increased noise floor from the lower bit-depth (the analyzer cannot assign a THD ratio for harmonic peaks it cannot see above the noise floor). From 20Hz to 1.5kHz, the 16/44.1 THD ratios are stable at 0.0002-0.0003%, while the 24/96 THD data for the right channel is extremely low at 0.00004%. The left channel at 24/96 was stable at 0.0001% from 20Hz to 1.5kHz. Just shy of and beyond 2kHz, there is a steep rise in all THD data, peaking beyond 1% at 20kHz for the 16/44.1 data. This is not a purely frequency dependent phenomenon, but rather an issue with distortion with an input signal magnitude of 0dBFS. We reached out to Lenbrook Industries (Bluesound's parent company) to ask if they were aware of this issue. They responded that they were aware, that overload at 0dBFS from steady-state signals (sinewaves) is a conseqeunce of their special filters (though overload with musical signals should not occur), and finally that the sofware team was looking at making some adjustments to fix the issue. Below are the same plots, but with a -2dBFS input signal. We see . . .

thd vs frequency 16 441 24 96 -2dBFS

. . . very different results beyond 2kHz, with THD ratios never exceeding 0.0002%.

THD ratio (unweighted) vs. output (analog)

thd ratio unweighted vs output

The chart above shows THD ratios measured at the balanced outputs of the Node Icon as a function of output voltage for the analog line level-input, with the volume at maximum. THD values start at 0.15% at 1mVrms, down to a low of 0.0002% at 1-2Vrms, then a steep rise past 3Vrms to the 1% THD mark at 3.8Vrms. For the Node Icon, clipping is occurring at the input of the ADC, not at the analog output stage.

THD+N ratio (unweighted) vs. output (analog)

thd ratio unweighted vs output

The chart above shows THD+N ratios measured at the balanced outputs of the Node Icon as a function of output voltage for the balanced line-level -input, with the volume control at maximum. THD+N values start at 1.5% at 1mVrms, down to a low of 0.0006% at 3Vrms, then a steep rise past 3Vrms to the 1% THD mark at 3.8Vrms.

THD ratio (unweighted) vs. output (16/44.1 and 24/96)

thd vs output 16 441 24 96

The chart above shows THD ratios measured at the balanced outputs of the Node Icon as a function of output voltage for the digital optical input, swept from -90dBFS to 0dBFS, with the volume control at maximum. Blue/red traces are for 16/44.1 data and purple/green for 24/96. For the 16/44.1 data, THD values start at 3%, and predictably, reach their low at the maximum output voltage of about 3.8Vrms, at 0.0002%. For the 24/96 data, THD ratios ranged from 0.05% down to 0.00005% (right channel) at the maximum output voltage.

THD+N ratio (unweighted) vs. output (16/44.1 and 24/96)

thd n vs output 16 441 24 96

The chart above shows THD+N ratios measured at the balanced outputs of the Node Icon as a function of output voltage for the digital optical input, swept from -90dBFS to 0dBFS, with the volume control at maximum. Blue/red traces are for 16/44.1 data and purple/green for 24/96. For the 16/44.1 data, THD+N values start at 30% and reach their low at the maximum output voltage of about 3.8Vrms, at 0.002%. For the 24/96 data, THD+N ratios ranged from 0.5% down to just above 0.0001% at the maximum output voltage.

Intermodulation distortion vs. generator level (SMPTE, 60Hz:4kHz, 4:1, 16/44.1, 24/96)

intermodulation distortion SMPTE vs generator level 441k 96k

The chart above shows intermodulation distortion (IMD) ratios measured at balanced output for 16/44.1 (blue/red) input data and 24/96 input data (purple/green), from -60dBFS to 0dBFS. Here, the SMPTE IMD method was used, where the primary frequency (F1 = 60Hz) and the secondary frequency (F2 = 7kHz) are mixed at a ratio of 4:1. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 16/44.1 data yields IMD ratios from 2% down to 0.002% at 0dBFS. The 24/96 data yields IMD ratios from 0.1% down to 0.0005% (right channel) at 0dBFS.

FFT spectrum – 1kHz (analog line-level input, XLR output)

FFT spectrum 1khz

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced outputs for the line-level input. We see that the signal’s second (2kHz) harmonic is at -120dBrA, or 0.0001%, the third (3kHz) harmonic is at -110dBrA, or 0.0003%, and the fifth (5kHz) harmonic is at -125dBrA, or 0.00006%. Other signal harmonics can be seen, but around the very low -140dBrA, or 0.00001%, level. Below 1kHz, there are essentially no peaks above the -150dBrA noise floor. Also evident are peaks at 47Kz and 49kHz, proof of the 48kHz sampling of the signal.

FFT spectrum – 1kHz (analog line-level input, RCA output)

FFT spectrum 1khz

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the unbalanced outputs for the line-level input. In terms of signal harmonics, this FFT is identical to the FFT above for balanced outputs. Noise artifacts are evident here (unlike the FFT above for the balanced outputs), but at very low levels (at and below -130dBrA, or 0.00003%).

FFT spectrum – 1kHz (digital input, 16/44.1 data at 0dBFS)

fft spectrum 1khz 1644 1 0dbfs

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced outputs for the optical digital input, sampled at 16/44.1. We only see one peak, the second (2kHz) signal harmonic for the left channel at -120dBrA, or 0.0001%.

FFT spectrum – 1kHz (digital input, 24/96 data at 0dBFS)

fft spectrum 1khz 2496 0dbfs

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced outputs for the optical digital input, sampled at 24/96. We see several spurious noise related noise peaks above the -160dBrA noise floor but at very low levels (-140 to -150dBrA). The second (2kHz) and fifth (5kHz) signal harmonic is at -135dBrA, or 0.00002%, while the third (3kHz) harmonic is at -120/-140dBrA (left/right), or 0.0001/0.00001%.

FFT spectrum – 1kHz (digital input, 16/44.1 data at -90dBFS)

fft spectrum 1khz 1644 1 90dbfs

Shown above is the FFT for a 1kHz -90dBFS dithered 16/44.1 input sinewave stimulus at the optical digital input, measured at the balanced outputs. We see the 1kHz primary signal peak, at the correct amplitude, and no signal or noise related harmonic peaks above the -140dBrA noise floor.

FFT spectrum – 1kHz (digital input, 24/96 data at -90dBFS)

fft spectrum 1khz 2496 90dbfs

Shown above is the FFT for a 1kHz -90dBFS dithered 24/96 input sinewave stimulus at the optical digital input, measured at the balanced outputs. We see the 1kHz primary signal peak, at the correct amplitude, and a signal related harmonic peak (left channel only) at 5kHz at around -145dBrA, or 0.000006%. Other noise-related peaks can be seen but at the extremely low -150dBrA, or 0.000003%, level.

FFT spectrum – 50Hz (line-level input)

fft spectrum 50hz

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the balanced outputs for the line-level input. The X axis is zoomed in from 40Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. The most predominant (non-signal) peaks are that of the signal’s third (150Hz) harmonic at -110dBrA, or 0.0003%. Other signal harmonic peaks can be seen at and below -120dBrA, or 0.0001%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, line-level input)

intermodulation distortion fft 18khz 19khz summed stimulus

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs for the line-level input. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 10W (0dBrA) into 8 ohms at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -120dBrA or 0.0001%, while the third-order modulation products, at 17kHz and 20kHz are at -110dBrA, or 0.0003%. This is a very clean IMD result.

Intermodulation distortion FFT (line-level input, APx 32 tone)

fft spectrum 32 tone

Shown above is the FFT of the balanced outputs of the Node Icon with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 2Vrms. The intermodulation products—i.e., the “grass” between the test tones—are distortion products from the preamplifier and are around the -130dBrA, or 0.00003%, level.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, coaxial digital input, 16/44.1)

intermodulation distortion fft 18khz 19khz summed stimulus 1644-1

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs for the digital optical input at 16/44.1. We find that the second-order modulation product (i.e., the difference signal of 1kHz) cannot be seen above the -135dBrA noise floor, while the third-order modulation products, at 17kHz and 20kHz, are at -125dBrA, or 0.00006%. We also see the main aliased peaks at 25.1kHz and 26.1kHz at around -30dBrA.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, coaxial digital input, 24/96)

intermodulation distortion fft 18khz 19khz summed stimulus 2496

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs for the digital optical input at 24/96. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -140/-150dBrA (left/right), or 0.00001/0.000003%, while the third-order modulation products, at 17kHz and 20kHz, are at -125dBrA or 0.00006%.

Square-wave response (10kHz)

square wave response 10kHz

Above is the 10kHz squarewave response using the analog line-level input, at roughly 2Vrms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the Node Icon’s slew-rate performance. Rather, it should be seen as a qualitative representation of its very limited  bandwidth. An ideal squarewave can be represented as the sum of a sinewave and an infinite series of its odd-order harmonics (e.g., 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot, and softening of the edges. Here, only the fundamental 10kHz frequency can be seen, as a sinewave.

Square-wave response (1kHz)

square wave response 1kHz

Above is the 1kHz squarewave response using the analog line-level input, at roughly 2Vrms. Due to the very limited bandwidth of the Node Icon due to its 48kHz sampling of analog signals, the 1kHz squarewave reproduction is poor, with significant ringing in the plateaus.

Diego Estan
Electronics Measurement Specialist

WiiM Ultra Streaming Preamplifier Measurements

Details
Parent Category: Products
Category: Preamplifier Measurements
Created: 01 May 2025

Link: reviewed by AJ Wykes on SoundStage! Simplifi on May 1, 2025

General Information

All measurements taken using an Audio Precision APx555 B Series analyzer.

The WiiM Ultra was conditioned for 30 minutes at 2Vrms in/out into 100k ohms before any measurements were taken.

The Ultra offers one set of line-level analog inputs (RCA), one set of moving-magnet (MM) phono-level analog inputs (RCA), one S/PDIF digital input (optical), one HDMI (ARC) input, as well as streaming over ethernet or WiFi and Bluetooth inputs. Available outputs are a set of analog line-level (RCA), S/PDIF digital over optical (TosLink) and coaxial (RCA), as well as a single subwoofer output (configurable with internal bass management). There is also a ⅛″ TRS headphone output jack on the front panel. For the purposes of these measurements, unless otherwise stated, the following inputs were evaluated: digital optical S/PDIF (RCA), analog line-level (RCA), as well as phono MM (RCA).

Most measurements were made with a 2Vrms line-level and 0dBFS digital input with the volume set to achieve 2Vrms at the output (volume at maximum). Of note is that the Ultra digitizes all incoming analog signals to perform volume, EQ, and bass management. The ADC’s bit depth and sample rate are user-selectable (unless otherwise stated, 24/192 was used). As a line-level analog preamp, the Ultra does not offer any gain (-0.05dB at max volume), and the ADC will clip with a 2.15Vrms input signal. As a phono preamp, the Ultra was set to maximum gain (51.6dB at max volume), which yielded an output of roughly 1.9Vrms for a 5mVrms input level.

The Ultra also offers seven different reconstruction filters for the DAC, they are:

  1. Brick Wall Filter (labelled Filter 1 in this report)
  2. Corrected Minimum Phase Fast Roll-Off (labelled Filter 2 in this report)
  3. Apodizing Fast Roll-Off (labelled Filter 3 in this report)
  4. Minimum Phase Slow Roll-Off (labelled Filter 4 in this report)
  5. Minimum Phase Fast Roll-Off (labelled Filter 5 in this report)
  6. Linear Phase Slow Roll-Off (labelled Filter 6 in this report)
  7. Linear Phase Fast Roll-Off (labelled Filter 7 in this report and the default filter unless otherwise stated)

The Ultra’s volume control operates in the digital domain, evidenced by our left-right channel tracking table below, which shows identical deviations (dB) at each sample volume level. Overall volume ranges from -59.6dB to -0.05dB (analog line-level in/out). Volume steps range from 1.5dB (lower volume levels) to 0.3dB (levels 50 through 100).

Volume-control accuracy (measured at preamp outputs): left-right channel tracking

Volume position Channel deviation
1 0.011dB
10 0.011dB
20 0.011dB
30 0.011dB
40 0.011dB
50 0.011dB
60 0.011dB
70 0.011dB
80 0.011dB
90 0.011dB
100 0.011dB

Published specifications vs. our primary measurements

The table below summarizes the measurements published by WiiM for the Ultra compared directly against our own. The published specifications are sourced from WiiM’s website, either directly or from the manual available for download, or a combination thereof. With the exception of frequency response, where the Audio Precision bandwidth was set at its maximum (DC to 1MHz), unless otherwise stated, assume a 1kHz sinewave at 0dBFS (24/96) at the input, 2Vrms at the output into 100k ohms, and a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channels.

Parameter Manufacturer SoundStage! Lab
SNR (1kHz at 24/96 0dBFS, A-weighted, 2Vrms out) 121dB 119dB
THD+N (1kHz at 24/96 0dBFS, A-weighted, 2Vrms out) 0.00018% 0.00019%
Headphone out SNR (A-weighted, 300 ohms) 119dB 117dB
Headphone out THD+N (A-weighted, 300 ohms) -99dB -106dB
Headphone out SNR (A-weighted, 32 ohms) 119dB 117dB
Headphone out THD+N (A-weighted, 32 ohms) -92dB -91dB

Our primary measurements revealed the following using the balanced line-level analog input and digital coaxial input (unless specified, assume a 1kHz sinewave at 2Vrms or 0dBFS, 2Vrms output, 100kohm loading, 10Hz to 22.4kHz bandwidth):

Parameter Left channel Right channel
Crosstalk, one channel driven (10kHz) -86dB -86dB
DC offset <-0.3mV <0.2mV
Gain (RCA in/out, maximum) -0.05dB -0.04dB
IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) <-93dB <-95dB
IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1 ) <-78dB <-79dB
Input impedance (line input, RCA) 22.6k ohms 22.7k ohms
Maximum output voltage (1% THD+N, due to ADC clipping) 2.12Vrms 2.12Vrms
Noise level (with signal, A-weighted)* <13uVrms <13uVrms
Noise level (with signal, 20Hz to 20kHz)* <20uVrms <20uVrms
Noise level (no signal, A-weighted, volume min)* <2.7uVrms <2.7uVrms
Noise level (no signal, 20Hz to 20kHz, volume min)* <3.3uVrms <3.3uVrms
Output impedance (RCA) 11.4 ohms 11.7 ohms
Signal-to-noise ratio (2Vrms out, A-weighted, 2Vrms in)* 105dB 105dB
Signal-to-noise ratio (2Vrms out, 20Hz to 20kHz, 2Vrms in)* 100dB 100dB
Dynamic range (2Vrms out, A-weighted, digital 24/96) 119dB 119dB
Dynamic range (2Vrms out, A-weighted, digital 16/44.1) 96dB 96dB
THD ratio (unweighted) <0.00036% <0.00031%
THD ratio (unweighted, digital 24/96) <0.00008% <0.00005%
THD ratio (unweighted, digital 16/44.1) <0.0004% <0.0004%
THD+N ratio (A-weighted) <0.00041% <0.00036%
THD+N ratio (A-weighted, digital 24/96) <0.00019% <0.00019%
THD+N ratio (A-weighted, digital 16/44.1) <0.0016% <0.0016%
THD+N ratio (unweighted) <0.00038% <0.00034%

Our primary measurements revealed the following using the MM phono-level input (unless specified, assume a 1kHz sinewave at 5mVrms, 1.9Vrms output, 100kohm loading, 10Hz to 22.4kHz bandwidth):

Parameter Left channel Right channel
Crosstalk, one channel driven (10kHz) -73dB -74dB
DC offset <-0.3mV <0.4mV
Gain (default phono preamplifier) 51.6dB 51.5dB
IMD ratio (18kHz and 19 kHz stimulus tones) <-50dB <-50dB
IMD ratio (3kHz and 4kHz stimulus tones) <-62dB <-62dB
Input impedance 38.3k ohms 38.1k ohms
Input sensitivity (1.9Vrms out, max volume) 5mVrms 5mVrms
Noise level (with signal, A-weighted) <210uVrms <210uVrms
Noise level (with signal, 20Hz to 20kHz) <1000uVrms <1000uVrms
Overload margin (relative 5mVrms input, 1kHz) 10.6dB 10.6dB
Signal-to-noise ratio (1.9Vrms out, A-weighted) 78dB 78dB
Signal-to-noise ratio (1.9Vrms out, 20Hz to 20kHz) 67dB 68dB
THD (unweighted) <0.012% <0.012%
THD+N (A-weighted) <0.018% <0.018%
THD+N (unweighted) <0.07% <0.07%

Our primary measurements revealed the following using the analog input at the headphone output (unless specified, assume a 1kHz sinewave, 0dBFS (24/96) input, 2Vrms output, 300 ohms loading, 10Hz to 22.4kHz bandwidth):

Parameter Left or Right channel
Maximum gain 7.1dB
Maximum output power into 600 ohms 33mW
Maximum output power into 300 ohms 66mW
Maximum output power into 32 ohms 164mW
Output impedance 41 ohms
Maximum output voltage (100k ohm load) 4.5Vrms
Noise level (with signal, A-weighted) <9uVrms
Noise level (with signal, 20Hz to 20kHz) <12uVrms
Noise level (no signal, A-weighted, volume min) <6uVrms
Noise level (no signal, 20Hz to 20kHz, volume min) <9uVrms
Dynamic range (A-weighted, 1% THD, 4.5Vrms out) 117dB
Dynamic range (20Hz - 20kHz, 1% THD, 4.5Vrms out) 114dB
THD ratio (unweighted) <0.0002%
THD+N ratio (A-weighted) <0.0005%
THD+N ratio (unweighted) <0.0006%

Frequency response (line-level input)

frequency response

In our measured frequency-response (relative to 1kHz) plot above, the Ultra is near flat within the audioband (-0.2dB at 20Hz and 20kHz). At the extremes, the Ultra is -2.5dB at 5Hz, and brickwall filtering just below 96kHz (due to the 24/192 ADC sampling). In the graph above and most of the graphs below, only a single trace may be visible. This is because the left channel (blue or purple trace) is performing identically to the right channel (red or green trace), and so they perfectly overlap, indicating that the two channels are ideally matched.

Frequency response vs. ADC sample rate (line-level analog input)

frequency response vs adc sample rate

In our measured frequency-response (relative to 1kHz) plot above, the blue/red traces are sampled at 44.1kHz, purple/green at 48kHz, pink/orange at 96kHz, and finally the two green traces are sampled using the default 192kHz (seen in the first graph above but limited to 80kHz). At high frequencies, there is nothing unusual, as we see brickwall type filtering around half the sample rate for all data. At low frequencies, strangely, we see different low-frequency extensions. The best results are with the 44.1kHz and 48kHz sample rates (-0.5dB at 5Hz), then 96kHz (-1.0dB at 5Hz), and finally the worst performer, the 192kHz sample rate (-2.5dB at 5Hz).

Frequency response with bass management (line-level analog input)

frequency response sub out

In our measured frequency response (relative to 1kHz for main output, relative to 20Hz for the subout) plot above, the blue trace is the subout, whereas the red trace is the main left output. The crossover frequency was chosen at 80Hz. We see 12dB/octave slopes, and a crossover point at the correct selected frequency.

Phase response (line-level analog input)

phase response

Above is the phase-response plot from 20Hz to 20kHz for the line-level input. The Ultra does not invert polarity, but because the signal is digitized, there is a significant amount of phase shift (350000 degrees of shift at 20kHz) due to the delay involved in performing the sampling.

Phase response (line-level analog input, excess only)

phase response excess

Above is the phase response plot from 20Hz to 20kHz for the line-level input, only showing excess phase shift (beyond the timing delays due to ADC sampling). We see just shy of +20 degrees at 20Hz, and no phase shift at 20kHz.

Frequency response vs. input type

frequency response vs input type

The chart above shows the Ultra’s frequency response (relative to 1kHz) as a function of input type. The green trace is the same (but limited to 80kHz) analog input data from the previous graph (sampled at 24/192). The blue trace is for a 16-bit/44.1kHz dithered digital input signal from 5Hz to 22kHz using the coaxial input, the purple trace is for a 24/96 dithered digital input signal from 5Hz to 48kHz, and finally pink is 24/192 from 5Hz to 96kHz. The behavior at low frequencies is the same for all the digital sample rates, perfectly flat down to 5Hz, as opposed to the -2.5dB at 5Hz response from the sampled analog input. The behavior at high frequencies for all three digital sample rates is as expected, offering brickwall type filtering around 22, 48, and 96kHz (half the respective sample rate). Also as expected, the analog input (sampled at 24/192), follows the exact same high frequency response as the 24/192 input data.

Frequency response vs. digital filter type (16/44.1)

frequency response vs filter type

The chart above shows the Ultra’s frequency response (relative to 1kHz) for three filter types with a 16/44.1 input signal. The blue trace is the Brick Wall filter, red is Corrected Minimum Phase Fast Roll-Off, and green is Apodizing Fast Roll-Off. The Brick Wall filter has a -3dB point at 19.9kHz and is near 0dB at 19kHz. The Corrected Minimum Phase Fast Roll-Off filter has a -3dB point at 19kHz and is at -0.5dB at 18kHz. The Apodizing Fast Roll-Off filter has a -3dB point just past 20kHz and is near 0dB at 20kHz, though shows a ripple response (+/-0.2dB) from 10kHz to 20kHz.

frequency response vs filter type

The chart above shows the Ultra’s frequency response (relative to 1kHz) for four filter types with a 16/44.1 input signal. The pink trace is the Minimum Phase Slow Roll-Off filter, blue is Minimum Phase Fast Roll-Off, purple is Linear Phase Slow Roll-Off, and orange is Linear Phase Fast Roll-Off (the default filter). The Minimum Phase Slow Roll-Off filter has a -3dB point at 19.1kHz and is at -0.5dB at 17kHz. The Minimum Phase Fast Roll-Off filter has a -3dB point at 21.1kHz and is near 0dB at 19kHz. The Linear Phase Slow Roll-Off filter has a -3dB point just past 19.7kHz and is at -0.5dB at 17.3kHz. The Linear Phase Fast Roll-Off filter has a -3dB point at 21.1kHz and is near 0dB at 19kHz, essentially identical to the Minimum Phase Fast Roll-Off filter.

Phase response vs filter type (16/44.1 input, excess only)

frequency response vs filter type

Above is the phase-response plot (excess) from 20Hz to 20kHz for the first three filter type with a 16/44.1 input signal. The blue trace is the Brick Wall filter, red is Corrected Minimum Phase Fast Roll-Off, and green is Apodizing Fast Roll-Off. The Brick Wall and Apodizing Fast Roll-Off filters have identical behaviour at -45 degrees at 20kHz. The Corrected Minimum Phase Fast Roll-Off filter shows a bump from 10-15kHz (-20 to -10 degrees), then a steep phase roll-off (-180 degrees at 20kHz).

frequency response vs filter type

Above is the phase response plot (excess) from 20Hz to 20kHz for the next four filter types with a 16/44.1 input signal. The pink trace is the Minimum Phase Slow Roll-Off filter, blue is Minimum Phase Fast Roll-Off, purple is Linear Phase Slow Roll-Off, and orange is Linear Phase Fast Roll-Off (the default filter). The Minimum Phase Slow Roll-Off and Minimum Phase Fast Roll-Off filters have very close behaviours, right around -180 degrees at 20kHz. The Linear Phase Slow Roll-Off and Linear Phase Fast Roll-Off filters are identical at -45 degrees at 20kHz.

Frequency response (MM input)

frequency response phono mm

The chart above shows the frequency response (relative to 1 kHz) for the MM phono input. What is shown is the deviation from the RIAA curve, where the input signal sweep is EQd with an inverted RIAA curve supplied by Audio Precision (i.e., zero deviation would yield a flat line at 0dB). The result shows relatively small maximum deviations within the audioband: about +0.2dB at 50kHz and +0.5dB at 20kHz. At low frequencies we see -1dB at 20Hz, and the -3dB point is at 15Hz.

Phase response (MM phono input)

phase response phono mm

Above is the phase response plot from 20Hz to 20kHz for the MM phono input. The Ultra does not invert polarity, but because the signal is digitized, there is a significant amount of phase shift (400000 degrees of shift at 20kHz) due to the delay involved in performing the sampling.

Phase response (MM phono input, excess only)

phase response phono mm excess

Above is the phase response plot from 20Hz to 20kHz for the phono MM input only showing excess phase shift (beyond the timing delays due to ADC sampling). For the phono input, since the RIAA equalization curve must be implemented, which ranges from +19.9dB (20Hz) to -32.6dB (90kHz), phase shift at the output is inevitable. We see typical phase shifts associated with the implementation of RIAA filters: +100 degrees at 20Hz, 0 degrees at 200Hz and 5-6kHz, +20 degrees at 1kHz.

Digital linearity (16/44.1 and 24/96 data)

digital linearity 1644 1 2496

The chart above shows the results of a linearity test for the coaxial digital input (the optical input performed identically) for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the outputs of the Ultra. For this test, the digital input is swept with a dithered 1kHz input signal from -120dBFS to 0dBFS, and the output is analyzed by the APx555. The ideal response would be a straight flat line at 0dB. At -120dBFS, the 16/44.1 data overshot by only 1-2dB, while the 24/96 remained perfect. To verify how well the 24/96 data would perform down to -140dBFS, we extended the sweep in the chart below.

digital linearity 1644 1 2496 extended

Here we can see that the 24/96 data only missed the mark by +1/-1dB (left/right) at -140dBFS. This is an exceptional digital-linearity test result.

Impulse response (24/48 data)

impulse response

The graph above shows the impulse response for a looped 24/44.1 test file that moves from digital silence to full 0dBFS (all “1”s) for one sample period then back to digital silence, measured at the outputs of the Ultra. The blue trace is the Brick Wall filter, red is Corrected Minimum Phase Fast Roll-Off, and green is Apodizing Fast Roll-Off. Both the Brick Wall and Apodizing Fast Roll-Off filters show typical symmetrical sinc functions with pre/post ringing. The Corrected Minimum Phase Fast Roll-Off filter minimizes pre-ringing but shows extensive post-ringing.

impulse response

This chart shows the remaining four filters. The pink trace is the Minimum Phase Slow Roll-Off filter, blue is Minimum Phase Fast Roll-Off, purple is Linear Phase Slow Roll-Off, and orange is Linear Phase Fast Roll-Off (the default filter). The Minimum Phase Slow Roll-Off filter shows no pre-ringing and minimum post-ringing. The Minimum Phase Fast Roll-Off filter shows no pre-ringing but significant post-ringing. The Linear Phase Slow Roll-Off filter is closest to an idealized impulse response: symmetrical with very little pre- and post-ringing. The Linear Phase Fast Roll-Off filter is another example of a typical symmetrical Sinc function with pre/post ringing.

J-Test (optical input)

jtest optical 2448

The chart above shows the results of the J-Test test for the optical digital input measured at the line-level output of the Ultra. TheJ-Test was developed by Julian Dunn the 1990s. It is a test signal—specifically, a -3dBFS undithered 12kHz squarewave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz squarewave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sinewave (the main peak). In addition, an undithered 250Hz squarewave at -144dBFS is mixed with the signal. This test file causes the 22 least-significant bits to constantly toggle, which produces strong jitter spectral components at the 250Hz rate and its odd harmonics. The test file shows how susceptible the DAC and delivery interface are to jitter, which would manifest as peaks above the noise floor at 500Hz intervals (e.g., 250Hz, 750Hz, 1250Hz, etc). Note that the alternating peaks are in the test file itself, but at levels of -144dBrA and below.  The test file can also be used in conjunction with artificially injected sinewave jitter by the Audio Precision, to show how well the DAC rejects jitter.

The optical SPDIF input of the Ultra shows a strong J-Test result, with spurious peaks at the -130dBrA and below level.

J-Test (optical, 2kHz sinewave jitter at 100ns)

jtest coaxial 2448 2khz 100ns

The chart above shows the results of the J-Test test for the optical digital input measured at the line-level output of the Ultra, but with an additional 100ns of 2kHz sinewave jitter injected by the APx555. The results are essentially the same as the J-Test result without additional jitter, with no visible peaks at the 10kHz and 14kHz positions. Further evidence of the Ultra’s strong jitter immunity.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (optical input, Filter 1)

wideband fft noise plus 19 1khz 1644 1kHz

The chart above shows a fast Fourier transform (FFT) of the Ultra’s outputs with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the optical digital input, sampled at 16/44.1, using the Brick Wall filter. The steep roll-off around 20kHz in the white-noise spectrum shows the use of a brickwall-type reconstruction filter. There are no obvious aliased images within the audioband. The primary aliasing signal at 25kHz is highly suppressed at -100dBrA.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (optical input, Filter 2)

wideband fft noise plus 19 1khz 1644 1kHz

The chart above shows a fast Fourier transform (FFT) of the Ultra’s outputs with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the optical digital input, sampled at 16/44.1, using the Corrected Minimum Phase Fast Roll-Off filter. The steep roll-off around 20kHz in the white-noise spectrum shows the use of a brickwall-type reconstruction filter. There are no obvious aliased images within the audioband. The primary aliasing signal at 25kHz is highly suppressed at -110dBrA.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (optical input, Filter 3)

wideband fft noise plus 19 1khz 1644 1kHz

The chart above shows a fast Fourier transform (FFT) of the Ultra’s outputs with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the optical digital input, sampled at 16/44.1, using the Apodizing Fast Roll-Off filter. The steep roll-off around 20kHz in the white-noise spectrum shows the use of a brickwall-type reconstruction filter. There are no obvious aliased images within the audioband. The primary aliasing signal at 25kHz is highly suppressed at -100dBrA.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (optical input, Filter 4)

wideband fft noise plus 19 1khz 1644 1kHz

The chart above shows a fast Fourier transform (FFT) of the Ultra’s outputs with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the optical digital input, sampled at 16/44.1, using the Minimum Phase Slow Roll-Off  filter. The slow roll-off around 20kHz in the white-noise spectrum matches the description of the reconstruction filter. There are no obvious aliased images within the audioband. The primary aliasing signal at 25kHz is barely suppressed at -30dBrA.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (optical input, Filter 5)

wideband fft noise plus 19 1khz 1644 1kHz

The chart above shows a fast Fourier transform (FFT) of the Ultra’s outputs with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the optical digital input, sampled at 16/44.1, using the Minimum Phase Fast Roll-Off filter. The steep roll-off around 20kHz in the white-noise spectrum shows the use of a brickwall-type reconstruction filter. There are no obvious aliased images within the audioband. The primary aliasing signal at 25kHz is highly suppressed at -110dBrA.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (optical input, Filter 6)

wideband fft noise plus 19 1khz 1644 1kHz

The chart above shows a fast Fourier transform (FFT) of the Ultra’s outputs with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the optical digital input, sampled at 16/44.1, using the Linear Phase Slow Roll-Off filter. The slow roll-off around 20kHz in the white-noise spectrum matches the description of the reconstruction filter. There are no obvious aliased images within the audioband. The primary aliasing signal at 25kHz is barely suppressed at -30dBrA.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (optical input, Filter 7)

wideband fft noise plus 19 1khz 1644 1kHz

The chart above shows a fast Fourier transform (FFT) of the Ultra’s outputs with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the optical digital input, sampled at 16/44.1, using the Linear Phase Fast Roll-Off filter. The steep roll-off around 20kHz in the white-noise spectrum shows the use of a brick-wall type reconstruction filter. There are no obvious aliased images within the audioband. The primary aliasing signal at 25kHz is highly suppressed at -110dBrA.

THD ratio (unweighted) vs. frequency vs. load (analog)

thd vs frequency vs load

The chart above shows THD ratios at the line-level output into 100k ohms (blue/red) and 600 ohms (purple/green) as a function of frequency for the analog inputs. The 100k-ohm THD data are identical to the 600-ohm data, an indication that the Ultra will have no issues with sub-1k ohm amplifier input impedances. THD data ranged from 0.05% at 20Hz, down to 0.003% from 200Hz to 4kHz, then up to 0.006% at 20kHz.

THD ratio (unweighted) vs. frequency vs. sample rate (16/44.1 and 24/96)

thd vs frequency 16 441 24 96

The chart above shows THD ratios at the line-level output into 100k ohms for a 16/44.1 (blue/red) dithered 1kHz signal at the optical input and a 24/96 (purple/green) signal, as a function of frequency. The 16/44.1 THD ratios were higher (around 0.0002%) than the 24/96 THD ratios due to the increased noise floor from the lower bit-depth (the analyzer cannot assign a THD ratio for harmonic peaks it cannot see above the noise floor). 24/96 THD ratios were lower (by about 5dB) for the right channel from 30Hz to 1kHz, ranging from 0.00003% to 0.00005%. At 20kHz, all THD ratios measured 0.0002%.

THD ratio (unweighted) vs. frequency (MM phono input)

thd ratio unweighted vs frequency phono mm

The graph above shows THD ratio as a function of frequency plot for the phono input. The input sweep is EQ’d with an inverted RIAA curve. The THD values ranged from around 0.3% (20Hz) down to around 0.003% (200Hz to 300Hz), then up to 0.06% at 20kHz.

THD ratio (unweighted) vs. output (analog)

thd ratio unweighted vs output

The chart above shows THD ratios measured at the outputs of the Ultra as a function of output voltage for the analog line-level input, with the volume control set to maximum. THD values start at 0.2% at 1mVrms, down to a low of 0.001% at 0.2-0.5Vrms, then a rise to 0.003% nearing 2Vrms, then the 1% THD mark, due to ADC clipping, at 2.15Vrms.

THD+N ratio (unweighted) vs. output (analog)

thd ratio unweighted vs output

The chart above shows THD+N ratios measured at the outputs of the Ultra as a function of output voltage for the analog line-level input, with the volume control at maximum. THD+N values start at 3% at 1mVrms, down to a low of 0.003% at 1-1.9Vrms, then a steep rise to the 1% THD mark, due to ADC clipping, at 2.15Vrms.

THD ratio (unweighted) vs. output (16/44.1 and 24/96)

thd vs output 16 441 24 96

The chart above shows THD ratios measured at the outputs of the Ultra as a function of output voltage for the digital optical S/PDIF input, swept from -90dBFS to 0dBFS, with the volume control at maximum. Blue/red traces are for 16/44.1 data, and purple/green for 24/96. For the 16/44.1 data, THD values start at 2%, and predictably, reach their low at the maximum output voltage of just over 2Vrms, at 0.0002%. For the 24/96 data, THD ratios ranged from 0.2% down to 0.00005% at the maximum output voltage.

THD+N ratio (unweighted) vs. output (16/44.1 and 24/96)

thd vs output 16 441 24 96

The chart above shows THD+N ratios measured at the outputs of the Ultra as a function of output voltage for the digital optical S/PDIF input, swept from -90dBFS to 0dBFS, with the volume control at maximum. Blue/red traces are for 16/44.1 data, and purple/green for 24/96. For the 16/44.1 data, THD+N values start at 20% and reach their low at the maximum output voltage of just over 2Vrms, at 0.002%. For the 24/96 data, THD+N ratios ranged from 1.5% down to 0.0002% at the maximum output voltage.

Intermodulation distortion vs. generator level (SMPTE, 60Hz:4kHz, 4:1, 16/44.1, 24/96)

intermodulation distortion SMPTE vs generator level 441k 96k

The chart above shows intermodulation distortion (IMD) ratios measured at output for 16/44.1 (blue/red) input data and 24/96 input data (purple/green), from -60dBFS to 0dBFS. Here, the SMPTE IMD method was used, where the primary frequency (F1 = 60Hz) and the secondary frequency (F2 = 7kHz) are mixed at a ratio of 4:1. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 16/44.1 data yields IMD ratios from 2% down to 0.003% at 0dBFS. The 24/96 data yields IMD ratios from 0.15% down to 0.0005% at 0dBFS.

FFT spectrum – 1kHz (analog line-level input)

FFT spectrum 1khz

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the outputs for the analog line-level input. We see that the signal’s second (2kHz), third (3kHz) and fifth (5kHz) harmonics dominate, at -90/-100/-115dBrA, or 0.003/0.001/0.0002%. Subsequent signal harmonics can be seen at and below the -120dBrA, or 0.0001%, level. Below 1kHz, we see two small peaks from power-supply noise artifacts at 60Hz and 180Hz, around the -130dBrA, or 0.00003%, level.

FFT spectrum – 1kHz (digital input, 16/44.1 data at 0dBFS)

fft spectrum 1khz 1644 1 0dbfs

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the outputs for the optical digital input, sampled at 16/44.1. We see the third (3kHz) signal harmonic just above the noise floor at -130dBrA, or 0.00003%. The noise floor is much higher due to the 16-bit depth limitation.

FFT spectrum – 1kHz (digital input, 24/96 data at 0dBFS)

fft spectrum 1khz 2496 0dbfs

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the outputs for the optical digital input, sampled at 24/96. We see the second (2kHz), third (3kHz) and fifth (5kHz) signal harmonics at the very low levels of -140/-130/-140dBrA, or 0.00003% to 0.00001%. Power-supply-related noise peaks can be seen at and below the very low level of -135dBrA, or 0.00002% at 60/180/300Hz.

FFT spectrum – 1kHz (digital input, 16/44.1 data at -90dBFS)

fft spectrum 1khz 1644 1 90dbfs

Shown above is the FFT for a 1kHz -90dBFS dithered 16/44.1 input sinewave stimulus at the optical digital input, measured at the analog outputs. We see the 1kHz primary signal peak, at the correct amplitude, and no signal- or noise-related harmonic peaks above the -135dBrA noise floor.

FFT spectrum – 1kHz (digital input, 24/96 data at -90dBFS)

fft spectrum 1khz 2496 90dbfs

Shown above is the FFT for a 1kHz -90dBFS dithered 24/96 input sinewave stimulus at the optical digital input, measured at the analog outputs. We see the 1kHz primary signal peak, at the correct amplitude, and signal- and noise-related peaks at around and below -140dBrA, or 0.00001%.

FFT spectrum – 1kHz (digital input, 24/96 data at -0dBFS by putting volume control at minimum)

fft spectrum 1khz 2496 60dbfs

Shown above is the FFT for a 1kHz -0dBFS dithered 24/96 input sinewave stimulus at the optical digital input, measured at the analog outputs, with the volume set to minimum, which yields a signal at -60dBrA. This FFT can be compared to the FFT below, which shows . . .

FFT spectrum – 1kHz (digital input, 24/96 data at -60dBFS by reducing analyzer level but with volume control at maximum)

fft spectrum 1khz 2496 60dbfs

. . . a 1kHz -60dBFS dithered 24/96 input sinewave stimulus but wth the volume set to maximum. We find that both FFTs are essentially the same, indicating that the digital volume control, even at the lowest levels, is transparent.

FFT spectrum – 1kHz (MM phono input)

fft spectrum 1khz phono mm

Shown above is the FFT for a 1kHz input sinewave stimulus, measured at the analog outputs for the MM phono input. The dominant signal related harmonics can be seen at 2/3/4/5/6/7/10kHz, at -80dBrA to -100dBrA, or 0.01% to 0.001%. The main noise-related peak can be seen around 80Hz at -70dBrA, or 0.03%.

FFT spectrum – 50Hz (line-level input)

fft spectrum 50hz

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the outputs for the line-level input. The X axis is zoomed in from 40 Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. The most predominant (non-signal) peaks are that of the signal’s second (100Hz) harmonic at -80dBrA, or 0.01%, and the third signal harmonic (150Hz) at -90dBrA, or 0.003%. A small power-supply-related noise peak can be seen at 180Hz at -130dBrA, or 0.00003%.

FFT spectrum – 50Hz (MM phono input)

fft spectrum 50hz phono mm

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the outputs for the MM phono input. The most predominant (non-signal) peaks are that of the signal’s second (100Hz) and third (150Hz) harmonics, as well as two noise-related peaks clustered just past 80Hz. All these peaks are around the -70dBrA level, or 0.03%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, line-level input)

intermodulation distortion fft 18khz 19khz summed stimulus

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the outputs for the analog line-level input. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, it would yield 10W (0dBrA) into 8 ohms at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -110dBRA, or 0.0003%, while the third-order modulation products, at 17kHz and 20kHz, are at the same level.

Intermodulation distortion FFT (line-level input, APx 32 tone)

fft spectrum 32 tone

Shown above is the FFT of the outputs of the Ultra with the APx 32-tone signal applied to the analog input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 2Vrms. The intermodulation products—i.e., the “grass” between the test tones—are distortion products from the preamplifier and are around the -100dBrA, or 0.001%, level from 20Hz to 100Hz, and below the -120dBrA, or 0.0001%, level from 300Hz to 20kHz.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, coaxial digital input, 16/44.1)

intermodulation distortion fft 18khz 19khz summed stimulus 1644-1

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the outputs for the digital optical input at 16/44.1. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is difficult to distinguish above the -135dBrA noise floor, while the third-order modulation products, at 17kHz and 20kHz, are at -125dBrA, or 0.00006%. We also see the main aliased peaks at 25.1kHz and 26.1kHz at around -120dBrA.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, coaxial digital input, 24/96)

intermodulation distortion fft 18khz 19khz summed stimulus 2496

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs for the digital coaxial input at 24/96. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is just below -130dBrA, or 0.00003%, while the third-order modulation products, at 17kHz and 20kHz, are at -125dBrA, or 0.00006%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, MM phono input)

intermodulation distortion FFT 18kHz 19kHz summed stimulus phono mm

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the outputs for the phono MM input. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -95dBrA or 0.002%, while the third-order modulation products, at 17kHz and 20kHz, are at -60dBrA, or 0.1%.

Square-wave response (10kHz)

square wave response 10kHz

Above is the 10kHz squarewave response using the analog line-level input, at roughly 2Vrms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the Ultra’s slew-rate performance. Rather, it should be seen as a qualitative representation of its limited bandwidth to the 24/912 digital sampling. An ideal squarewave can be represented as the sum of a sinewave and an infinite series of its odd-order harmonics (e.g., 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot, and softening of the edges. The Ultra’s reproduction of the 10kHz square wave is relatively clean for a digitized input, but due to the limited bandwidth, rippling can be seen in the plateaus of the squarewave.

Diego Estan
Electronics Measurement Specialist

Eversolo Audio DMP-A8 Streaming Preamplifier Measurements

Details
Parent Category: Products
Category: Preamplifier Measurements
Created: 01 November 2024

Link: reviewed by Roger Kanno on SoundStage! Simplifi on November 1, 2024

General Information

All measurements taken using an Audio Precision APx555 B Series analyzer.

The Eversolo Audio DMP-A8 was conditioned for 30 minutes at 2Vrms in/out into 200k ohms before any measurements were taken.

The DMP-A8 offers a multitude of inputs, both digital and analog (balanced and unbalanced), and line-level analog outputs over balanced XLR and unbalanced over RCA. For the purposes of these measurements, unless otherwise stated, the following inputs were evaluated: digital coaxial S/PDIF (RCA) and analog balanced (XLR). Comparisons were made between unbalanced (RCA) and balanced (XLR) line inputs and outputs, and no appreciable differences were seen in terms of gain and THD+N (FFTs for different configurations can be seen in this report).

Most measurements were made with a 2Vrms line-level and 0dBFS digital input with the volume set to achieve 2Vrms at the output. The signal-to-noise ratio (SNR) measurements were made with the same input signal values, and, for comparison, on the line-level input, a SNR measurement was also made with the volume at maximum, but with a lower input voltage to achieve the same 2Vrms output.

The DMP-8 offers a wide range of DSP functions that can only be applied to the digital inputs. All DSP functions were turned off for these measurements. There are also six different DAC reconstruction filters that can be selected (unless otherwise noted, the Sharp Roll-off filter was used for these measurements):

  1. Sharp Roll-off
  2. Slow Roll-off
  3. Short Delay Sharp Roll-off
  4. Short Delay Slow Roll-off
  5. Super Slow Roll-off (emulation of NOS)
  6. Low Dispersion Short Delay

The DMP-A8 also offers a range of volume step settings: 0.5/1/2/3dB. Unless otherwise stated, the 0.5dB setting was used for all measurements. Based on the accuracy and random results of the left/right volume channel matching (see table below), the DMP-A8 volume control is likely digitally controlled in the analog domain. The overall range is from -89.3dB to 9.9dB for the line-level inputs.

Volume-control accuracy (measured at preamp outputs): left-right channel tracking

Volume position Channel deviation
-89.5dB 0.003dB
-80dB 0.001dB
-70dB 0.006dB
-60dB 0.003dB
-50dB 0.006dB
-40dB 0.010dB
-30dB 0.010dB
-20dB 0.005dB
-10dB 0.014dB
0dB 0.013dB
+5dB 0.010dB
+10dB 0.014dB

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Eversolo for the DMP-A8 compared directly against our own. The published specifications are sourced from Eversolo’s website, either directly or from the manual available for download, or a combination thereof. With the exception of frequency response, where the Audio Precision bandwidth was set at its maximum (DC to 1MHz), unless otherwise stated, assume a 1kHz sinewave at 2Vrms or 0dBFS at the input, 2Vrms at the output into 200k ohms, and a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channels.

Parameter Manufacturer SoundStage! Lab
Output level (0dBFS, XLR) 4.2Vrms 4.35Vrms
Output level (0dBFS, RCA) 2.1Vrms 2.17Vrms
Frequency response 20Hz-20kHz (±0.25dB) 20Hz-20kHz (0/-0.03dB)
Dynamic range (24/96@0dBFS, max output XLR, Awgt) >128dB 129.3dB
Dynamic range (24/96@0dBFS, max output RCA, Awgt) >125dB 128.3dB
SnR (24/96@0dBFS, max output XLR, Awgt) >128dB 128.5dB
SnR (24/96@0dBFS, max output RCA, Awgt) >125dB 127.9dB
THD+N (1kHz, 2Vrms in/out, XLR, Awgt) <0.00009% <0.00008%
THD+N (1kHz, 2Vrms in/out, RCA, Awgt) <0.0001% <0.000094%
Crosstalk (1kHz, 24/96@0dBFS in/2Vrms out) >-121dB -122dB

Our primary measurements revealed the following using the balanced line-level analog input and digital coaxial input (unless specified, assume a 1kHz sinewave at 2Vrms or 0dBFS when a sample rate is specified, 2Vrms output, 200k-ohm loading, 10Hz to 22.4kHz bandwidth):

Parameter Left channel Right channel
Crosstalk, one channel driven (10kHz) -146dB -143dB
DC offset <-0.24mV <0.14mV
Gain (RCA in/out) 9.86dB 9.88dB
Gain (XLR in/out) 9.92dB 9.94dB
IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) <-110dB <-111dB
IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1 ) <-105dB <-107dB
Input impedance (line input, RCA) 11.7k ohms 11.7k ohms
Input impedance (line input, XLR) 23.9k ohms 24.4k ohms
Maximum output voltage (at clipping 1% THD+N) 16.9Vrms 16.9Vrms
Maximum output voltage (at clipping 1% THD+N into 600 ohms) 11Vrms 11Vrms
Noise level (with signal, A-weighted)* <1.35uVrms <1.35uVrms
Noise level (with signal, 20Hz to 20kHz)* <1.69uVrms <1.69uVrms
Noise level (with signal, A-weighted, RCA)* <1.29uVrms <1.29uVrms
Noise level (no signal, A-weighted, volume min)* <0.75uVrms <0.75uVrms
Noise level (no signal, 20Hz to 20kHz, volume min)* <0.92uVrms <0.92uVrms
Noise level (no signal, A-weighted, volume min, RCA)* <0.55uVrms <0.55uVrms
Output impedance (RCA) 51.8 ohms 51.9 ohms
Output impedance (XLR) 102 ohms 102 ohms
Signal-to-noise ratio (2Vrms out, A-weighted, 2Vrms in)* 128.1dB 127.8dB
Signal-to-noise ratio (2Vrms out, 20Hz to 20kHz, 2Vrms in)* 126.2dB 125.8dB
Signal-to-noise ratio (2Vrms out, A-weighted, max volume)* 123.1dB 122.9dB
Signal-to-noise ratio (2Vrms out, A-weighted, 2Vrms in, RCA)* 127.5dB 128.1dB
Dynamic range (2Vrms out, A-weighted, digital 24/96)* 125.3dB 125.4dB
Dynamic range (2Vrms out, A-weighted, digital 16/44.1)* 96.1dB 96.1dB
THD ratio (unweighted) <0.00002% <0.00002%
THD ratio (unweighted, digital 24/96) <0.0001% <0.00007%
THD ratio (unweighted, digital 16/44.1) <0.0004% <0.0004%
THD+N ratio (A-weighted) <0.000078% <0.000078%
THD+N ratio (A-weighted, digital 24/96) <0.00014% <0.00011%
THD+N ratio (A-weighted, digital 16/44.1) <0.0016% <0.0016%
THD+N ratio (unweighted) <0.00012% <0.00012%

* due to very low noise of DUT, analyzer self-noise has been removed from measurement to more accurately report value

Frequency response (line-level input)

frequency response

In our measured frequency response (relative to 1kHz) plot above, the DMP-A8 is near perfectly flat within the audioband (0dB at 20Hz, -0.05dB at 20kHz). At the extremes, the DMP-A8 is 0dB at 5Hz, -0.6dB at 100kHz, and -5dB just before 200kHz. The DMP-A8 appears to be DC-coupled, as there is no attenuation at low frequencies, even at 5Hz. In the graph above and most of the graphs below, only a single trace may be visible. This is because the left channel (blue or purple trace) is performing identically to the right channel (red or green trace), and so they perfectly overlap, indicating that the two channels are ideally matched.

Phase response (line-level input)

phase response

Above is the phase response plot from 20Hz to 20kHz for the balanced line-level input. The DMP-A8 does not invert polarity and exhibits, at worst, less than -10 degrees (at 20kHz) of phase shift within the audioband.

Frequency response vs. input type (left channel only, Sharp Roll-off filter)

frequency response vs input type

The chart above shows the DMP-A8’s frequency response (relative to 1kHz) as a function of input type. The green trace is the same (but limited to 80kHz) analog input data from the previous graph. The blue trace is for a 16-bit/44.1kHz dithered digital input signal from 5Hz to 22kHz using the coaxial input, the purple trace is for a 24/96 dithered digital input signal from 5Hz to 48kHz, and finally pink is 24/192 from 5Hz to 96kHz. The behavior at low frequencies is the same for all the digital sample rates, as well as the analog input—flat down to 5Hz. The behavior at high frequencies for all three digital sample rates is as expected, offering filtering around 22k, 48k, and 96kHz (half the respective sample rates). The 44.1kHz sampled input signal exhibits typical “brick-wall”-type behavior, with a -3dB point at 20.9kHz. The -3dB point for the 96kHz sampled data is at 45.5kHz, and 68.9kHz for the 192kHz sampled data.

Frequency response vs. filter type (16/44.1, Sharp Roll-off, Slow Roll-off, Super Slow Roll-off)

frequency response vs filter type

The plots above show frequency-response for a -3dBFS input signal sampled at 16/44.1 for the Sharp Roll-off filter (blue), the Slow Roll-off filter (red), and the Super Slow Roll-off filter (green) into a 200k ohm-load for the left channel only. The graph is zoomed in from 1kHz to 22kHz to highlight the various responses of the three filters. We can see that the Sharp Roll-Off  filter provides a “brick-wall”-type response, and the Slow Roll-Off filter shows gentle attenuation (-1dB at 12.8kHz), and the Super Slow Roll-Off filter shows the gentlest attenuation (-1dB at 11kHz). The -3dB points for all three filters are: 20.9kHz (blue), 15.8kHz (red), and 18.7kHz (green).

Frequency response vs. filter type (16/44.1, Short Delay Sharp Roll-off, Short Delay Slow Roll-off, Low Dispersion Short Delay)

frequency response vs filter type

The plots above show frequency-response for a -3dBFS input signal sampled at 16/44.1 for the Short Delay Sharp Roll-off filter (blue), the Short Delay Slow Roll-off filter (red), and the Low Dispersion Short Delay filter (green) into a 200k ohm-load for the left channel only. The graph is zoomed in from 1kHz to 22kHz to highlight the various responses of the three filters. We can see that both the Short Delay Sharp Roll-Off and Low Dispersion Short Delay filters provide a “brick-wall”-type response very similar to the default Sharp Roll-off filter, while the Short Delay Slow Roll-off filter shows gentle attenuation (-1dB at 12.8kHz). The -3dB points for all three filters are: 20.9kHz (blue), 15.8kHz (red), and 20.8kHz (green).

Phase response vs. filter type (16/44.1, all filters)

phase response vs filter type

Above are the phase response plots from 20Hz to 20kHz for a -3dBFS input signal sampled at 16/44.1 for the Sharp Roll-off filter (blue), the Slow Roll-off filter (red), the Short Delay Sharp Roll-off filter (green), the Short Delay Slow Roll-off filter (pink), the Super Slow Roll-off filter (purple), and the Low Dispersion Short Delay filter (orange) into a 200k ohm-load for the left channel only. We find that only the two short-delay filters show any phase shift within the audioband: -180 and -80 degrees at 20kHz for the Sharp and Slow Roll-off short-delay filters, respectively.

Digital linearity (16/44.1 and 24/96 data)

digital linearity 1644 1 2496

The chart above shows the results of a linearity test for the coaxial digital input (the optical input performed identically) for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the balanced outputs of the DMP-A8. The digital input was swept with a dithered 1kHz input signal from -120dBFS to 0dBFS, and the output was analyzed by the APx555. The ideal response would be a straight flat line at 0dB. At -120dBFS, the 16/44.1 data overshot by only 1dB, while the 24/96 remained perfect. To verify how well the 24/96 data would perform down to -140dBFS, we extended the sweep . . .

digital linearity 1644 1 2496 extended

. . . in the chart above. Here we can see that the 24/96 data only undershot the mark by -1dB (left) at -140dBFS. This is an exceptional, and essentially perfect, digital-linearity test result.

Impulse response (24/48 data)

impulse response 2448

The graph above shows the impulse responses for a looped 24/44.1 test file that moves from digital silence to full 0dBFS (all “1”s) for one sample period then back to digital silence, measured at the balanced outputs of the DMP-A8. The blue plot is for the Sharp Roll-off filter, red for Slow Roll-off, and green for Short Delay Sharp Roll-off. The Sharp Roll-off filter shows a typical symmetrical sinc function response. The Slow Roll-off filter also shows a symmetrical response but with much less pre- and post-ringing. The Short Delay Sharp Roll-off filter shows no pre-ringing but extensive post-ringing.

Impulse response (24/48 data)

impulse response 2448

The graph above shows the impulse responses for a looped 24/44.1 test file that moves from digital silence to full 0dBFS (all “1”s) for one sample period then back to digital silence, measured at the balanced outputs of the DMP-A8. The blue plot is for the Short Delay Slow Roll-off filter, red for Super Slow Roll-off, and green for Low Dispersion Short Delay. The Short Delay Slow Roll-off filter shows no pre-ringing and minimized pots-ringing. The Super Slow Roll-Off filter shows a near ideal response, close to what would be expected from an NOS DAC—a sharp impulse with no pre- and post-ringing. The Low Dispersion Short Delay filter shows minimized pre-ringing with more post-ringing.

J-Test (coaxial input)

jtest coaxial 2448

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level output of the DMP-A8. J-Test was developed by Julian Dunn in the 1990s. It is a test signal—specifically, a -3dBFS undithered 12kHz squarewave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz squarewave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sinewave (the main peak). In addition, an undithered 250Hz squarewave at -144dBFS is mixed with the signal. This test file causes the 22 least-significant bits to constantly toggle, which produces strong jitter spectral components at the 250Hz rate and its odd harmonics. The test file shows how susceptible the DAC and delivery interface are to jitter, which would manifest as peaks above the noise floor at 500Hz intervals (e.g., 250Hz, 750Hz, 1250Hz, etc). Note that the alternating peaks are in the test file itself, but at levels of -144dBrA and below.  The test file can also be used in conjunction with artificially injected sinewave jitter by the Audio Precision, to show how well the DAC rejects jitter.

The coaxial S/PDIF input of the DMP-A8 shows a near-perfect J-Test result, with only a few small peaks in the audioband at a extraordinarily low -155dBrA.

J-Test (optical input)

jtest optical 2448

The chart above shows the results of the J-Test test for the optical digital input measured at the balanced outputs of the DMP-A8. The results here are very similar to the result from the coaxial input above.

J-Test (coaxial, 2kHz sinewave jitter at 10ns)

jtest coaxial 2448 2khz 10ns

The chart above shows the results of the TJ-Test test for the coaxial digital input measured at the line level output of the DMP-A8, with an additional 10ns of 2kHz sinewave jitter injected by the APx555. The results are essentially the same as the J-Test result without additional jitter. The same was true for the optical input.

J-Test (coaxial, 2kHz sinewave jitter at 100ns)

jtest coaxial 2448 2khz 100ns

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level output of the DMP-A8, with an additional 100ns of 2kHz sinewave jitter injected by the APx555. The results are essentially the same as the J-Test result without additional jitter. The same was true for the optical input. This is an exceptional result.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (coaxial input, Sharp Roll-off filter)

wideband fft noise plus 19 1khz 1644 1kHz

The chart above shows a fast Fourier transform (FFT) of the DMP-A8’s balanced outputs with white noise at -4 dBFS (blue/red), and a 19.1 kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1, using the Sharp Roll-off filter. The steep roll-off around 20kHz in the white-noise spectrum shows that this filter is of the brick-wall-type variety. There are no obvious aliased images within the audioband. The primary aliasing signal at 25kHz is highly suppressed at -120dBrA, while the second and third distortion harmonics (38.2, 57.3kHz) of the 19.1kHz signal are at similar levels.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (coaxial input, Slow Roll-off filter)

wideband fft noise plus 19 1khz 1644 1kHz

The chart above shows a fast Fourier transform (FFT) of the DMP-A8’s balanced outputs with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1, using the Slow Roll-off filter. The slow roll-off around 20kHz matches Eversolo’s description for this filter. There is one aliased image within the audioband, around 13kHz at -130dBrA. The primary aliasing signal at 25kHz is barely suppressed at -20dBrA, while the second and third distortion harmonics (38.2, 57.3kHz) of the 19.1kHz signal are much lower at -120dBrA and below.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (coaxial input, Short Delay Sharp Roll-off filter)

wideband fft noise plus 19 1khz 1644 1kHz

The chart above shows a fast Fourier transform (FFT) of the DMP-A8’s balanced outputs with white noise at -4 dBFS (blue/red, and a 19.1 kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1, using the Short Delay Sharp Roll-off filter. The steep roll-off around 20kHz in the white-noise spectrum shows that this filter is of the brick-wall-type variety. There are no obvious aliased images within the audioband. The primary aliasing signal at 25kHz is completely suppressed, while the second and third distortion harmonics (38.2, 57.3kHz) of the 19.1kHz signal are at and below -110dBrA.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (coaxial input, Short Delay Slow Roll-off filter)

wideband fft noise plus 19 1khz 1644 1kHz

The chart above shows a fast Fourier transform (FFT) of the DMP-A8’s balanced outputs with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1, using the Short Delay Slow Roll-off filter. The slow roll-off around 20kHz matches Eversolo’s description for this filter. There is one aliased image within the audioband, around 13kHz at -130dBrA. The primary aliasing signal at 25kHz is barely suppressed at -20dBrA, while the second and third distortion harmonics (38.2, 57.3kHz) of the 19.1kHz signal are much lower at -120dBrA and below.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (coaxial input, Super Slow Roll-off filter)

wideband fft noise plus 19 1khz 1644 1kHz

The chart above shows a fast Fourier transform (FFT) of the DMP-A8’s balanced outputs with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at -1dBFS fed to the coaxial digital input, sampled at 16/44.1, using the Super Slow Roll-Off filter. The very gentle roll-off around 20kHz and repeating nature in the noise spectrum at intervals of the 44.1kHz sample rate are indicative of a filter emulating a NOS DAC. There are obvious aliased images within the audioband at 13.2/7.3/1.4kHz at -80/-100/-115dBrA. The primary aliasing signal at 25kHz is barely suppressed at -10dBrA.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (coaxial input, Low Dispersion Short Delay filter)

wideband fft noise plus 19 1khz 1644 1kHz

The chart above shows a fast Fourier transform (FFT) of the DMP-A8’s balanced outputs with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1, using the Low Dispersion Short Delay filter. The medium-steep roll-off around 20kHz in the white noise spectrum shows that this filter is of the brick-wall type variety. There are no obvious aliased images within the audioband. The primary aliasing signal at 25kHz is at -50dBrA, while the second and third distortion harmonics (38.2, 57.3kHz) of the 19.1kHz signal are at and below -110dBrA.

THD ratio (unweighted) vs. frequency vs. load (analog)

thd vs frequency vs load

The chart above shows THD ratios at the balanced line-level output into 200k ohms (blue/red) and 600 ohms (purple/green) as a function of frequency for the analog balanced inputs. The 200k-ohm THD data are lower than the 600-ohm data by roughly 5dB. The 200k-ohm data range from 0.00003% at 20Hz, down to an astonishingly low 0.00002% at 40Hz to 1kHz, then a rise to 0.0002% at 20kHz. It’s important to note that these THD values are less than twice as high as the AP’s signal generator, thereby pushing the limits of the analyzer capabilities.

THD ratio (unweighted) vs. frequency vs. sample rate (16/44.1 and 24/96)

thd vs frequency 16 441 24 96

The chart above shows THD ratios at the balanced line-level output into 200k ohms for a 16/44.1 (blue/red) dithered 1kHz signal at the coaxial input and a 24/96 (purple/green) signal, as a function of frequency. The 16/44.1 THD ratios were higher (0.0003% to 0.0002%) than the 24/96 THD ratios (0.00006% to 0.0001%) from 20Hz to 6kHz, due to the increased noise floor from the lower bit-depth (the analyzer cannot assign a THD ratio for harmonic peaks it cannot see above the noise floor). At 20kHz, all THD ratios measured 0.0005%.

THD ratio (unweighted) vs. output (analog)

thd ratio unweighted vs output

The chart above shows THD ratios measured at the balanced outputs of the DMP-A8 as a function of output voltage for the balanced line level-input, with the volume control at maximum. THD values start at 0.02% at 1mVrms, down to a low of 0.00003/0.00005% (left/right) at 1-3Vrms, then a steep rise past 12Vrms to the 1% THD mark at 16.9Vrms.

THD+N ratio (unweighted) vs. output (analog)

thd ratio unweighted vs output

The chart above shows THD+N ratios measured at the balanced outputs of the DMP-A8 as a function of output voltage for the balanced line level-input, with the volume control at maximum. THD+N values start at 0.2% at 1mVrms, down to a low of 0.0001% at 2-10Vrms, then a steep rise past 12Vrms to the 1% THD mark at 16.9Vrms.

THD ratio (unweighted) vs. output (16/44.1 and 24/96)

thd vs output 16 441 24 96

The chart above shows THD ratios measured at the balanced outputs of the DMP-A8 as a function of output voltage for the digital coaxial S/PDIF input, swept from -90dBFS to 0dBFS, with the volume control at maximum. Blue/red traces are for 16/44.1 data, and purple/green for 24/96. For the 16/44.1 data, THD values start at 2%, and predictably, reach their low at the maximum output voltage of about 4.3Vrms, at 0.0002%. For the 24/96 data, THD ratios ranged from 0.05% down to 0.00008% at the maximum output voltage.

THD+N ratio (unweighted) vs. output (16/44.1 and 24/96)

thd vs output 16 441 24 96

The chart above shows THD+N ratios measured at the balanced outputs of the DMP-A8 as a function of output voltage for the digital coaxial S/PDIF input, swept from -90dBFS to 0dBFS, with the volume control at maximum. Blue/red traces are for 16/44.1 data, and purple/green for 24/96. For the 16/44.1 data, THD+N values start at 20% and reach their low at the maximum output voltage of about 4.3Vrms, at 0.002%. For the 24/96 data, THD ratios ranged from 0.5% down to 0.00015% at the maximum output voltage.

Intermodulation distortion vs. generator level (SMPTE, 60Hz:4kHz, 4:1, 16/44.1, 24/96)

intermodulation distortion SMPTE vs generator level 441k 96k

The chart above shows intermodulation distortion (IMD) ratios measured at balanced output for 16/44.1 (blue/red) input data and 24/96 input data (purple/green), from -60dBFS to 0dBFS. Here, the SMPTE IMD method was used, where the primary frequency (F1 = 60Hz) and the secondary frequency (F2 = 7kHz) are mixed at a ratio of 4:1. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 16/44.1 data yields IMD ratios from 2% down to 0.002% at 0dBFS. The 24/96 data yields IMD ratios from 0.1% down to around 0.0005% near 0dBFS.

FFT spectrum – 1kHz (XLR line-level input)

FFT spectrum 1khz

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced outputs for the balanced line-level input. Because both the noise and signal related peaks are so vanishingly low for the DMP-A8, we must directly compare against the loopback (AP signal generator’s outputs connected to the analyzer’s inputs using the same XLR cables used with the DUT) FFT spectrum below. For the signal related harmonics, we find roughly the same levels at the second (2kHz) position: -150/-140dBrA (left/right), or 0.000003/0.00001%. We therefore attribute these peaks to the signal generator and not the DMP-A8. At the third (3kHz) position, we find that the signal generator contributes -150dBrA (0.000003%) of the -140dBrA (0.00001%) levels seen above. Subtracting one from the other yields a level of 0.000007% distortion at the 3kHz position for the DMP-A8. The only significant signal-related harmonic peak remaining for both channels is at the fifth (5kHz) harmonic position at -150dBrA, or 0.000003%. This means that the true THD ratio for the DMP-A8 is at the absurdly low 0.00001% (-140dBrA) level. For the noise-related peaks, we can ignore the peaks at the 60 and 120Hz positions, as these are also seen in the loopback FFT below. What we are left with are a few spurious peaks (mostly right channel) at near and below -150dBrA, or 0.000003%. This is an exceptionally quiet device. In fact, the residual rms noise (volume at minimum, A-weighted) we measured from the DMP-A8 (0.55uVrms for RCA output, 0.75uVrms XLR) essentially equals the lowest noise we have ever seen in a device (RME ADI-2 DAC IEM headphone output). These levels are right around the noise levels inherent to the AP’s signal generator (0.66 uVrms, A-weighted). A truly phenomenal result, which translates to a signal-to-noise ratio (A-weighted, relative to 2Vrms) of 128dB!

FFT spectrum – 1kHz (XLR line-level input)

FFT spectrum 1khz loopback

Shown above is the fast Fourier transform (FFT) for a 1kHz 2Vrms input sinewave stimulus, measured by directly connecting the AP signal generator to the analyzer using our balanced XLR interconnects.

FFT spectrum – 1kHz (RCA line-level input)

FFT spectrum 1khz

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the unbalanced outputs for the unbalanced line-level input. We see close to the same results as with the balanced input FFT above. The main difference is the higher 2kHz signal harmonic peak at -130dBrA instead of -140dBrA.

FFT spectrum – 1kHz (digital input, 16/44.1 data at 0dBFS)

fft spectrum 1khz 1644 1 0dbfs

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced outputs for the coaxial digital input, sampled at 16/44.1. We see the second (2kHz) and third (3kHz) signal harmonics at roughly -130dBrA, or 0.00003%, and -120dBrA, or 0.0001%, respectively.  The noise floor is much higher due to the 16-bit depth limitation.

FFT spectrum – 1kHz (digital input, 24/96 data at 0dBFS)

fft spectrum 1khz 2496 0dbfs

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced outputs for the coaxial digital input, sampled at 24/96. We see power-supply-related noise peaks but only near and below the -150dBrA, or 0.000003%, level. The second (2kHz) and third (3kHz) signal harmonics dominate at -130dBrA, or 0.00003%, and -120dBrA, or 0.0001%. Higher signal harmonics can be seen at -140dBrA, or 0.00001%, and below.

FFT spectrum – 1kHz (digital input, 16/44.1 data at -90dBFS)

fft spectrum 1khz 1644 1 90dbfs

Shown above is the FFT for a 1kHz -90dBFS dithered 16/44.1 input sinewave stimulus at the coaxial digital input, measured at the balanced outputs. We see the 1kHz primary signal peak, at the correct amplitude, and no signal or noise-related harmonic peaks above the -140dBrA noise floor.

FFT spectrum – 1kHz (digital input, 24/96 data at -90dBFS)

fft spectrum 1khz 2496 90dbfs

Shown above is the FFT for a 1kHz -90dBFS dithered 24/96 input sinewave stimulus at the coaxial digital input, measured at the balanced outputs. We see the 1kHz primary signal peak, at the correct amplitude, and no signal-related harmonic peaks. We see power-supply-related noise peaks but only near and below the -150dBrA, or 0.000003%, level.

FFT spectrum – 50Hz (line-level input)

fft spectrum 50hz

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the balanced outputs for the balanced line-level input. The X axis is zoomed in from 40 Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. The most predominant (non-signal) peak is that of the signal’s third (150Hz) harmonic at -135dBrA or 0.00002%. Power-supply-related peaks can be seen but only near and below the -150dBrA, or 0.000003%, level.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, line-level input)

intermodulation distortion fft 18khz 19khz summed stimulus

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs for the balanced line-level input. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 10W (0dBrA) into 8 ohms at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -125dBRA or 0.00006%, while the third-order modulation products, at 17kHz and 20kHz, are at -120dBrA or 0.0001%. This is a very clean IMD result.

Intermodulation distortion FFT (line-level input, APx 32 tone)

fft spectrum 32 tone

Shown above is the FFT of the balanced outputs of the DMP-A8 with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 2Vrms. The intermodulation products—i.e., the “grass” between the test tones—are distortion products from the preamplifier and are around the extremely low -150dBrA, or 0.000003%, level.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, coaxial digital input, 16/44.1)

intermodulation distortion fft 18khz 19khz summed stimulus 1644-1

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs for the digital coaxial input at 16/44.1. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -130dBrA (left channel), or 0.00003%, while the third-order modulation products, at 17kHz and 20kHz, are at -120dBrA, or 0.0001%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, coaxial digital input, 24/96)

intermodulation distortion fft 18khz 19khz summed stimulus 2496

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs for the digital coaxial input at 24/96. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -130/140dBrA (left/right), or 0.00003/0.00001%, while the third-order modulation products, at 17kHz and 20kHz, are at -120dBrA or 0.0001%.

Square-wave response (10kHz)

square wave response 10kHz

Above is the 10kHz squarewave response using the analog line-level input, at roughly 2Vrms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the DMP-A8’s slew-rate performance. Rather, it should be seen as a qualitative representation of its extended bandwidth. An ideal squarewave can be represented as the sum of a sinewave and an infinite series of its odd-order harmonics (e.g., 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot, and softening of the edges. The DMP-A8’s reproduction of the 10kHz square wave is very clean, with only extremely mild softening in the edges.

Diego Estan
Electronics Measurement Specialist

Simaudio Moon 891 Streaming Preamplifier Measurements

Details
Parent Category: Products
Category: Preamplifier Measurements
Created: 01 October 2024

Link: reviewed by Aron Garrecht on SoundStage! Ultra on October 1, 2024

General Information

All measurements taken using an Audio Precision APx555 B Series analyzer.

The Simaudio Moon 891 was conditioned for 30 minutes at 2Vrms in/out into 200k ohms before any measurements were taken.

The 891 offers a multitude of inputs, both digital and analog (balanced and unbalanced), as well as line-level analog balanced outputs over XLR and unbalanced over RCA. For the purposes of these measurements, unless otherwise stated, the following inputs were evaluated: digital coaxial S/PDIF (RCA), analog balanced (XLR), and phono RCA, configured using the default settings for moving-magnet (MM) and moving-coil (MC) cartridges. Comparisons were made between unbalanced (RCA) and balanced (XLR) line inputs and outputs, and no appreciable differences were seen in terms of gain and THD+N (FFTs for different configurations can be seen in this report).

Most measurements were made with a 2Vrms line-level analog and 0dBFS digital input with the volume set to achieve 2Vrms at the output. For the phono input, a 5mVrms MM level and 0.5mVrms MC level was used to achieve 1Vrms at the output. The signal-to-noise ratio (SNR) measurements were made with the same input signal values and, for comparison, on the line-level input, a SNR measurement was also made with the volume at maximum, but with a lower input voltage to achieve the same 2Vrms output.

The 891 also offers a range of gain settings (40 in total) by using the Offset feature in the configuration menu. The menu allows for a setting between -10dB to +10dB, in 0.5dB steps, individually assignable to each input. Note that this changes the gain for the input, it does not offset the volume level. Also of note, despite the -10dB to +10dB in the menu, actual gain varies from roughly -6dB to +14dB. The default setting in the menu is +6dB (+10dB of actual gain), which is, unless otherwise stated, what was used for these measurements.

The 891 offers two different volume steps in the setup menu: 0.5dB and 0.1dB. Based on the accuracy and random results of the left/right volume channel matching (see table below), the 891 volume control is likely digitally controlled but operating in the analog domain.

The 891 offers an incredible 620 (using the 0.1dB setting) volume steps from -69dB to 9.7dB for the line-level inputs. The first 20 steps (0 to 20dB) are in 1dB increments, and then the 20dB to 80dB volume positions can be changed in 0.1dB increments. Turning the volume knob quickly will increase the volume step sizes. It is also worth highlighting the superb channel matching in the table below. The worst-case deviation seen was 0.006dB, and very often throughout the volume range, left/right channel matching was measured at 0.000dB. This superb channel matching, along with the 620 steps and 0.1dB resolution, coupled with the volume knob’s silky smooth “feel” (also replicated on the remote control), add-up to what is arguably the finest volume control on any consumer audio device anywhere.

Volume-control accuracy (measured at preamp outputs): left-right channel tracking

Volume position Channel deviation
1 0.005dB
10 0.004dB
20 0.003dB
25 0.002dB
30 0.000dB
35 0.002dB
40 0.003dB
45 0.002dB
50 0.001dB
55 0.000dB
60 0.000dB
65 0.001dB
70 0.001dB
75 0.000dB
80 0.006dB

Published specifications vs. our primary measurements

The table below summarizes the measurements published by SimAudio for the 891 compared directly against our own. The published specifications are sourced from SimAudio’s website, either directly or from the manual available for download, or a combination thereof. With the exception of frequency response, where the Audio Precision bandwidth was set at its maximum (DC to 1MHz), unless otherwise stated, assume a 1kHz sinewave at 2Vrms or 0dBFS at the input, 2Vrms at the output into 200k ohms, and a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channels.

Parameter Manufacturer SoundStage! Lab
Input impedance (line level, RCA) 22k ohms 25.5k ohms
Maximum gain (line level) 13.5dB 13.6dB
Phono gain 40/54/60/66dB 40.4/54.1/60.1/66.5dB
Phono input resistance 10/100/470/1k/47k ohms 11.7/100/466/999/44.3k ohms
Output impedance (RCA) 50 ohms 51 ohms
Crosstalk (1kHz) -125dB -137dB
Frequency response (line-level) 2Hz-200kHz (0, -3dB) 2Hz-200kHz (0, -3dB)
SNR (line level, A-weighted, 4Vrms out) 125dB 125.5dB
Dynamic range (digital input, 24/96, fixed output) 125dB 124dB
THD+N (at 1kHz, 10Hz to 22.4kHz bandwidth) 0.0003% 0.00017%
IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) 0.00006% 0.0003%

Our primary measurements revealed the following using the balanced line-level analog input and digital coaxial input (unless specified, assume a 1kHz sinewave at 2Vrms or 0dBFS, 2Vrms output, 200kohm loading, 10Hz to 22.4kHz bandwidth):

Parameter Left channel Right channel
Crosstalk, one channel driven (10kHz) -122.5dB -119.3dB
DC offset <1.3mV <-1.0mV
Gain (RCA in/out, default) 9.6dB 9.6dB
Gain (XLR in/out, default) 9.7dB 9.7dB
Gain (RCA in/out, maximum) 13.6dB 13.6dB
Gain (XLR in/out, maximum) 13.7dB 13.7dB
IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) <-118dB <-118dB
IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1 ) <-106dB <-108dB
Input impedance (line input, RCA) 25.5k ohms 25.1k ohms
Input impedance (line input, XLR) 53.4k ohms 53.4k ohms
Maximum output voltage (at clipping 1% THD+N) 19.1Vrms 19.1Vrms
Maximum output voltage (at clipping 1% THD+N into 600 ohms) 14.8Vrms 14.8Vrms
Noise level (with signal, A-weighted)* <2.2uVrms <2.2uVrms
Noise level (with signal, 20Hz to 20kHz)* <2.8uVrms <2.8uVrms
Noise level (with signal, A-weighted, RCA)* <2.6uVrms <2.6uVrms
Noise level (no signal, A-weighted, volume min)* <0.97uVrms <0.97uVrms
Noise level (no signal, 20Hz to 20kHz, volume min)* <1.23uVrms <1.23uVrms
Noise level (no signal, A-weighted, volume min, RCA)* <1.78uVrms <1.78uVrms
Output impedance (RCA) 51 ohms 51 ohms
Output impedance (XLR) 97 ohms 97 ohms
Signal-to-noise ratio (2Vrms out, A-weighted, 2Vrms in)* 120.1dB 119.9dB
Signal-to-noise ratio (2Vrms out, 20Hz to 20kHz, 2Vrms in)* 118.1dB 118.0dB
Signal-to-noise ratio (2Vrms out, A-weighted, max volume)* 116.1dB 116.0dB
Signal-to-noise ratio (2Vrms out, A-weighted, 2Vrms in, RCA)* 117.9dB 117.9dB
Dynamic range (2Vrms out, A-weighted, digital 24/96)* 120.5dB 120.7dB
Dynamic range (2Vrms out, A-weighted, digital 16/44.1)* 96.1dB 96.1dB
THD ratio (unweighted) <0.00004% <0.00005%
THD ratio (unweighted, digital 24/96) <0.00018% <0.00015%
THD ratio (unweighted, digital 16/44.1) <0.0004% <0.0004%
THD+N ratio (A-weighted) <0.00012% <0.00012%
THD+N ratio (A-weighted, digital 24/96) <0.00022% <0.00019%
THD+N ratio (A-weighted, digital 16/44.1) <0.0016% <0.0016%
THD+N ratio (unweighted) <0.00017% <0.00017%

*due to very low noise of DUT, analyzer self-noise has been removed from measurement to more accurately report value.

Our primary measurements revealed the following using the phono-level input, MM configuration (unless specified, assume a 1kHz sinewave at 5mVrms, 1Vrms output, 200kohm loading, 10Hz to 22.4kHz bandwidth):

Parameter Left channel Right channel
Crosstalk, one channel driven (10kHz) -99dB -101dB
DC offset <-1.4mV <-1.2mV
Gain (default phono preamplifier) 40.4dB 40.4dB
IMD ratio (18kHz and 19 kHz stimulus tones) <-103dB <-103dB
IMD ratio (3kHz and 4kHz stimulus tones) <-100dB <-100dB
Input impedance 44.4k ohms 44.3k ohms
Input sensitivity (1Vrms out, max volume) 3.45mVrms 3.45mVrms
Noise level (with signal, A-weighted) 19uVrms 19uVrms
Noise level (with signal, 20Hz to 20kHz) 45uVrms 45uVrms
Overload margin (relative 5mVrms input, 1kHz) 22.9dB 22.9dB
Signal-to-noise ratio (1Vrms out, A-weighted) 93.1dB 93.2dB
Signal-to-noise ratio (1Vrms out, 20Hz to 20kHz) 86.7dB 87.0dB
THD (unweighted) <0.0004% <0.0004%
THD+N (A-weighted) <0.0019% <0.0019%
THD+N (unweighted) <0.005% <0.005%

Our primary measurements revealed the following using the phono-level input, MC configuration (unless specified, assume a 1kHz sinewave at 0.5mVrms, 1Vrms output, 200kohm loading, 10Hz to 22.4kHz bandwidth):

Parameter Left channel Right channel
Crosstalk, one channel driven (10kHz) -90.9dB -91.4dB
DC offset <-3mV <-2mV
Gain (default phono preamplifier) 58.5dB 58.5dB
IMD ratio (18kHz and 19 kHz stimulus tones) <-85dB <-85dB
IMD ratio (3kHz and 4kHz stimulus tones) <-80dB <-80dB
Input impedance 99.4 ohms 99.7 ohms
Input sensitivity (1Vrms out, max volume) 0.393mVrms 0.393mVrms
Noise level (with signal, A-weighted) <230uVrms <230uVrms
Noise level (with signal, 20Hz to 20kHz) <500uVrms <500uVrms
Overload margin (relative 0.5mVrms input, 1kHz) 24.0dB 24.0dB
Signal-to-noise ratio (1Vrms out, A-weighted) 71.6dB 71.5dB
Signal-to-noise ratio (1Vrms out, 20Hz to 20kHz) 66.0dB 65.6dB
THD (unweighted) <0.0035% <0.0035%
THD+N (A-weighted) <0.023% <0.023%
THD+N (unweighted) <0.06% <0.06%

Frequency response (line-level input)

frequency response

In our measured frequency response (relative to 1kHz) plot above, the 891 is near perfectly flat within the audioband (0dB at 20Hz, -0.05dB at 20kHz). At the extremes, the 891 is 0dB at 5Hz, -0.8dB at 100kHz, and -3dB just past 200kHz. These data corroborate SimAudio’s claim of 2Hz to 200kHz (0/-3dB). The 891 appears to be DC-coupled, as there is no attenuation at low frequencies, even at 5Hz. In the graph above and most of the graphs below, only a single trace may be visible. This is because the left channel (blue or purple trace) is performing identically to the right channel (red or green trace), and so they perfectly overlap, indicating that the two channels are ideally matched.

Phase response (line-level input)

phase response

Above is the phase response plot from 20Hz to 20kHz for the balanced line-level input. The 891 does not invert polarity and exhibits, at worst, less than -10 degrees (at 20kHz) of phase shift within the audioband.

Frequency response vs. input type (left channel only)

frequency response vs input type

The chart above shows the 891’s frequency response (relative to 1kHz) as a function of input type. The green trace is the same (but limited to 80kHz) analog input data from the previous graph. The blue trace is for a 16-bit/44.1kHz dithered digital input signal from 5Hz to 22kHz using the coaxial input, the purple trace is for a 24/96 dithered digital input signal from 5Hz to 48kHz, and, finally, pink is 24/192 from 5Hz to 96kHz. The behavior at low frequencies is the same for all the digital sample rates, as well as the analog input: flat down to 5Hz. The behavior at high frequencies for all three digital sample rates is as expected, offering filtering around 22kHz, 48kHz, and 96kHz (half the respective sample rate). The 44.1kHz sampled input signal exhibits typical “brick-wall”-type behavior, with a -3dB point at 21kHz. The -3dB point for the 96kHz sampled data is at 46kHz, and 91kHz for the 192kHz sampled data.

Frequency response (MM input)

frequency response phono mm

The chart above shows the frequency response (relative to 1 kHz) for the phono input (MM configuration). What is depicted is the deviation from the RIAA curve, where the input signal sweep is EQd with an inverted RIAA curve supplied by Audio Precision (i.e., zero deviation would yield a flat line at 0dB).  The result shows extremely small maximum deviations within the audioband: about +0.1dB from 5kHz to 20kHz and -0.05dB at 20Hz. This is an example of excellent RIAA tracking implemented in the analog domain.

Frequency response (MC input)

frequency response phono mc

The chart above shows the frequency response for the phono input (MC configuration). We see essentially the same result as with the MM configuration.

Phase response (MM and MC phono inputs)

phase response phono mm

Above is the phase response plot from 20Hz to 20kHz for the phono input (MM and MC configurations behaved identically). For the phono input, since the RIAA equalization curve must be implemented, which ranges from +19.9dB (20Hz) to -32.6dB (90kHz), phase shift at the output is inevitable. Here we find a worst case of about -60 degrees at 200Hz and -90 degrees at 20kHz.

Digital linearity (16/44.1 and 24/96 data)

digital linearity 1644 1 2496

The chart above shows the results of a linearity test for the coaxial digital input (the optical input performed identically) for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the balanced outputs of the 891. In this test, the digital input is swept with a dithered 1kHz input signal from -120dBFS to 0dBFS and the output is analyzed by the APx555. The ideal response would be a straight flat line at 0dB. At -120dBFS, the 16/44.1 data overshot by only 1-2dB, while the 24/96 remained perfect. To verify how well the 24/96 data would perform down to -140dBFS, we extended the sweep, shown in the chart below.

digital linearity 1644 1 2496 extended

Here we can see that the 24/96 data only overshot the mark by +1dB (left/right) at -140dBFS. This is an exceptionally good digital-linearity test result.

Impulse response (24/48 data)

impulse response 2448

The graph above shows the impulse response for a looped 24/44.1 test file that moves from digital silence to full 0dBFS (all “1”s) for one sample period then back to digital silence, measured at the balanced outputs of the 891. We can see that the 891 utilizes a reconstruction filter that favors no pre-ringing and significant post-ringing.

J-Test (coaxial input)

jtest coaxial 2448

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level output of the 891. J-Test was developed by Julian Dunn the 1990s. It is a test signal—specifically, a -3dBFS undithered 12kHz squarewave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz squarewave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sinewave (the main peak). In addition, an undithered 250Hz squarewave at -144dBFS is mixed with the signal. This test file causes the 22 least significant bits to constantly toggle, which produces strong jitter spectral components at the 250Hz rate and its odd harmonics. The test file shows how susceptible the DAC and delivery interface are to jitter, which would manifest as peaks above the noise floor at 500Hz intervals (e.g., 250Hz, 750Hz, 1250Hz, etc). Note that the alternating peaks are in the test file itself, but at levels of -144dBrA and below.  The test file can also be used in conjunction with artificially injected sinewave jitter by the Audio Precision, to show how well the DAC rejects jitter.

The coaxial S/PDIF input of the 891 shows a near-perfect J-Test result, with only two very small peaks on either side of the 12kHz fundamental at an extraordinarily low -155dBrA.

J-Test (optical input)

jtest optical 2448

The chart above shows the results of the J-Test test for the optical digital input measured at the balanced outputs of the 891. The results here are similar but not quite as pristine as the coaxial input above. Here, the peaks adjacent to the 12kHz fundamental reach -145dBrA.

J-Test (AES-EBU input)

jtest aes ebu 2448

The chart above shows the results of the J-Test test for the AES-EBU digital input measured at the balanced outputs of the 891. The results here are essentially identical to those found on the coaxial input.

J-Test (coaxial, 2kHz sinewave jitter at 10ns)

jtest coaxial 2448 2khz 10ns

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level output of the 891, with an additional 10ns of 2kHz sinewave jitter injected by the APx555. The results are essentially the same as the J-Test result without additional jitter. The same was true for the optical input (not shown).

J-Test (coaxial, 2kHz sinewave jitter at 100ns)

jtest coaxial 2448 2khz 100ns

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level output of the 891, with an additional 100ns of 2kHz sinewave jitter injected by the APx555. This time, the tell-tale peaks at 10kHz and 12kHz can be seen; however, they are very small in amplitude, just below -120dBrA.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (coaxial input)

wideband fft noise plus 19 1khz 1644 1kHz

The chart above shows a fast Fourier transform (FFT) of the 891’s balanced outputs with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1. The steep roll-off around 20kHz in the white-noise spectrum shows that the 891 uses a brick-wall-type reconstruction filter. There are no obvious aliased images within the audioband. The primary aliasing signal at 25kHz is highly suppressed at -95dBrA, while the second and third distortion harmonics (38.2, 57.3kHz) of the 19.1kHz tone are even lower.

THD ratio (unweighted) vs. frequency vs. load (analog)

thd vs frequency vs load

The chart above shows THD ratios at the balanced line-level output into 200k ohms (blue/red) and 600 ohms (purple/green) as a function of frequency for the analog balanced inputs. The 200k-ohm THD data are lower than the 600-ohm data by 5-10dB. The 200k-ohm data range from 0.0004% at 20Hz, down to an astonishingly low 0.00003% at 300Hz to 1.5kHz (left channel). It’s important to note that these THD values are only about twice as high as the AP’s signal generator, so they are pushing the limits of the analyzer’s capabilities. The right channel did exhibit about 5dB higher THD compared to the left channel above 1kHz. At 20kHz, the left channel into 200k ohms was at 0.0002%, while the right channel was at 0.0003%. The right channel into 600 ohms was at 0.0004% at 20kHz.

THD ratio (unweighted) vs. frequency vs. sample rate (16/44.1 and 24/96)

thd vs frequency 16 441 24 96

The chart above shows THD ratios at the balanced line-level output into 200k ohms for a 16/44.1 (blue/red) dithered 1kHz signal at the coaxial input and a 24/96 (purple/green) signal, as a function of frequency. The 16/44.1 THD ratios were slightly higher (0.0003% to 0.0002%) than the 24/96 THD ratios (0.00007% to 0.0002%) from 20Hz to 2kHz, due to the increased noise floor from the lower bit-depth (the analyzer cannot assign a THD ratio for harmonic peaks it cannot see above the noise floor). At 20kHz, all THD ratios measured 0.001%.

THD ratio (unweighted) vs. frequency (phono input, MM and MC)

thd ratio unweighted vs frequency phono mm mc

The graph above shows THD ratio as a function of frequency plots for the phono input. The MM configuration is shown in blue/red (left/right) and MC in purple/green (left/right). The input sweep was EQ’d with an inverted RIAA curve. The THD values for the MM configuration vary from around 0.004% (20Hz) down to around 0.0002% (1kHz to 10kHz), then up to 0.0003% at 20kHz.  The MC THD values were higher, although these are limited by the higher noise floor (the analyzer cannot assign a THD ratio for harmonic peaks it cannot see above the noise floor). THD ratios ranged from 0.04% (20Hz), down to 0.0006% (8kHz), then up to 0.0015% at 20kHz.

THD ratio (unweighted) vs. output (analog)

thd ratio unweighted vs output

The chart above shows THD ratios measured at the balanced outputs of the 891 as a function of output voltage for the balanced line-level input. THD values start at 0.05% at 1mVrms, down to a low of 0.00004% at 2-3Vrms, then a steep rise past 10Vrms to the 1% THD mark at 20Vrms.

THD+N ratio (unweighted) vs. output (analog)

thd ratio unweighted vs output

The chart above shows THD+N ratios measured at the balanced outputs of the 891 as a function of output voltage for the balanced line level-input, with the volume control at maximum. THD+N values start at 0.4% at 1mVrms, down to a low of 0.00015% at 5Vrms, then a steep rise past 10Vrms to the 1% THD mark at 20Vrms.

THD ratio (unweighted) vs. output (16/44.1 and 24/96)

thd vs output 16 441 24 96

The chart above shows THD ratios measured at the balanced outputs of the 891 as a function of output voltage for the digital coaxial S/PDIF input, swept from -90dBFS to 0dBFS, with the volume control at maximum. Blue/red traces are for 16/44.1 data and purple/green for 24/96. For the 16/44.1 data, THD values start at 3%, and predictably, reach their low at the maximum output voltage of about 6.5Vrms, at 0.0002%. For the 24/96 data, the right channel outperformed the left by 5dB, and THD ratios ranged from 0.2% down to 0.0001% at the maximum output voltage.

THD+N ratio (unweighted) vs. output (16/44.1 and 24/96)

thd vs output 16 441 24 96

The chart above shows THD+N ratios measured at the balanced outputs of the 891 as a function of output voltage for the digital coaxial S/PDIF input, swept from -90dBFS to 0dBFS, with the volume control at maximum. Blue/red traces are for 16/44.1 data, and purple/green for 24/96. For the 16/44.1 data, THD+N values start at 30% and reach their low at the maximum output voltage of about 6.5Vrms, at 0.002%. For the 24/96 data, THD ratios ranged from 1% down to 0.0002% at the maximum output voltage.

Intermodulation distortion vs. generator level (SMPTE, 60Hz:4kHz, 4:1, 16/44.1, 24/96)

intermodulation distortion SMPTE vs generator level 441k 96k

The chart above shows intermodulation distortion (IMD) ratios measured at balanced output for 16/44.1 (blue/red) input data and 24/96 input data (purple/green), from -60dBFS to 0dBFS. Here, the SMPTE IMD method was used, where the primary frequency (F1 = 60Hz) and the secondary frequency (F2 = 7kHz) are mixed at a ratio of 4:1. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 16/44.1 data yields IMD ratios from 2% down to 0.003% at 0dBFS. The 24/96 data yields IMD ratios from 0.15% down to 0.0005% to 0.001% near 0dBFS.

FFT spectrum – 1kHz (XLR line-level input)

FFT spectrum 1khz

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced outputs for the balanced line-level input. We see that the signal’s second harmonic, at 2kHz, is at -130dBrA or 0.00003%, and lower at -135dBrA or 0.00002% at the third (3kHz) harmonic. A few other signal harmonics are at a vanishingly low -140dBrA, or 0.00001%, and below. Below 1kHz, we see two small peaks from power-supply noise artifacts at 60Hz and 120Hz. These are extraordinarily low at around -150dBrA, or 0.000003%, and it must be highlighted that they are actually from the Audio Precision’s sinewave generator, and not inherent to the 891. We can say this with confidence for two reasons: these same peaks can be seen at roughly the same amplitudes when the Audio Precision’s sinewave generator is connected directly to the inputs of its analyzer (loopback), and these peaks are non-existent in the digital 24/96 FFT below, where the Audio Precision’s DAC generator is used instead of its analog sinewave generator. It should also be stressed how extraordinarily low the 891’s noise floor is—quite possibly the quietest preamp we’ve ever measured. The 891 does not seem to have any correlated power-supply-related noise (60Hz and harmonics related, which we would describe as “hum”). The residual A-weighted noise from the 891 (volume control set to minimum with no signal) due to non-correlated thermal noise (what we would describe as “hiss”) was measured at 0.97uVrms, compared to the analyzer’s self-noise of 0.66uVrms. Even with the volume set to the reference position (70.5dB) for our measurements and with a 2Vrms output signal present (notched out by the analyzer), A-weighted noise was measured at 2.2uVrms. It’s important to highlight here that Simaudio have managed to reduce both THD and noise when compared to their 791 model preamp, which measures extraordinarily well. In terms of THD, the main improvement is in the third harmonic, where the 791 yielded -115dBrA (0.0002%), compared to the 891’s -135dBrA (0.00002%). In terms of residual noise, the 791 measured an already astounding 1.23uVrms, only to be outdone by the 891’s 0.97uVrms! We can only commend Simaudio for producing what must be one of the most transparent, if not the most transparent, analog audio products in existence.

FFT spectrum – 1kHz (RCA line-level input)

FFT spectrum 1khz

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the unbalanced outputs for the unbalanced line-level input. We see close to the same results as with the balanced input FFT above, except for a slightly higher 3kHz signal harmonic peak at -130dBrA instead of -135dBrA.

FFT spectrum – 1kHz (digital input, 16/44.1 data at 0dBFS)

fft spectrum 1khz 1644 1 0dbfs

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced outputs for the coaxial digital input, sampled at 16/44.1. We see the second (2kHz) and third (3kHz) signal harmonics at roughly -120dBrA, or 0.0001%. The second harmonic peak for the right channel is at -130dBrA, or 0.00003%. The noise floor is much higher due to the 16-bit depth limitation.

FFT spectrum – 1kHz (digital input, 24/96 data at 0dBFS)

fft spectrum 1khz 2496 0dbfs

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced outputs for the coaxial digital input, sampled at 24/96. We see no power-supply-related noise peaks above the -160dBrA noise floor, and the second (2kHz) and third (3kHz) signal harmonics at roughly -120dBrA, or 0.0001%. The second harmonic peak for the right channel is at -130dBrA, or 0.00003%. Higher signal harmonics can be seen at -130dBrA, or 0.00003%, and below.

FFT spectrum – 1kHz (digital input, 16/44.1 data at -90dBFS)

fft spectrum 1khz 1644 1 90dbfs

Shown above is the FFT for a 1kHz -90dBFS dithered 16/44.1 input sinewave stimulus at the coaxial digital input, measured at the balanced outputs. We see the 1kHz primary signal peak, at the correct amplitude, and no signal- or noise-related harmonic peaks above the -140dBrA noise floor.

FFT spectrum – 1kHz (digital input, 24/96 data at -90dBFS)

fft spectrum 1khz 2496 90dbfs

Shown above is the FFT for a 1kHz -90dBFS dithered 24/96 input sinewave stimulus at the coaxial digital input, measured at the balanced outputs. We see the 1kHz primary signal peak, at the correct amplitude, and signal related harmonic peaks at 2kHz and 4kHz at around -140dBrA, or 0.00001%. Other signal related harmonics can be seen but at the extremely low -150dBrA, or 0.000003%, level.

FFT spectrum – 1kHz (MM phono input)

fft spectrum 1khz phono mm

Shown above is the FFT for a 1kHz input sinewave stimulus, measured at the balanced outputs across for the phono input, configured for MM (default 40dB of gain in phono stage). The dominant signal-related harmonic can be seen at 2kHz, at -115dBrA, or 0.0002%. Power-supply-related noise peaks can be seen at the -95dBrA, or 0.002%, level at 60Hz, and at 180Hz at -105dBrA (right channel), or 0.0006%.

FFT spectrum – 1kHz (MC phono input)

fft spectrum 1khz phono mc

Shown above is the FFT for a 1kHz input sinewave stimulus, measured at the balanced outputs across for the phono input, configured for MC (default 60dB of gain in phono stage). There are no visible signal-related harmonic peaks above the -110 to -120dBrA noise floor. Power-supply-related noise peaks can be seen at the -75dBrA, or 0.02%, level at 60Hz, and at 180Hz at -85dBrA (right channel), or 0.006%.

FFT spectrum – 50Hz (line-level input)

fft spectrum 50hz

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the balanced outputs for the balanced line-level input. The X axis is zoomed in from 40 Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. The most predominant (non-signal) peaks are that of the signal’s second (100Hz) harmonic at -115dBrA or 0.0002%, and the third signal harmonic (150Hz) at -125dBrA, or 0.00006%. Power-supply-related peaks can be seen at 60Hz (-140dBrA or 0.00001%) and 120Hz (-150dBrA or 0.000003%), but as discussed above, these are inherent to the Audio Precision sinewave generator.

FFT spectrum – 50Hz (MM phono input)

fft spectrum 50hz phono mm

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the balanced outputs for the phono input configured for MM. The most predominant (non-signal) peaks are that of the signal’s second (100Hz) harmonic at -110dBrA or 0.0003%, and the primary (60Hz) power-supply-noise harmonic at -95dBrA, or 0.002%.

FFT spectrum – 50Hz (MC phono input)

fft spectrum 50hz phono mc

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the balanced outputs for the phono input configured for MC. The most predominant (non-signal) peak is that of the primary (60Hz) power-supply-noise harmonic at -75dBrA, or 0.02%. No signal harmonic peaks can be seen above the noise floor.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, line-level input)

intermodulation distortion fft 18khz 19khz summed stimulus

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs for the balanced line-level input. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 10W (0dBrA) into 8 ohms at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -125dBrA or 0.00006%, while the third-order modulation products, at 17kHz and 20kHz, are at -120dBrA or 0.0001%. This is a very clean IMD result.

Intermodulation distortion FFT (line-level input, APx 32 tone)

fft spectrum 32 tone

Shown above is the FFT of the balanced outputs of the 891 with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 2Vrms. The intermodulation products—i.e., the “grass” between the test tones—are distortion products from the preamplifier and are around the extremely low -150dBrA, or 0.000003%, level.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, coaxial digital input, 16/44.1)

intermodulation distortion fft 18khz 19khz summed stimulus 1644-1

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs for the digital coaxial input at 16/44.1. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -130dBrA (left channel), or 0.00003%, while the third-order modulation products, at 17kHz and 20kHz, are at -120dBrA, or 0.0001%. We also see the main aliased peaks at 25.1kHz and 26.1kHz at around -100dBrA.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, coaxial digital input, 24/96)

intermodulation distortion fft 18khz 19khz summed stimulus 2496

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs for the digital coaxial input at 24/96. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -130dBrA (left), or 0.00003%, and -150dBrA (right), or 0.000003%, while the third-order modulation products, at 17kHz and 20kHz, are at -120dBrA, or 0.0001%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, MM phono input)

intermodulation distortion FFT 18kHz 19kHz summed stimulus phono mm

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs across for the phono input configured for MM. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -105dBrA, or 0.0006%, while the third-order modulation products, at 17kHz and 20kHz, are at -120dBrA, or 0.0001%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, MC phono input)

intermodulation distortion FFT 18kHz 19kHz summed stimulus phono mc

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs across for the phono input configured for MC. We find that the second-order modulation product (i.e., the difference signal of 1kHz) for the left channel is just barely noticeable above the noise floor at -105dBrA, or 0.0006%, while the third-order modulation products, at 17kHz and 20kHz, are at around -120dBrA, or 0.0001%, but only visible at 20kHz.

Square-wave response (10kHz)

square wave response 10kHz

Above is the 10kHz squarewave response using the analog line-level input, at roughly 2Vrms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the 891’s slew-rate performance. Rather, it should be seen as a qualitative representation of its extended bandwidth. An ideal squarewave can be represented as the sum of a sinewave and an infinite series of its odd-order harmonics (e.g., 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot, and softening of the edges. The 891 reproduction of the 10kHz square wave is very clean, with only extremely mild softening in the edges.

Diego Estan
Electronics Measurement Specialist

Sonic Frontiers SFL-2 Preamplifier Measurements

Details
Parent Category: Products
Category: Preamplifier Measurements
Created: 01 October 2024

Link: written about by Jason Thorpe on SoundStage! Ultra on October 1, 2024

General information

The Sonic Frontiers SFL-2 was released in about 1995. The SFL-2 under test here was owned by SoundStage! writer Jason Thorpe. He bought it in the late 1990s and had used it in his system until late 2023. He’s since sold it to a consumer. But before he sold it, we wanted to measure it to gauge its current level of performance. It was measured in March 2024. Jason reports that the tubes at time of measurements had under 2000 hours on them, which he felt meant they had plenty of life left since they were rated for about 10,000 hours.

All measurements taken using an Audio Precision APx555 B Series analyzer.

The SFL-2 was conditioned for 30 minutes at 2Vrms at the output before any measurements were taken. All measurements were taken with both channels driven.

The SFL-2 offers five sets of line-level unbalanced (RCA) inputs, one set of line-level balanced (XLR) inputs, and two sets each of unbalanced and balanced outputs. The balanced outputs offer 6dB more gain than the unbalanced outputs. The volume control is a stepped attenuator with 23 positions (including the minimum position, which is grounded). Based on the accuracy and repeatable nature of the channel deviation (table below), the volume control is in the analog domain but must have been, at one time, carefully level matched between channels at each volume position due to the high level of accuracy. The stepped attenuator offers, for the most part, 3dB increments throughout the entire range. There’s an additional 0dB/-1.5dB switch that can enable 1.5dB of attenuation, for a finer adjustment.  

There is a significant difference in terms of THD between unbalanced and balanced inputs and outputs in the SFL-2 (see both the main table and FFTs below). The lowest THD configuration measured was balanced in/balanced out, although the left channel exhibits much higher THD than the right channel. Unless otherwise stated, measurements were made with the volume set to unity gain, using the XLR inputs and outputs, with a 2Vrms input.

Volume-control accuracy (measured at preamp outputs): left-right channel tracking

{

Volume position Channel deviation
1 0.144
2 0.138
3 0.140
4 0.146
5 0.145
6 0.142
7 0.144
8 0.143
9 0.144
10 0.142
11 0.144
12 0.145
13 0.143
14 0.143
15 0.143
16 0.144
17 0.146
18 0.149
19 0.149
20 0.144
21 0.146
22 0.143

Primary measurements

Our primary measurements revealed the following using the balanced line-level inputs (unless otherwise specified, assume a 1kHz sinewave, 2Vrms input and output into 200k ohms load, 10Hz to 22.4kHz bandwidth):

Parameter Left channel Right channel
Crosstalk, once channel driven (10kHz) -58.9dB -59.7dB
DC offset <0.3mV <0.7mV
Gain (default) 28.7dB 28.5dB
IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) <-76dB <-89dB
IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1 ) <-67dB <-88dB
Input impedance (balanced) 116k ohms 123k ohms
Input impedance (unbalanced) 57.4k ohms 54.0k ohms
Maximum output voltage (at clipping 1% THD+N) 85.9Vrms 90Vrms
Maximum output voltage (at clipping 1% THD+N into 600 ohms) 1.2Vrms 1.5Vrms
Noise level (with signal, A-weighted) <84uVrms <62uVrms
Noise level (with signal, 20Hz to 20kHz) <125uVrms <101uVrms
Noise level (no signal, A-weighted, volume min) <84uVrms <62uVrms
Noise level (no signal, 20Hz to 20kHz, volume min) <125uVrms <101uVrms
Output impedance (balanced) 797 ohms 786 ohms
Output impedance (unbalanced) 442 ohms 430 ohms
Signal-to-noise ratio (A-weighted, 2Vrms out) 86.7dB 89.1dB
Signal-to-noise ratio (20Hz-20kHz), 2Vrms out) 83.7dB 86.0dB
Signal-to-noise ratio (max volume, 2Vrms out, A-weighted) 85.9dB 87.8dB
THD (unweighted, balanced) <0.014% <0.0006%
THD (unweighted, unbalanced) <0.081% <0.118%
THD+N (A-weighted) <0.014% <0.0032%
THD+N (unweighted) <0.014% <0.006%

Frequency response

frequency response

In our measured frequency-response plot above, the SFL-2 is essentially perfectly flat within the audioband, and 0dB at 5Hz and -1.5dB at 200kHz. The SFL-2 appears to be DC-coupled, as it yielded 0dB of deviation at 5Hz. In the graph above and most of the graphs below, only a single trace may be visible. This is because the left channel (blue or purple trace) is performing identically to the right channel (red or green trace), and so they perfectly overlap, indicating that the two channels are ideally matched.

Phase response

phase response

Above is the phase-response plot from 20Hz to 20kHz. The blue/red traces are with the phase switch set to 0 degrees, the purple/green traces are with the phase switch set to 180 degrees. The SFL-2 does not invert polarity (except when the phase switch is set to 180 degrees), and it yielded a worst-case 5 degrees or so of phase shift at 20kHz.

THD ratio (unweighted) vs. frequency

thd vs frequency vs load

The plot above shows THD ratios at the output as a function of frequency (20Hz to 20kHz) for a sinewave input stimulus. The blue and red plots are for left and right channels into 200k ohms, while purple/green (L/R) are into 600 ohms. As previously mentioned, there are gross differences in THD ratios between the left and right channels into 200k ohms. The right channel ranged from 0.003% at 20Hz, down to 0.0004% at 1 to 2kHz, then up to 0.003% at 20kHz. It should be noted that because this is a tube-based preamp, noise levels are higher than what would be seen in a well-designed solid-state preamp. As such, the limiting factor in assigning THD ratios for the right channel into 200k ohms is largely due to the noise floor (the analyzer cannot assign a THD ratio below the noise floor). THD ratios for the left channel into 200k ohms were relatively constant through the audioband, just above 0.01%. The 600-ohm THD ratio were considerable higher, ranging from 0.03/0.1% (left/right) at 20Hz, then up to 0.3/0.4% (left/right) from 200Hz to 20kHz. The SFL-2 is significantly impacted by a lower impedance load, due to the high output impedance (about 800 ohms) tube outputs. This is also evidenced by the maximum output signals (at 1% THD) measured at the balanced outputs into 200k ohms and 600 ohms: a staggering 90Vrms versus 1.5Vrms.

THD ratio (unweighted) vs. output voltage

thd ratio unweighted vs output

The plot above shows THD ratios measured at the output of the SFL-2 as a function of output voltage into 200k ohms with a 1kHz input sinewave. At the 10mVrms level, THD values measured around 0.1% at 10mVrms, dipping down to around 0.0003% at 2Vrms for the right channel and 0.003% at 0.4Vrms for the left channel. Between 1 and 30Vrms at the output, we find a 10 to 25dB difference in THD between the left and right channels. The 1% THD point is reached at around 90Vrms. It’s also important to mention that anything above 2-4Vrms is not typically required to drive most power amps to full power.

THD+N ratio (unweighted) vs. output voltage

thd n ratio unweighted vs output

The plot above shows THD+N ratios measured at the output of the SFL-2 as a function of output voltage into 200k ohms with a 1kHz input sinewave. At the 10mVrms level, THD+N values measured around 1.5%, dipping down to around 0.003% at 5-7Vrms for the right channel, and 0.01% at 1-2Vrms for the left channel. Between 3 and 30Vrms at the output, we find a 10 to 25dB difference in THD+N between the left and right channels.

FFT spectrum – 1kHz (balanced in, balanced out)

fft spectrum 1khz bal in bal out

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output into a 200k-ohm load, for the balanced inputs and outputs. We see that the signal’s second harmonic, at 2kHz, is at around -80dBrA, or 0.01%, for the left channel, and only -120dBrA, or 0.0001% for the right channel. The third harmonic, at 3 kHz, is at -115dBrA, or 0.0002%, for both channels. There are no power-supply-related noise peaks above the relatively high noise floor, which varies from -100dBrA at low frequencies, down to -130dBrA at 20kHz.

FFT spectrum – 1kHz (unbalanced in, balanced out)

fft spectrum 1khz unbal in bal out

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output into a 200k-ohm load, for the unbalanced inputs and balanced outputs. We see that the signal’s second harmonic, at 2kHz, is at around -70/-75dBrA (left/right), or 0.03/0.02%. The third harmonic, at 3kHz, is at -110dBrA, or 0.0003%, for both channels. There are no significant power-supply-related noise peaks above the relatively high noise floor, but for a very small -110dBrA (left channel), or 0.0003%, peak at 120Hz.

FFT spectrum – 1kHz (unbalanced in, unbalanced out)

fft spectrum 1khz unbal in bal out

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output into a 200k-ohm load, for the unbalanced inputs and outputs. We see that the signal’s second harmonic, at 2kHz, is at around -70/-60dBrA (left/right), or 0.03/0.1%. The third harmonic, at 3 kHz, is at -100dBrA, or 0.001%, for both channels. There are no significant power-supply-related noise peaks above the relatively high noise floor, except for the very small -110dBrA (right channel), or 0.0003%, peak at 120Hz.

FFT spectrum – 1kHz (balanced in, unbalanced out)

fft spectrum 1khz unbal in unbal out

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output into a 200k-ohm load, for the balanced inputs and unbalanced outputs. We see that the signal’s second harmonic, at 2kHz, is at around -60dBrA, or 0.1%. The third harmonic, at 3kHz, is at -100dBrA, or 0.001%, for both channels. Using the unbalanced outputs appears to yield the worst-case THD ratios. Again, there are no significant power-supply-related noise peaks above the relatively high noise floor, except for a very small -110dBrA (right channel), or 0.0003%, peak at 120Hz.

FFT spectrum – 50Hz

fft spectrum 50hz

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output into a 200k-ohm load. The X axis is zoomed in from 40Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. The only significant non-signal peak is from the signal’s second harmonic (100Hz) at -80/-105dBrA (left/right), or 0.01/0.0006%. The third signal harmonic (150Hz) is at -115dBrA, or 0.0002%. As above, there are no significant-power-supply related noise peaks above the relatively high noise floor, except for the very small -110dBrA (left channel), or 0.0003%, peak at 120Hz.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus)

intermodulation distortion fft 18khz 19khz summed stimulus

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output into a 200k-ohm load. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 2Vrms (0dBrA) at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -85/-95dBrA (left/right), or 0.006/0.002%, while the third-order modulation products, at 17kHz and 20kHz, are at -115dBrA, or 0.0002%.

Intermodulation distortion FFT (line-level input, APx 32 tone)

fft spectrum 32 tone

Shown above is the FFT of the balanced outputs of the SFL-2 with the APx 32-tone signal applied to the analog balanced input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 2Vrms. The intermodulation products—i.e., the “grass” between the test tones—are distortion products from the amplifier and are around the -115dBrA level for the left channel and the -130dBrA level for the right channel (below 15kHz or so).

Square-wave response (10kHz)

square wave response 10kHz

Above is the 10kHz squarewave response at the output into 200k ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the SFL-2’s slew-rate performance. Rather, it should be seen as a qualitative representation of its very high bandwidth. An ideal squarewave can be represented as the sum of a sinewave and an infinite series of its odd-order harmonics (e.g., 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot, and softening of the edges. The SFL-2’s reproduction of the 10kHz squarewave is essentially perfect, with sharp corners and no ringing.

Diego Estan
Electronics Measurement Specialist

NAD Masters M66 Streaming Preamplifier Measurements

Details
Parent Category: Products
Category: Preamplifier Measurements
Created: 01 September 2024

Link: reviewed by Roger Kanno on SoundStage! Hi-Fi on September 1, 2024

General Information

All measurements taken using an Audio Precision APx555 B Series analyzer.

The NAD Masters M66 was conditioned for 30 minutes 2Vrms in/out into 200k ohms before any measurements were taken.

The M66 offers a multitude of inputs, both digital and analog (balanced and unbalanced), and line-level/subwoofer analog balanced outputs over XLR and unbalanced over RCA. Also offered is a ¼″ TRS headphone output on the front panel. For the purposes of these measurements, unless otherwise stated, the following inputs were evaluated: digital coaxial S/PDIF (RCA), analog balanced (XLR), as well as phono moving magnet (MM) and moving coil (MC). Comparisons were made between unbalanced (RCA) and balanced (XLR) line inputs and outputs, other than the 6dB of extra gain over the balanced outputs, there were no appreciable differences in terms THD+N (FFTs for different configurations can be seen in this report). Unless otherwise stated, the Analog Direct mode was used for the analog inputs.

Most measurements were made with a 2Vrms line-level and 0dBFS digital input with the volume set to achieve 2Vrms at the output. For the phono input, a 6.4mVrms MM level and 0.5mVrms MC level was used to achieve 1Vrms at the output. Of note is the low preamp gain in Analog Direct mode (-3.6/2.7dB RCA/XLR out). Using the MM input over the balanced output, 1Vrms could not be achieved with the volume at maximum with a standard 5mVrms input. Also noteworthy is the early onset of clipping when Analog Direct is turned off (which is required to use DSP functions). The ADC clipped with a 1.92Vrms input (RCA and XLR). The signal-to-noise ratio (SNR) measurements were made with the same input signal values and for comparison, on the line-level input, a SNR measurement was also made with the volume at maximum, but with a lower input voltage to achieve the same 2Vrms output.

The M66 offers 80 volume steps in 1dB increments, from -76dB to +2.7dB (line-level, XLR in/out). Based on the random and non-repeatable channel deviation values observed below, the M66 utilizes a digitally controlled volume control operating in the analog domain.

Volume-control accuracy (measured at preamp outputs): left-right channel tracking

Volume position Channel deviation
-79 0.010dB
-70 0.003dB
-60 0.016dB
-50 0.003dB
-40 0.017dB
-30 0.011dB
-20 0.002dB
-10 0.011dB
0 0.002dB

Published specifications vs. our primary measurements

The table below summarizes the measurements published by NAD for the M66 compared directly against our own. The published specifications are sourced from NAD’s website, either directly or from the manual available for download, or a combination thereof. Except for frequency response, where the Audio Precision bandwidth was set at its maximum (DC to 1MHz), unless otherwise stated, assume a 1kHz sinewave at 2Vrms or 0dBFS at the input, 2Vrms at the output into 200k ohms, and a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channels.

Parameter (Analog Direct off unless specified) Manufacturer SoundStage! Lab
Line-level input    
THD (20Hz-20kHz, 2Vrms out) <0.001% <0.0002%
Signal-to-noise ratio (A-wgt, 0.5Vrms out) >105dB 111dB
Channel separation (1kHz) >116dB 136dB
Channel separation (10kHz) >106dB 125dB
Input impedance 56k ohms 59k ohms
Maximum input signal (0.1% THD) >5.6Vrms 1.92Vrms
Maximum input signal (0.1% THD, Analog Direct mode) >8Vrms 11.18Vrms
Output impedance 22 ohms 22.9 ohms
Gain (unbalanced, Analog Direct off) 8.63dB 2.6dB
Gain (balanced, Analog Direct off) 14.89dB 8.6dB
Frequency response (Analog Direct off) ±0.2dB (20Hz-20kHz) 0/-0.09dB(20Hz/20kHz)
Frequency response (Analog Direct on) ±0.2dB (20Hz-80kHz) 0/-0.09dB(20Hz/80kHz)
Max voltage output (0.1%THD, RCA, Analog Direct off) 5Vrms 2.6Vrms
Max voltage output (0.1%THD, RCA, Analog Direct on) 10Vrms 7.6Vrms
Max voltage output (0.1%THD, XLR, Analog Direct off) 10Vrms 5.2Vrms
Max voltage output (0.1%THD, XLR, Analog Direct on) 20Vrms 15.5Vrms
Digital input    
THD (20Hz-20kHz, 2Vrms out) <0.0005% <0.0003%
Channel separation (1kHz) >126dB 138dB
Channel separation (10kHz) >115dB 122dB
Input sensitivity (0.5Vrms out, max volume) -20.25dBFS -20.32dBFS
Subwoofer outputs    
THD (20Hz-200Hz, 2Vrms out) <0.005% <0.005%
Signal-to-noise ratio (A-wgt, 0.5Vrms out, RCA) >84dB 85dB
Signal-to-noise ratio (A-wgt, 0.5Vrms out, XLR) >80dB 79dB
Output impedance 480 ohms 433 ohms
Phono input    
THD (MM, 20Hz-20kHz, 2Vrms out) <0.008% <0.02%
THD (MC, 20Hz-20kHz, 2Vrms out) <0.02% <0.3%
Signal-to-noise ratio (MM, A-wgt, 0.5Vrms out) >82dB 75dB
Signal-to-noise ratio (MC, A-wgt, 0.5Vrms out) >75dB 54dB
Input impedance (MM) 56k ohms 59.9k ohms
Input impedance (MC) 100 ohms 139 ohms
Input sensitivity (MM, 0.5Vrms out, max volume) 1.7mVrms 3.2mVrms (1.6Vrms XLR)
Input sensitivity (MM, 0.5Vrms out, max volume) 0.123mVrms 0.254mVrms (0.127mVrms XLR)
Frequency response ±0.2dB (20Hz-20kHz) 0/+0.2dB(20Hz/20kHz)
Maximum input signal (MM, 0.1% THD, 1kHz) >80mVrms 94mVrms
Maximum input signal (MM, 0.1% THD, 1kHz) >7mVrms 7.4mVrms
Headphone output (Analog Direct off)    
THD (20Hz-20kHz, 1Vrms out) <0.002% <0.0003%
Signal-to-noise ratio (A-wgt, 0.5Vrms out, 32-ohm load) >98dB 99dB
Frequency response ±0.3dB (20Hz-20kHz) 0/-0.03dB(20Hz/20kHz)
Channel separation (1kHz) >62dB 105dB
Output impedance 4.7 ohms 5.4 ohms

Our primary measurements revealed the following using the balanced line-level analog input and digital coaxial input (unless specified, assume a 1kHz sinewave at 2Vrms or 0dBFS, 2Vrms output, 200k-ohm loading, 10Hz to 22.4kHz bandwidth):

Parameter Left channel Right channel
Crosstalk, one channel driven (10kHz) -126dB -126dB
DC offset <-0.7mV <-0.6mV
Gain (RCA in/out) -3.6dB -3.6dB
Gain (XLR in/out) 2.7dB 2.7dB
IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) <-117dB <-117dB
IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1 ) <-107dB <-108dB
Input impedance (line input, RCA) 59.0k ohms 58.3k ohms
Input impedance (line input, XLR) 119.0k ohms 119.1k ohms
Maximum output voltage (at clipping 1% THD+N) 15.8Vrms 15.8Vrms
Maximum output voltage (at clipping 1% THD+N into 600 ohms) 14.7Vrms 14.7Vrms
Noise level (with signal, A-weighted)* <2.6uVrms <2.6uVrms
Noise level (with signal, 20Hz to 20kHz)* <3.3uVrms <3.3uVrms
Noise level (with signal, A-weighted, RCA)* <1.9uVrms <1.9uVrms
Noise level (no signal, A-weighted, volume min)* <1.7uVrms <1.7uVrms
Noise level (no signal, 20Hz to 20kHz, volume min)* <2.1uVrms <2.1uVrms
Noise level (no signal, A-weighted, volume min, RCA)* <1.2uVrms <1.2uVrms
Output impedance (RCA) 22.4 ohms 22.9 ohms
Output impedance (XLR) 44.4 ohms 44.4 ohms
Signal-to-noise ratio (2Vrms out, A-weighted, 2Vrms in)* 118.3dB 118.1dB
Signal-to-noise ratio (2Vrms out, 20Hz to 20kHz, 2Vrms in)* 116.2dB 116.1dB
Signal-to-noise ratio (2Vrms out, A-weighted, max volume)* 118.1dB 117.9dB
Signal-to-noise ratio (2Vrms out, A-weighted, 2Vrms in, RCA)* 122.5dB 122.5dB
Dynamic range (2Vrms out, A-weighted, digital 24/96)* 122.1dB 121.9dB
Dynamic range (2Vrms out, A-weighted, digital 16/44.1)* 96.0dB 96.0dB
THD ratio (unweighted) <0.00006% <0.00006%
THD ratio (unweighted, digital 24/96) <0.00018% <0.00018%
THD ratio (unweighted, digital 16/44.1) <0.0004% <0.0004%
THD+N ratio (A-weighted) <0.00015% <0.00015%
THD+N ratio (A-weighted, digital 24/96) <0.00022% <0.00026%
THD+N ratio (A-weighted, digital 16/44.1) <0.0016% <0.0016%
THD+N ratio (unweighted) <0.0002% <0.0002%

*due to very low noise of DUT, analyzer self-noise has been removed from measurement to more accurately report value.

Our primary measurements revealed the following using the phono-level input, MM configuration (unless specified, assume a 1kHz sinewave at 6.4mVrms, 1Vrms output, 200k-ohm loading, 10Hz to 22.4kHz bandwidth):

Parameter Left channel Right channel
Crosstalk, one channel driven (10kHz) -97.3dB -96.5dB
DC offset <1.1mV <1.1mV
Gain (default phono preamplifier) 41.2dB 41.2dB
IMD ratio (18kHz and 19 kHz stimulus tones) <-92dB <-92dB
IMD ratio (3kHz and 4kHz stimulus tones) <-90dB <-90dB
Input impedance 58.6k ohms 59.9k ohms
Input sensitivity (1Vrms out, max volume) 6.4mVrms 6.4mVrms
Noise level (with signal, A-weighted) <70uVrms <70uVrms
Noise level (with signal, 20Hz to 20kHz) <650uVrms <650uVrms
Overload margin (relative 5mVrms input, 1kHz) 25.7dB 25.7dB
Signal-to-noise ratio (1Vrms out, A-weighted) 81.8dB 81.6dB
Signal-to-noise ratio (1Vrms out, 20Hz to 20kHz) 69.6dB 69.4dB
THD (unweighted) <0.0007% <0.0007%
THD+N (A-weighted) <0.007% <0.007%
THD+N (unweighted) <0.08% <0.08%

Our primary measurements revealed the following using the phono-level input, MC configuration (unless specified, assume a 1kHz sinewave at 0.5mVrms, 1Vrms output, 200k-ohm loading, 10Hz to 22.4kHz bandwidth)

Parameter Left channel Right channel
Crosstalk, one channel driven (10kHz) -94.2dB -91.3dB
DC offset <-1.3mV <-1.3mV
Gain (default phono preamplifier) 63.3dB 63.3dB
IMD ratio (18kHz and 19 kHz stimulus tones) <74dB <74dB
IMD ratio (3kHz and 4kHz stimulus tones) <72dB <72dB
Input impedance 139 ohms 139 ohms
Input sensitivity (1Vrms out, max volume) 0.5mVrms 0.5mVrms
Noise level (with signal, A-weighted) <700uVrms <700uVrms
Noise level (with signal, 20Hz to 20kHz) <7mVrms <7mVrms
Overload margin (relative 0.5mVrms input, 1kHz) 23.6dB 23.6dB
Signal-to-noise ratio (1Vrms out, A-weighted) 62.3dB 61.7dB
Signal-to-noise ratio (1Vrms out, 20Hz to 20kHz) 50.4dB 49.9dB
THD (unweighted) <0.005% <0.005%
THD+N (A-weighted) <0.07% <0.07%
THD+N (unweighted) <0.8% <0.8%

Our primary measurements revealed the following using the analog input at the headphone output (unless specified, assume a 1kHz sinewave, 2Vrms input/output, 300-ohm loading, 10Hz to 22.4kHz bandwidth, Analog Direct Mode on):

Parameter Left and right channels
Maximum gain 6.2dB
Maximum output power into 600 ohms 180mW
Maximum output power into 300 ohms 347mW
Maximum output power into 32 ohms 1.75W
Output impedance 5.4 ohms
Maximum output voltage (100k ohm load) 10.6Vrms
Noise level (with signal, A-weighted) <6.0uVrms
Noise level (with signal, 20Hz to 20kHz) <7.3uVrms
Noise level (no signal, A-weighted, volume min) <5.6uVrms
Noise level (no signal, 20Hz to 20kHz, volume min) <6.8uVrms
Signal-to-noise ratio (A-weighted, 1% THD, 10.2Vrms out) 124.3dB
Signal-to-noise ratio (20Hz - 20kHz, 1% THD, 10.2Vrms out) 122.7dB
THD ratio (unweighted) <0.00008%
THD+N ratio (A-weighted) <0.0003%
THD+N ratio (unweighted) <0.0004%

Frequency response (line-level input)

frequency response

In our measured frequency-response (relative to 1kHz) chart above, the M66 is essentially perfectly flat within the audioband (0dB at 20Hz and 20kHz). At the extremes, the M66 is 0dB at 5Hz, and -0.2dB at 200kHz. These data corroborate NAD’s claim of ±0.2dB (20Hz-80kHz). With Analog Direct mode, the M66 can be considered an extremely wide-bandwidth audio device. The M66 also appears to be DC-coupled, as there is no attenuation at low frequencies, even at 5Hz. In the graph above and most of the graphs below, only a single trace may be visible. This is because the left channel (blue or purple trace) is performing identically to the right channel (red or green trace), and so they perfectly overlap, indicating that the two channels are ideally matched.

Frequency response (line-level input, subwoofers active)

frequency response

Above are the frequency-response (relative to 1kHz and 20Hz) plots for the M66 balanced line-level outputs (red/blue) with two subwoofers (purple/green) engaged and the crossover set to 120Hz. In order to implement bass management, Analog Direct must be turned off to enable DSP functions. The line-level outputs show sharp attenuation just past 30kHz, with a -3dB point just past 40kHz, suggesting that the M66 samples incoming analog signals at 96kHz. The high- and low-pass slopes appear to be second-order (12dB/octave), with the crossover point at -6dB. The subwoofer outputs are perfectly flat down to 5Hz.

Phase response (line-level input)

phase response

Above is the phase response plot from 20Hz to 20kHz for the balanced line-level input. The M66 does not invert polarity and exhibits, at worst, less than -5 degrees (at 20kHz) of phase shift within the audioband.

Frequency response vs. input type (left channel only)

frequency response vs input type

The chart above shows the M66’s frequency response (relative to 1kHz) as a function of input type. The green trace is the same (but limited to 80kHz) analog input data from the first frequency- response graph. The blue trace is for a 16-bit/44.1kHz dithered digital input signal from 5Hz to 22kHz, the purple trace is for a 24/96 dithered digital input signal from 5Hz to 48kHz, and finally pink is 24/192 from 5Hz to 96kHz—all using the coaxial input. The behavior at low frequencies is the same for all the digital sample rates, as well as the analog input: flat down to 5Hz. The behavior at high frequencies for all three digital sample rates is as expected, offering filtering around 22k, 48k, and 96kHz (half the respective sample rate). All three digital data plots show “brick-wall”-type behavior, with-3dB points at 21.8kHz (16/44.1), 47kHz (24/96), and 94kHz (24/192).

Frequency response (MM input)

frequency response phono mm

The chart above shows the frequency response (relative to 1 kHz) for the MM phono input. What is shown is the deviation from the RIAA curve, where the input signal sweep is EQ’d with an inverted RIAA curve supplied by Audio Precision (i.e., zero deviation would yield a flat line at 0dB). The chart shows extremely small maximum deviations within the audioband: 0dB at 20Hz and about +0.2dB at 20kHz. This is an example of exceptional RIAA tracking implemented in the analog domain (Analog Direct is turned on). At the extremes, there is sharp attenuation at low frequencies (-3dB at 5Hz) and a rise at high frequencies (+0.5dB at 50kHz).

Frequency response (MC input)

frequency response phono mc

The chart above shows the frequency response for the MC phono input. We see essentially the same result as with the MM configuration.

Phase response (MM and MC phono inputs)

phase response phono mm

Above is the phase-response plot from 20Hz to 20kHz for the phono input (MM and MC behaved identically). For the phono input, since the RIAA equalization curve must be implemented, which ranges from +19.9dB (20Hz) to -32.6dB (90kHz), phase shift at the output is inevitable. Here we find a worst case of about -60 degrees at 200Hz and -90 degrees at 20kHz.

Digital linearity (16/44.1 and 24/96 data)

digital linearity 1644 1 2496

The chart above shows the results of a linearity test for the coaxial digital input (the optical input performed identically) for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the balanced outputs of the M66. The digital input is swept with a dithered 1kHz input signal from -120dBFS to 0dBFS, and the output is analyzed by the APx555. The ideal response would be a straight flat line at 0dB. At -120dBFS, the 16/44.1 data overshot by only 1-2dB, while the 24/96 remained perfect. To verify how well the 24/96 data would perform down to -140dBFS, we extended the sweep in the chart below.

digital linearity 1644 1 2496 extended

Here we can see that the 24/96 data only undershot the mark by 2dB (left/right) at -140/-130dBFS. This is an exceptional digital linearity-test result.

Impulse response (24/48 data)

impulse response 2448

The graph above shows the impulse response for a looped 24/44.1 test file that moves from digital silence to full 0dBFS (all “1”s) for one sample period then back to digital silence, measured at the balanced outputs of the M66. We can see that the M66 utilizes a reconstruction filter that favors no pre-ringing.

J-Test (coaxial input)

jtest coaxial 2448

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level pre-outputs of the M66 where 0dBFS is set to 1Vrms. J-Test was developed by Julian Dunn the 1990s. It is a test signal—specifically, a -3dBFS undithered 12kHz squarewave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz squarewave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sinewave (the main peak). In addition, an undithered 250Hz squarewave at -144dBFS is mixed with the signal. This test file causes the 22 least significant bits to constantly toggle, which produces strong jitter spectral components at the 250Hz rate and its odd harmonics. The test file shows how susceptible the DAC and delivery interface are to jitter, which would manifest as peaks above the noise floor at 500Hz intervals (e.g., 250Hz, 750Hz, 1250Hz, etc.). Note that the alternating peaks are in the test file itself, but at levels of -144dBrA and below.  The test file can also be used in conjunction with artificially injected sinewave jitter by the Audio Precision, to show how well the DAC rejects jitter.

The coaxial S/PDIF input of the M66 shows a strong but not perfect J-Test result, with small peaks in the audioband at a low -135dBrA and below level.

J-Test (optical input)

jtest optical 2448

The chart above shows the results of the J-Test test for the optical digital input measured at the balanced outputs of the M66. The results here are similar but not quite as strong as the coaxial input above. Here, the peaks reach the -125dBrA level.

J-Test (AES-EBU input)

jtest optical 2448

The chart above shows the results of the J-Test test for the AES-EBU balanced digital input measured at the balanced outputs of the M66. The results here are very similar to those found on the coaxial input.

J-Test (coaxial, 2kHz sinewave jitter at 10ns)

jtest coaxial 2448 2khz 10ns

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level output of the M66, with an additional 10ns of 2kHz sinewave jitter injected by the APx555. The results are close to the same as the J-Test result without additional jitter. The same was true for the optical input. No peaks can be seen at the 10kHz and 14kHz positions.

J-Test (coaxial, 2kHz sinewave jitter at 100ns)

jtest coaxial 2448 2khz 100ns

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level output of the M66, with an additional 100ns of 2kHz sinewave jitter injected by the APx555. The tell-tale peaks at 10kHz and 14kHz still cannot be seen. This is a strong result showing that the M66 DAC has very good jitter immunity.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (coaxial input)

wideband fft noise plus 19 1khz 1644 1kHz

The chart above shows a fast Fourier transform (FFT) of the M66’s balanced outputs with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1. The steep roll-off around 20kHz in the white-noise spectrum shows that the M66 uses a brick-wall-type reconstruction filter. There are no obvious aliased images within the audioband. The primary aliasing signal at 25kHz is highly suppressed at -70dBrA, while the second and third distortion harmonics (38.2, 57.3kHz) of the 19.1kHz tone are even lower.

THD ratio (unweighted) vs. frequency vs. load (analog)

thd vs frequency vs load

The chart above shows THD ratios at the balanced line-level output into 200k ohms (blue/red) and 600 ohms (purple/green) as a function of frequency for the analog balanced inputs. The 200k-ohm THD data are lower than the 600-ohm data by 5-10dB from 20Hz to 200Hz. The 200k-ohm data range from 0.00015% at 20Hz, down to an astonishingly low 0.00003% at 200Hz, then a steady climb to 0.002% near 20kHz. It’s important to note that these THD values are only about twice as high as the AP’s signal generator, thereby pushing the limits of the analyzer’s capabilities.

THD ratio (unweighted) vs. frequency vs. sample rate (16/44.1 and 24/96)

thd vs frequency 16 441 24 96

The chart above shows THD ratios at the balanced line-level output into 200k ohms for a 16/44.1 (blue/red) dithered 1kHz signal at the coaxial input and a 24/96 (purple/green) signal, as a function of frequency. The 16/44.1 THD ratios were slightly higher (0.0003% to 0.0002%) than the 24/96 THD ratios, which were more of a constant 0.0002% across most of the audioband.

THD ratio (unweighted) vs. frequency (phono input, MM and MC)

thd ratio unweighted vs frequency phono mm mc

The graph above shows THD ratio as a function of frequency plots for the phono input. The MM input is shown in blue/red (left/right channels) and MC in purple/green (left/right channels). The input sweep is EQ’d with an inverted RIAA curve. The THD values for the MM configuration vary from around 0.02% (20Hz) down to around 0.0003% (2kHz to 3kHz), then up to 0.0005% at 20kHz. The MC THD values were higher, although these are limited by the higher noise floor (the analyzer cannot assign a THD ratio for harmonic peaks it cannot see above the noise floor). THD ratios ranged from 0.3% (20Hz), down to 0.002% (2kHz to 20kHz). It should be pointed out that the higher THD ratios at low frequencies are also likely limited and driven by the higher noise floor.

THD ratio (unweighted) vs. output (analog)

thd ratio unweighted vs output

The chart above shows THD ratios measured at the balanced outputs of the M66 as a function of output voltage for the balanced line-level input, with the volume control at maximum. THD values start at 0.03% at 1mVrms, down to a low of 0.00004% at 2-3Vrms, then a steep rise past 10Vrms to the 1% THD mark at 15.5Vrms.

THD+N ratio (unweighted) vs. output (analog)

thd ratio unweighted vs output

The chart above shows THD+N ratios measured at the balanced outputs of the M66 as a function of output voltage for the balanced line level-input, with the volume control at maximum. THD+N values start at 0.4% at 1mVrms, down to a low of 0.00015% at 4-5Vrms, then a steep rise past 10Vrms to the 1% THD mark at 15.5Vrms.

THD ratio (unweighted) vs. output (16/44.1 and 24/96)

thd vs output 16 441 24 96

The chart above shows THD ratios measured at the balanced outputs of the M66 as a function of output voltage for the digital coaxial S/PDIF input, swept from -90dBFS to 0dBFS, with the volume control at maximum. Blue/red traces are for 16/44.1 data, and purple/green for 24/96. For the 16/44.1 data, THD values start at 2%, and predictably, reach their low at the maximum output voltage of about 5Vrms, at 0.0003%. For the 24/96 data, THD ratios ranged from 0.1% down to 0.0001% near the maximum output voltage.

THD+N ratio (unweighted) vs. output (16/44.1 and 24/96)

thd vs output 16 441 24 96

The chart above shows THD+N ratios measured at the balanced outputs of the M66 as a function of output voltage for the digital coaxial S/PDIF input, swept from -90dBFS to 0dBFS, with the volume control at maximum. Blue/red traces are for 16/44.1 data, and purple/green for 24/96. For the 16/44.1 data, THD+N values start at 20% and reach their low at the maximum output voltage of about 5Vrms, at 0.002%. For the 24/96 data, THD ratios ranged from 0.5% down to 0.0002% near the maximum output voltage.

Intermodulation distortion vs. generator level (SMPTE, 60Hz:4kHz, 4:1, 16/44.1, 24/96)

intermodulation distortion SMPTE vs generator level 441k 96k

The chart above shows intermodulation distortion (IMD) ratios measured at the balanced output for 16/44.1 (blue/red) input data and 24/96 input data (purple/green), from -60dBFS to 0dBFS. Here, the SMPTE IMD method was used, where the primary frequency (F1 = 60Hz) and the secondary frequency (F2 = 7kHz) are mixed at a ratio of 4:1. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 16/44.1 data yields IMD ratios from 2% down to 0.003% at 0dBFS. The 24/96 data yields IMD ratios from 0.1% down to 0.0005% at 0dBFS.

FFT spectrum – 1kHz (XLR line-level input)

FFT spectrum 1khz

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced outputs for the balanced line-level input. We see that the signal’s third harmonic, at 3kHz, is at -130dBrA, or 0.00003%. There are a multitude of high-order signal harmonics, but they are all at the extremely low -140dBrA, or 0.00001%, level. Below 1kHz, we see two small peaks from power-supply noise artifacts at 60Hz and 120Hz. These are extraordinarily low at around -150dBrA, or 0.000003%, and it must be highlighted that they are actually from the Audio Precision’s sinewave generator, and not inherent to the M66. We can say this with confidence for two reasons: these same peaks can be seen at roughly the same amplitudes when the Audio Precision’s sinewave generator is connected directly to the inputs of its analyzer (loopback), and these peaks are non-existent in the digital 24/96 FFT below, where the Audio Precision’s DAC generator is used instead of its analog sine wave generator. It should also be stressed how extraordinarily low the M66’s noise floor is. The M66 does not seem to have any correlated power-supply-related (60Hz and harmonics) noise (what we would describe as “hum”). The residual A-weighted noise from the M66 (volume control set to minimum with no signal) due to non-correlated thermal noise (what we would describe as “hiss”) was measured at 1.7uVrms, compared to the analyzer’s self-noise of 0.66 uVrms. FFTs don’t get much cleaner than this!

FFT spectrum – 1kHz (RCA line-level input)

FFT spectrum 1khz

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the unbalanced outputs for the unbalanced line-level input. We see essentially the same result as with the balanced input FFT above.

FFT spectrum – 1kHz (XLR line-level input, Analog Direct off)

FFT spectrum 1khz

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced outputs for the balanced line-level input, with Analog Direct turned off. Here we see signal harmonics at higher levels compared to when the signal path is purely analog. There are a multitude of signal harmonics ranging from -120dBrA, or 0.0001%, down to -140dBrA, or 0.00001%. We also see the IMD peaks between the 1kHz signal and the ADC’s 96kHz sample rate at 95 and 97kHz.

FFT spectrum – 1kHz (digital input, 16/44.1 data at 0dBFS)

fft spectrum 1khz 1644 1 0dbfs

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced outputs for the coaxial digital input, sampled at 16/44.1. We see the third (3kHz) signal harmonic dominate at roughly -120dBrA, or 0.0001%.

FFT spectrum – 1kHz (digital input, 24/96 data at 0dBFS)

fft spectrum 1khz 2496 0dbfs

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced outputs for the coaxial digital input, sampled at 24/96. We see no power-supply-related noise peaks above the -155dBrA noise floor, and the third (3kHz) signal harmonic dominates at roughly -120dBrA, or 0.0001%. A multitude of signal harmonics can also be seen below this level down to -140dBrA, or 0.00001%.

FFT spectrum – 1kHz (digital input, 16/44.1 data at -90dBFS)

fft spectrum 1khz 1644 1 90dbfs

Shown above is the FFT for a 1kHz -90dBFS dithered 16/44.1 input sinewave stimulus at the coaxial digital input, measured at the balanced outputs. We see the 1kHz primary signal peak, at the correct amplitude, and no signal or noise-related harmonic peaks above the -140dBrA noise floor.

FFT spectrum – 1kHz (digital input, 24/96 data at -90dBFS)

fft spectrum 1khz 2496 90dbfs

Shown above is the FFT for a 1kHz -90dBFS dithered 24/96 input sinewave stimulus at the coaxial digital input, measured at the balanced outputs. We see the 1kHz primary signal peak, at the correct amplitude, and signal related harmonic peaks at 2kHz and 4kHz at and below -140dBrA, or 0.00001%.

FFT spectrum – 1kHz (MM phono input)

fft spectrum 1khz phono mm

Shown above is the FFT for a 1kHz input sinewave stimulus, measured at the balanced outputs, for the MM phono input. It is very difficult to see any signal-related harmonics above the noise floor and the myriad of noise-related peaks, at -80dBrA, or 0.01%, and below. Given how clean the M66’s line-level analog FFT is, the MM phono FFT is not the cleanest and somewhat disappointing by comparison.

FFT spectrum – 1kHz (MC phono input)

fft spectrum 1khz phono mc

Shown above is the FFT for a 1kHz input sinewave stimulus, measured at the balanced outputs, for the MC phono input. Again, it is very difficult to see any signal-related harmonics above the noise floor and the myriad of noise-related peaks, at -60dBrA, or 0.1%, and below. Once again, given how clean the M66’s line-level analog FFT is, the MC phono FFT is not the cleanest and is disappointing by comparison.

FFT spectrum – 50Hz (line-level input)

fft spectrum 50hz

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the balanced outputs for the balanced line-level input. The X axis is zoomed in from 40 Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. The most predominant (non-signal) peaks are that of the signal’s second (100Hz) harmonic at -120dBrA, or 0.0001%, and the third signal harmonic (150Hz) at -130dBrA or 0.00003%. Power-supply-related peaks can be seen at 60Hz (-140dBrA, or 0.00001%) and 120Hz (-150dBrA, or 0.000003%), but as discussed above, these are inherent to the Audio Precision sinewave generator.

FFT spectrum – 50Hz (MM phono input)

fft spectrum 50hz phono mm

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the balanced outputs for the MM phono input. There are a myriad of noise-related peaks at -80dBrA, or 0.01%, and below.

FFT spectrum – 50Hz (MC phono input)

fft spectrum 50hz phono mc

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the balanced outputs for the MC phono input. There are, again, numerious noise-related peaks at -60dBrA, or 0.01%, and below.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, line-level input)

intermodulation distortion fft 18khz 19khz summed stimulus

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs for the balanced line-level input. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 10W (0dBrA) into 8 ohms at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -125dBRA, or 0.00006%, while the third-order modulation products, at 17kHz and 20kHz are at roughly the same level. This is a very clean IMD result.

Intermodulation distortion FFT (line-level input, APx 32 tone)

fft spectrum 32 tone

Shown above is the FFT of the balanced outputs of the M66 with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference and corresponds to 2Vrms. The intermodulation products—i.e., the “grass” between the test tones—are distortion products from the preamplifier and are around the extremely low -150dBrA, or 0.000003%, level.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, coaxial digital input, 16/44.1)

intermodulation distortion fft 18khz 19khz summed stimulus 1644-1

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs for the digital coaxial input at 16/44.1. We find that the second-order modulation product (i.e., the difference signal of 1kHz) cannot be seen above the -135dBrA noise floor, while the third-order modulation products, at 17kHz and 20kHz, are at -125dBrA, or 0.00006%. We also see the main aliased peaks at 25.1kHz and 26.1kHz at around -80dBrA.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, coaxial digital input, 24/96)

intermodulation distortion fft 18khz 19khz summed stimulus 2496

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs for the digital coaxial input at 24/96. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -135dBrA, or 0.00002%, while the third-order modulation products, at 17kHz and 20kHz, are at -125dBrA or 0.00006%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, MM phono input)

intermodulation distortion FFT 18kHz 19kHz summed stimulus phono mm

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs across for the MM phono input. We find it difficult to distinguish the second-order and third-order IMD peaks amongst all the noise-related peaks.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, MC phono input)

intermodulation distortion FFT 18kHz 19kHz summed stimulus phono mc

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs across for the MC phono input. Once again, we find it difficult to distinguish the second-order and third-order IMD peaks amongst all the noise-related peaks.

Square-wave response (10kHz)

square wave response 10kHz

Above is the 10kHz squarewave response using the analog line-level input, at roughly 2Vrms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the M66’s slew-rate performance. Rather, it should be seen as a qualitative representation of its extended bandwidth. An ideal squarewave can be represented as the sum of a sinewave and an infinite series of its odd-order harmonics (e.g., 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot, and softening of the edges. The M66 reproduction of the 10kHz square wave is very clean, with no softening in the edges.

Diego Estan
Electronics Measurement Specialist

Meitner Audio PRE Preamplifier Measurements

Details
Parent Category: Products
Category: Preamplifier Measurements
Created: 15 January 2024

Link: reviewed by Phil Gold on SoundStage! Hi-Fi on January 15, 2024

General information

All measurements taken using an Audio Precision APx555 B Series analyzer.

The PRE was conditioned for 30 minutes at 2Vrms at the output before any measurements were taken. All measurements were taken with both channels driven.

The PRE offers two sets of line-level unbalanced (RCA) inputs, one set of line-level balanced (XLR) inputs, one set each of unbalanced (RCA) and balanced (XLR) outputs (both always on). The PRE offers a maximum of 6dB of gain from input to output for the same input type. That is to say, if the volume is set to unity gain, an input of 2Vrms will yield 2Vrms at the output for the unbalanced input/output scenario, and the balanced input/output scenario. For the unbalanced in/balanced out scenario, 12dB gain is available. For the balanced in/unbalanced out scenario, 0dB of gain is available.

Based on the accuracy and non-repeatable nature of the channel deviation (table below), the volume control is in the analog domain, but digitally controlled. It offers between 2 and 3dB step increments for the first 12 volume steps. From steps 12 to 22, 1dB steps were measured. Beyond level 22 up to 100, the volume control offers 0.5 dB steps.  Overall gain was measured at -68.7dB for volume step one, up to +6dB at the maximum position (100). Volume channel tracking proved exquisite, ranging from 0.000dB to 0.008dB.

There is a difference in terms of THD and noise between unbalanced and balanced signals in the PRE (see both the main table and FFTs below). The balanced outputs have about 6dB more uncorrelated thermal noise, whereas using the balanced inputs yields about 10dB less THD compared to the unbalanced inputs. Unfortunately, the lower distortion is only apparent in the FFTs, because they allow averages over multiple data runs, which averages out and lowers the noise floor, making the very low distortion peaks visible. During normal real-time THD measurements, the analyzer is set to measure for 2-3 seconds (maximum) and cannot assign a THD value below the measured noise floor. This explains why in the primary table below, THD appears lower for the unbalanced input/output compared to the balanced input/output. The true THD ratio figure for the balanced configuration, based on the balanced input/output FFT, is an astounding 0.00002% (about -135dB), compared to the 0.00007% (about -123dB) or so for the unbalanced input.

Unless otherwise stated, balanced input and output was evaluated, with an input and output of 2Vrms into a 200k ohm-load, with the analyzer’s input bandwidth filter set to 10Hz to 22.4kHz (exceptions include FFTs and THD vs frequency sweeps where the bandwidth is extended to 90kHz, and frequency and squarewave response where the bandwidth is extended from DC to 1MHz).

Volume-control accuracy (measured at preamp outputs): left-right channel tracking

Volume position Channel deviation
1 0.003dB
10 0.000dB
20 0.008dB
30 0.001dB
40 0.001dB
50 0.003dB
60 0.005dB
70 0.005dB
80 0.004dB
90 0.002dB
100 0.001dB

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Meitner for the PRE compared directly against our own. The published specifications are sourced from Meitner’s website, either directly or from the manual available for download, or a combination thereof. With the exception of frequency response, where the Audio Precision bandwidth was set at its maximum (DC to 1MHz), assume, unless otherwise stated, a 1kHz sinewave, 2Vrms input and output into 200k ohms load, 10Hz to 22.4kHz bandwidth, and the worst-case measured result between the left and right channels.

Parameter Manufacturer SoundStage! Lab
SNR (4Vrms output, 20Hz-20kHz BW) >116dB *108.1dB
Gain control range 74dB 74.6dB
THD (1kHz) 0.004% <0.0001%
Frequency range 0Hz-200kHz 0Hz-200kHz (0/-0.14dB)
System gain 6dB 6dB
Maximum input level 6.2Vrms 13.5Vrms
Input impedance (XLR) 20k ohms 47.9k ohms
Input impedance (RCA) 10k ohms 11.6k ohms
Output impedance (XLR) 150 ohms 149.4 ohms
Output impedance (RCA) 75 ohms 150.7 ohms

*SNR measured with unbalanced in/out = 115.3dB
*SNR calculated with residual noise (volume at 0) and unbalanced in/out = 118.6dB

Our primary measurements revealed the following using the balanced line-level inputs (unless otherwise specified, assume a 1kHz sinewave, 2Vrms input and output into 200k ohms load, 10Hz to 22.4kHz bandwidth):

Parameter Left channel Right channel
Crosstalk, once channel driven (10kHz) -122.2dB -89.1dB
DC offset <-1.7mV <0.6mV
Gain (default) 6dB 6dB
IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) <-113dB <-113dB
IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1 ) <-100dB <-100dB
Input impedance (balanced) 47.9k ohms 47.9k ohms
Input impedance (unbalanced) 11.5k ohms 11.6k ohms
Maximum output voltage (at clipping 1% THD+N) 20.2Vrms 20.2Vrms
Maximum output voltage (at clipping 1% THD+N into 600 ohms) 16Vrms 16Vmrs
Noise level (with signal, A-weighted) <12uVrms <12uVrms
Noise level (with signal, 20Hz to 20kHz) <15uVrms <15uVrms
Noise level (no signal, volume min, A-weighted) <7.9uVrms <7.9uVrms
Noise level (no signal, volume min, 20Hz to 20kHz) <10uVrms <10uVrms
Output impedance (balanced) 149.4 ohms 149.9 ohms
Output impedance (unbalanced) 150.7 ohms 150.7 ohms
Signal-to-noise ratio (2Vrms out, A-weighted, 2Vrms in) 104.2dB 104.1dB
Signal-to-noise ratio (2Vrms out, 20Hz to 20kHz, 2Vrms in) 102.1dB 102.3dB
Signal-to-noise ratio (2Vrms out, A-weighted, max volume) 100.7dB 100.7dB
THD (unweighted, balanced) <0.0001% <0.0001%
THD (unweighted, unbalanced) <0.00009% <0.00009%
THD+N (A-weighted) <0.0006% <0.0006%
THD+N (unweighted) <0.00082% <0.00082%

Frequency response

frequency response

In our measured frequency-response plot above, the PRE is perfectly flat within the audioband (0dB at 20Hz and 20kHz). The PRE appears to be DC-coupled, as it yielded 0dB of deviation at 5Hz. The PRE can certainly be considered an extended-bandwidth audio device, as it is only 0.14dB down at 200kHz. In the graph above and most of the graphs below, only a single trace may be visible. This is because the left channel (blue or purple trace) is performing identically to the right channel (red or green trace), and so they perfectly overlap, indicating that the two channels are ideally matched.

Phase response

phase response

Above is the phase-response plot from 20Hz to 20kHz. The PRE does not invert polarity, and exhibited zero phase shift within the audioband.

THD ratio (unweighted) vs. frequency

thd vs frequency vs load

The plot above shows THD ratios at the output as a function of frequency (20Hz to 20kHz) for a sinewave input stimulus. The blue and red plots are for the left and right channels into 200k ohms, while purple/green (L/R) are into 600 ohms. THD values were flat across most of the audioband at 0.0001% into 600 ohms and 200k ohms, with a small rise to 0.0002% at 20kHz. This shows that the PRE’s outputs are robust and would yield identical THD performance feeding an amplifier with either a high or low input impedance.

THD ratio (unweighted) vs. output voltage

thd ratio unweighted vs output

The plot above shows THD ratios measured at the output of the PRE as a function of output voltage into 200k ohms with a 1kHz input sinewave. At the 10mVrms level, THD values measured around 0.03%, dipping down to around 0.00006% at 6-8Vrms, followed by a rise to 0.0003% at around 18Vrms. The 1% THD point is reached at 20.2Vrms. It’s also important to mention that anything above 2-4Vrms is not typically required to drive most power amps to full power.

THD+N ratio (unweighted) vs. output voltage

thd n ratio unweighted vs output

The plot above shows THD+N ratios measured at the output of the PRE as a function of output voltage into 200k ohms with a 1kHz input sinewave. At the 10mVrms level, THD+N values measured around 0.2%, dipping down to around 0.0003% at 10-18Vrms.

FFT spectrum – 1kHz (balanced in, balanced out)

fft spectrum 1khz bal in bal out

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output into a 200k-ohm load, for the balanced inputs and outputs. We see that the signal’s second harmonic, at 2kHz, is extremely low at around -135dBrA, or 0.00002%, and subsequent signal harmonics are not visible above the -145dBrA noise floor. Below 1kHz, we can see very small peaks at 60, 120, 148, 180, and 300Hz. These peaks are all below the -130dBrA, or 0.00003%, level. This is a very clean FFT.

FFT spectrum – 1kHz (unbalanced in, balanced out)

fft spectrum 1khz unbal in bal out

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output into a 200k-ohm load for the unbalanced inputs and balanced outputs. The main difference here compared to the FFT above is the higher second signal harmonic, at -125dBRa, or 0.00006%, versus the -135dBrA 2kHz peak seen when the balanced inputs are used. Noise peaks left of the signal peak are at similar levels as the FFT above.

FFT spectrum – 1kHz (unbalanced in, unbalanced out)

fft spectrum 1khz unbal in bal out

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output into a 200k-ohm load for the unbalanced inputs and outputs. The same distortion profile with the higher 2kHz peaks can be seen here as with the FFT above. The common denominator is the use of the unbalanced inputs. The overall noise floor is at its lowest here, at -150dBrA.

FFT spectrum – 1kHz (balanced in, unbalanced out)

fft spectrum 1khz unbal in unbal out

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output into a 200k-ohm load for the balanced inputs and unbalanced outputs. The same distortion profile with the lower 2kHz peaks can be seen here as with the first FFT above. The common denominator is the use of the balanced inputs.

FFT spectrum – 50Hz

fft spectrum 50hz

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output into a 200k-ohm load. The X axis is zoomed in from 40Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. Signal-related peaks can be seen at the second (100Hz) and third (150Hz) harmonics, at an extremely low -140dBrA, or 0.00001%. Noise-related peaks are all below -135dBrA, or 0.00002%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus)

intermodulation distortion fft 18khz 19khz summed stimulus

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output into a 200k-ohm load. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 2Vrms (0dBrA) at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -125dBrA, or 0.0006%, while the third-order modulation products, at 17kHz and 20kHz, are at roughly the same level. This, like the 1kHz FFTs, is an extremely clean result.

Intermodulation distortion FFT (line-level input, APx 32 tone)

fft spectrum 32 tone

Shown above is the FFT of the output of the PRE into 200k ohms with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 2Vrms into 200k ohms. The intermodulation products—i.e., the “grass” between the test tones—are distortion products from the amplifier. Distortion products are at a vanishingly low -140dBrA, or 0.00001%. Thus, even with a complex input signal, the PRE does not add any audible coloration to the input signal.

Square-wave response (10kHz)

square wave response 10kHz

Above is the 10kHz squarewave response at the output into 200k ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the PRE’s slew-rate performance. Rather, it should be seen as a qualitative representation of its relatively high bandwidth. An ideal squarewave can be represented as the sum of a sinewave and an infinite series of its odd-order harmonics (e.g., 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot, and softening of the edges. The PRE’s reproduction of the 10kHz squarewave is extremely clean with sharp corners.

Diego Estan
Electronics Measurement Specialist

Simaudio Moon 791 Streaming Preamplifier Measurements

Details
Parent Category: Products
Category: Preamplifier Measurements
Created: 01 December 2023

Link: reviewed by Doug Schneider on SoundStage! Hi-Fi on December 1, 2023

General Information

All measurements taken using an Audio Precision APx555 B Series analyzer.

The Simaudio Moon North Collection 791 was conditioned for 30 minutes with 2Vrms in/out into 200k ohms before any measurements were taken.

The 791 offers a multitude of inputs, both digital and analog (balanced and unbalanced), and line-level analog balanced outputs over XLR and unbalanced over RCA. For the purposes of these measurements, unless otherwise stated, the following inputs were evaluated: digital coaxial S/PDIF (RCA), analog balanced (XLR), as well as phono (RCA), configured both using the default settings for moving-magnet (MM) and moving-coil (MC) cartridges. Comparisons were made between unbalanced (RCA) and balanced (XLR) line inputs and outputs, and no appreciable differences were seen in terms of gain and THD+N (FFTs for different configurations can be seen in this report).

Most measurements were made with a 2Vrms line-level and 0dBFS digital input with the volume set to achieve 2Vrms at the output.  For the phono input, a 5mVrms MM level and 0.5mVrms MC level were used to achieve 1Vrms at the output. The signal-to-noise ratio (SNR) measurements were made with the same input-signal values and, for comparison, on the line-level input, a SNR measurement was also made with the volume at maximum, but with a lower input voltage to achieve the same 2Vrms output.

The 791 also offers a range of gain settings (40 in total) by using the Offset feature in the onscreen menu system. The menu allows for a setting between -10dB to +10dB, in 0.5dB steps, individually assignable to each input. Note that this changes the gain for the input, it does not offset the volume to level. Also of note, despite the -10dB to +10dB in the menu, actual gain varies from roughly -6dB to +14dB. The default setting in the menu is +6dB (+10dB of actual gain), which is, unless otherwise stated, what was used for these measurements.

Based on the accuracy and random results of the left/right volume channel matching (see table below), the 791 volume control is likely digitally controlled but operating in the analog domain. The 791 offers 140 volume steps from -69dB to 9.8dB for the line-level inputs. The first 20 steps (0 to 20dB) are in 1dB increments, and then the 20dB to 80dB volume positions can be changed in 0.5dB increments.

Volume-control accuracy (measured at preamp outputs): left-right channel tracking

Volume position Channel deviation
1 0.019dB
10 0.014dB
20 0.014dB
30 0.000dB
40 0.022dB
50 0.000dB
60 0.014dB
70 0.016dB
80 0.005dB

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Simaudio for the 791 compared directly against our own. The published specifications are sourced from Simaudio’s website, either directly or from the manual available for download, or a combination thereof. With the exception of frequency response, where the Audio Precision bandwidth was set at its maximum (DC to 1MHz), assume, unless otherwise stated, assume a 1kHz sinewave at 2Vrms or 0dBFS at the input, 2Vrms at the output into 200k ohms, and a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channels.

Parameter Manufacturer SoundStage! Lab
Input impedance (line level, RCA) 22k ohms 25.8k ohms
Maximum gain (line level) 10dB 9.8dB (default), 13.7dB (max)
Phono gain 40/54/60/66dB 40.3/54/60/66.4dB
Phono input resistance 10/100/470/1k/47k ohms 11.7/99.8/466/0.97k/46k
Output impedance (RCA) 50 ohms 50.8 ohms
Crosstalk (1kHz) -125dB -141dB
Frequency response (line-level) 2Hz-200kHz (0, -3dB) 2Hz-200kHz (0, -3dB)
SNR (line-level, A-weighted, 2Vrms out) 120dB 119.8dB
Dynamic range (digital input, 24/96, fixed output) 125dB 124/125dB (L/R)
THD+N (at 1kHz, 10Hz to 22.4kHz bandwidth) 0.0004% 0.00025%
IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) 0.0003% 0.00015%

Our primary measurements revealed the following using the balanced line-level analog input and digital coaxial input (unless specified, assume a 1kHz sinewave at 2Vrms or 0dBFS, 2Vrms output, 200kohm loading, 10Hz to 22.4kHz bandwidth):

Parameter Left channel Right channel
Crosstalk, one channel driven (10kHz) -145dB -137dB
DC offset <0.1mV <0.1mV
Gain (RCA in/out, default) 9.7dB 9.7dB
Gain (XLR in/out, default) 9.8dB 9.8dB
Gain (RCA in/out, maximum) 13.6dB 13.6dB
Gain (XLR in/out, maximum) 13.7dB 13.7dB
IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) <-117dB <-117dB
IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1 ) <-115dB <-115dB
Input impedance (line input, RCA) 25.8k ohms 25.7k ohms
Input impedance (line input, XLR) 53.2k ohms 53.2k ohms
Maximum output voltage (at clipping 1% THD+N) 20Vrms 20Vrms
Maximum output voltage (at clipping 1% THD+N into 600 ohms) 16Vrms 16Vrms
Noise level (with signal, A-weighted)* 2.4uVrms 2.4uVrms
Noise level (with signal, 20Hz to 20kHz)* 3.0uVrms 3.0uVrms
Noise level (no signal, A-weighted, volume min)* 1.23uVrms 1.23uVrms
Noise level (no signal, 20Hz to 20kHz, volume min)* 1.58uVrms 1.58uVrms
Output impedance (RCA) 50.7 ohms 50.8 ohms
Output impedance (XLR) 96.6 ohms 96.7 ohms
Signal-to-noise ratio (2Vrms out, A-weighted, 2Vrms in)* 119.8dB 119.9dB
Signal-to-noise ratio (2Vrms out, 20Hz to 20kHz, 2Vrms in)* 117.8dB 117.9dB
Signal-to-noise ratio (2Vrms out, A-weighted, max volume)* 116.7dB 116.7dB
Dynamic range (2Vrms out, A-weighted, digital 24/96)* 119.2dB 119.8dB
Dynamic range (2Vrms out, A-weighted, digital 16/44.1)* 96.0dB 96.0dB
THD ratio (unweighted) <0.00019% <0.00019%
THD ratio (unweighted, digital 24/96) <0.00021% <0.00019%
THD ratio (unweighted, digital 16/44.1) <0.0004% <0.0004%
THD+N ratio (A-weighted) <0.00025% <0.00025%
THD+N ratio (A-weighted, digital 24/96) <0.00027% <0.00025%
THD+N ratio (A-weighted, digital 16/44.1) <0.0016% <0.0016%
THD+N ratio (unweighted) <0.00025% <0.00025%

*due to very low noise of DUT, analyzer self-noise has been removed from measurement to more accurately report value.

Our primary measurements revealed the following using the phono-level input, MM configuration (unless specified, assume a 1kHz sinewave at 5mVrms, 1Vrms output, 200kohm loading, 10Hz to 22.4kHz bandwidth):

Parameter Left channel Right channel
Crosstalk, one channel driven (10kHz) -101dB -103dB
DC offset <0.2mV <0.2mV
Gain (default phono preamplifier) 40.3dB 40.3dB
IMD ratio (18kHz and 19 kHz stimulus tones) <-101dB <-101dB
IMD ratio (3kHz and 4kHz stimulus tones) <-101dB <-101dB
Input impedance 45.5k ohms 46.0k ohms
Input sensitivity (1Vrms out, max volume) 3.15mVrms 3.15mVrms
Noise level (with signal, A-weighted) <19uVrms <19uVrms
Noise level (with signal, 20Hz to 20kHz) <40uVrms <40uVrms
Overload margin (relative 5mVrms input, 1kHz) 22.2dB 22.2dB
Signal-to-noise ratio (1Vrms out, A-weighted) 93.3dB 93.5dB
Signal-to-noise ratio (1Vrms out, 20Hz to 20kHz) 88.0dB 87.7dB
THD (unweighted) <0.0004% <0.0004%
THD+N (A-weighted) <0.0019% <0.0019%
THD+N (unweighted) <0.005% <0.005%

Our primary measurements revealed the following using the phono-level input, MC configuration (unless specified, assume a 1kHz sinewave at 0.5mVrms, 1Vrms output, 200kohm loading, 10Hz to 22.4kHz bandwidth):

Parameter Left channel Right channel
Crosstalk, one channel driven (10kHz) -94dB -93dB
DC offset <0.6mV <0.6mV
Gain (default phono preamplifier) 60dB 60dB
IMD ratio (18kHz and 19 kHz stimulus tones) <-85dB <-85dB
IMD ratio (3kHz and 4kHz stimulus tones) <-80dB <-80dB
Input impedance 99.5 ohms 99.8 ohms
Input sensitivity (1vrms out, max volume) 0.39mVrms 0.39mVrms
Noise level (with signal, A-weighted) <250uVrms <250uVrms
Noise level (with signal, 20Hz to 20kHz) <520uVrms <520uVrms
Overload margin (relative 0.5mVrms input, 1kHz) 24.1dB 24.1dB
Signal-to-noise ratio (2Vrms out, A-weighted) 71.1dB 71.2dB
Signal-to-noise ratio (2Vrms out, 20Hz to 20kHz) 65.9dB 66.1dB
THD (unweighted) <0.004% <0.004%
THD+N (A-weighted) <0.025% <0.025%
THD+N (unweighted) <0.06% <0.06%

Frequency response (line-level input)

frequency response

In our measured frequency response (relative to 1kHz) plot above, the 791 is near perfectly flat within the audioband (0dB at 20Hz, -0.05dB at 20kHz). At the extremes the 791 is 0dB at 5Hz, and -0.8dB at 100kHz, and -3dB just past 200kHz. These data corroborate Simaudio’s claim of 2Hz to 100kHz (0/-3dB). The 791 appears to be DC-coupled, as there is no attenuation at low frequencies, even at 5Hz. In the graph above and most of the graphs below, only a single trace may be visible. This is because the left channel (blue or purple trace) is performing identically to the right channel (red or green trace), and so they perfectly overlap, indicating that the two channels are ideally matched.

Phase response (line-level input)

phase response

Above is the phase response plot from 20Hz to 20kHz for the balanced line-level input. The 791 does not invert polarity and exhibits, at worst, less than -10 degrees (at 20kHz) of phase shift within the audioband.

Frequency response vs. input type (left channel only)

frequency response vs input type

The chart above shows the 791’s frequency response (relative to 1kHz) as a function of input type. The green trace is the same (but limited to 80kHz) analog input data from the previous graph. The blue trace is for a 16bit/44.1kHz dithered digital input signal from 5Hz to 22kHz, the purple trace is for a 24/96 dithered digital input signal from 5Hz to 48kHz, and finally pink is 24/192 from 5Hz to 96kHz (using the coaxial input). The behavior at low frequencies is the same for all the digital sample rates, as well as the analog input—flat down to 5Hz. The behavior at high frequencies for all three digital sample rates is as expected, offering filtering around 22, 48, and 96kHz (half the respective sample rate). The 44.1kHz sampled input signal exhibits typical “brick-wall”-type behavior, with a -3dB point at 21kHz. The -3dB point for the 96kHz sampled data is at 46kHz, and 91kHz for the 192kHz sampled data.

Frequency response (MM input)

frequency response phono mm

The chart above shows the frequency response (relative to 1 kHz) for the phono input (MM configuration) and shows extremely small maximum deviations within the audioband of about +0.05 (100-200Hz) and -0.1dB (20kHz). What is shown is the deviation from the RIAA curve, where the input signal sweep is EQ’d with an inverted RIAA curve supplied by Audio Precision (i.e., zero deviation would yield a flat line at 0dB). This is an example of exceptionally accurate RIAA tracking.

Frequency response (MC input)

frequency response phono mc

The chart above shows the frequency response for the phono input (MC configuration). We see essentially the same result as with the MM configuration.

Phase response (MM and MC phono inputs)

phase response phono mm

Above is the phase response plot from 20Hz to 20kHz for the phono input (MM and MC configurations behaved identically). For the phono input, since the RIAA equalization curve must be implemented, which ranges from +19.9dB (20Hz) to -32.6dB (90kHz), phase shift at the output is inevitable. Here we find a worst case of about +40 degrees at 20Hz and +20 degrees at 1kHz.

Digital linearity (16/44.1 and 24/96 data)

digital linearity 1644 1 2496

The chart above shows the results of a linearity test for the coaxial digital input (the optical input performed identically) for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the balanced outputs of the 791. For this test, the digital input is swept with a dithered 1kHz input signal from -120dBFS to 0dBFS and the output is analyzed by the APx555. The ideal response would be a straight flat line at 0dB. At -120dBFS, the 16/44.1 data overshot by only 1-2dB, while the 24/96 remained perfect. To verify how well the 24/96 data would perform down to -140dBFS, we extended the sweep in the chart below.

digital linearity 1644 1 2496 extended

Here we can see that the 24/96 data only overshot the mark by +2/+1dB (left/right) at -140dBFS. These tests show exceptional digital-linearity results for 16/44.1 and 24/96 data.

Impulse response (24/48 data)

impulse response 2448

The graph above shows the impulse response for a looped 24/44.1 test file that moves from digital silence to full 0dBFS (all “1”s) for one sample period then back to digital silence, measured at the balanced outputs of the 791. We can see that the 791 utilizes a reconstruction filter that favors no pre-ringing.

J-Test (coaxial input)

jtest coaxial 2448

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level output of the 791. J-Test was developed by Julian Dunn the 1990s. It is a test signal—specifically a -3dBFS undithered 12kHz squarewave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz squarewave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sinewave (the main peak). In addition, an undithered 250Hz squarewave at -144dBFS is mixed with the signal. This test file causes the 22 least-significant bits to constantly toggle, which produces strong jitter spectral components at the 250Hz rate and its odd harmonics. The test file shows how susceptible the DAC and delivery interface are to jitter, which would manifest as peaks above the noise floor at 500Hz intervals (e.g., 250Hz, 750Hz, 1250Hz, etc). Note that the alternating peaks are in the test file itself, but at levels of -144dBrA and below.  The test file can also be used in conjunction with artificially injected sinewave jitter by the Audio Precision, to show how well the DAC rejects jitter.

The coaxial S/PDIF input of the 791 shows a near-perfect J-Test result, with only two very small peaks on either side of the 12kHz fundamental at a vanishingly low -155dBrA.

J-Test (optical input)

jtest optical 2448

The chart above shows the results of the J-Test test for the optical digital input measured at the balanced outputs of the 791. The results here are similar but not quite as pristine as the coaxial input above. Here, the peaks adjacent to the 12kHz fundamental reach -145dBrA.

J-Test (coaxial, 2kHz sinewave jitter at 10ns)

jtest coaxial 2448 2khz 10ns

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level output of the 791, with an additional 10ns of 2kHz sinewave jitter injected by the APx555. The results are essentially the same as the J-Test result without additional jitter. The same was true for the optical input.

J-Test (coaxial, 2kHz sinewave jitter at 100ns)

jtest coaxial 2448 2khz 100ns

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level output of the 791, with an additional 100ns of 2kHz sinewave jitter injected by the APx555. This time, the tell-tale peaks at 10kHz and 12kHz can be seen; however, they are very small in amplitude, just below -120dBrA.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (coaxial input)

wideband fft noise plus 19 1khz 1644 1kHz

The chart above shows a fast Fourier transform (FFT) of the 791’s balanced outputs with white noise at -4dBFS (blue/red) and a 19.1kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1. The steep roll-off around 20kHz in the white-noise spectrum shows that the 791 uses a brick-wall-type reconstruction filter. There are no aliased images within the audioband. The primary aliasing signal at 25kHz is highly suppressed at -95dBrA, while the second and third distortion harmonics (38.2, 57.3kHz) of the 19.1kHz tone are even lower.

THD ratio (unweighted) vs. frequency vs. load (analog)

thd vs frequency vs load

The chart above shows THD ratios at the balanced line-level output into 200k ohms (blue/red) and 600 ohms (purple/green) as a function of frequency for the analog balanced inputs. The 200k and 600 ohms data are identical throughout the audioband, which is in indication that the 791’s outputs are robust and can handle loads below 1k ohms with no difficultly. THD ratios are very low, from 0.0003% to 0.0002%.

THD ratio (unweighted) vs. frequency vs. sample rate (16/44.1 and 24/96)

thd vs frequency 16 441 24 96

The chart above shows THD ratios at the balanced line-level output into 200k ohms for a 16/44.1 (blue/red) dithered 1kHz signal at the coaxial input and a 24/96 (purple/green) signal, as a function of frequency. The 16/44.1 THD ratios were slightly higher than the 24/96, at 0.0005% to 0.0002%.  The 24/96 data ranged from 0.0003% down to 0.00015%.

THD ratio (unweighted) vs. frequency (phono input, MM and MC)

thd ratio unweighted vs frequency phono mm mc

The graph above shows THD ratio as a function of frequency plot for the phono input. The MM configuration is shown in blue/red (left/right channels), and MC in purple/green (left/right channels). The input sweep is EQ’d with an inverted RIAA curve. The THD values for the MM configuration vary from around 0.003% (20Hz) down to just above and below 0.0002% (1kHz to 2kHz), then up to 0.0005% at 20kHz. The MC THD values were higher, although these are limited by the higher noise floor (the analyzer cannot assign a THD ratio for harmonic peaks it cannot see above the noise floor). THD ratios ranged from 0.03% (20Hz) down to 0.002% (2kHz to 20kHz).

THD ratio (unweighted) vs. output (analog)

thd ratio unweighted vs output

The chart above shows THD ratios measured at the balanced outputs of the 791 as a function of output voltage for the balanced line-level input. THD values start at 0.05% at 1mVrms, down to a low of 0.00005% at 3Vrms, then a steep rise past 10Vrms to the 1% THD mark at 20Vrms.

THD+N ratio (unweighted) vs. output (analog)

thd ratio unweighted vs output

The chart above shows THD+N ratios measured at the balanced outputs of the 791 as a function of output voltage for the balanced line level-input, with the volume control at maximum. THD+N values start at 0.4% at 1mVrms, down to a low of 0.00015% at 5Vrms, then a steep rise past 10Vrms to the 1% THD mark at 20Vrms.

THD ratio (unweighted) vs. output (16/44.1 and 24/96)

thd vs output 16 441 24 96

The chart above shows THD ratios measured at the balanced outputs of the 791 as a function of output voltage for the digital coaxial S/PIF input, swept from -90dBFS to 0dBFS, with the volume control at maximum. Blue/red traces are for 16/44.1 data, and purple/green for 24/96. For the 16/44.1 data, THD values start at 3%, and predictably, reach their low at the maximum output voltage of about 6.5Vrms, at 0.0002%. For the 24/96 data, the right channel outperformed the left by 5-10dB, and THD ratios ranged from 0.2% down to 0.0001% at the maximum output voltage.

THD+N ratio (unweighted) vs. output (16/44.1 and 24/96)

thd vs output 16 441 24 96

The chart above shows THD+N ratios measured at the balanced outputs of the 791 as a function of output voltage for the digital coaxial S/DPIF input, swept from -90dBFS to 0dBFS, with the volume control at maximum. Blue/red traces are for 16/44.1 data, and purple/green for 24/96. For the 16/44.1 data, THD+N values start at 20% and reach their low at the maximum output voltage of about 6.5Vrms, at 0.002%. For the 24/96 data, THD ratios ranged from 1% down to 0.0002% at the maximum output voltage.

Intermodulation distortion vs. generator level (SMPTE, 60Hz:4kHz, 4:1, 16/44.1, 24/96)

intermodulation distortion SMPTE vs generator level 441k 96k

The chart above shows intermodulation distortion (IMD) ratios measured at balanced output for 16/44.1 (blue/red) and 24/96 input data (purple/green) from -60dBFS to 0dBFS. Here, the SMPTE IMD method was used, where the primary frequency (F1 = 60Hz) and the secondary frequency (F2 = 7kHz) are mixed at a ratio of 4:1. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 16/44.1 data yields IMD ratios from 2% down to 0.002% at 0dBFS. The 24/96 data yields IMD ratios from 0.15% down to 0.0004% near 0dBFS.

FFT spectrum – 1kHz (XLR line-level input)

FFT spectrum 1khz

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced outputs for the balanced line-level input. We see that the signal’s second harmonic, at 2kHz, is at -130dBrA, or 0.00003%, and around -115dBrA, or 0.0002%, at the third (3kHz) harmonic. The subsequent signal harmonics are at a vanishingly low -140dBrA, or 0.00001%, and below. Below 1kHz, we see two small peaks from power-supply noise artifacts at 60Hz and 120Hz. These are extraordinarily low at around -150dBrA, or 0.000003%, and it must be highlighted that they are actually from the Audio Precision’s sinewave generator, and not inherent to the 791. We can say this with confidence for two reasons: these same peaks can be seen at roughly the same amplitudes when the Audio Precision’s sine wave generator is connected directly to the in inputs of its analyzer (loopback), and, these peaks are non-existent in the digital 24/96 FFT below, where the Audio Precision’s DAC generator is used instead of its analog sine wave generator. It should also be stressed how extraordinarily low the 791’s noise floor is—quite possibly the quietest preamp we’ve ever measured. The 791 does not seem to have any correlated power-supply (60Hz and harmonics) related noise (what we would describe as “hum”). The residual A-weighted noise from the 791 (volume control set to minimum with no signal) due to non-correlated thermal noise (what we would describe as “hiss”) was measured at 1.2 uVrms, compared to the analyzer’s self-noise of 0.66uVrms. Even with the volume set to the reference position (70.5dB) for our measurements and with a 2Vrms output signal present (notched out by the analyzer), A-weighted noise was measured at 2.4uVrms. Given all of the digital circuitry inside the 791, this is an impressive feat accomplished by Simaudio.

FFT spectrum – 1kHz (RCA line-level input)

FFT spectrum 1khz

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced outputs for the unbalanced line-level input. We see effectively the same results as with the balanced input FFT above, except for a slightly lower 2kHz signal harmonic peak for the left channel (-140dBrA instead of -130dBrA).

FFT spectrum – 1kHz (digital input, 16/44.1 data at 0dBFS)

fft spectrum 1khz 1644 1 0dbfs

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced outputs for the coaxial digital input, sampled at 16/44.1. We see similar results in terms of the second (2kHz) and third (3kHz) signal harmonics compared to the FFTs above. The noise floor is much higher due to the 16-bit depth limitation.

FFT spectrum – 1kHz (digital input, 24/96 data at 0dBFS)

fft spectrum 1khz 2496 0dbfs

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced outputs for the coaxial digital input, sampled at 24/96. We see essentially the same signal harmonic profile within the audioband as with the balanced analog FFT above. There are zero noise-related peaks to be seen above the -160dBrA noise floor.

FFT spectrum – 1kHz (digital input, 16/44.1 data at -90dBFS)

fft spectrum 1khz 1644 1 90dbfs

Shown above is the FFT for a 1kHz -90dBFS dithered 16/44.1 input sinewave stimulus at the coaxial digital input, measured at the balanced outputs. We see the 1kHz primary signal peak, at the correct amplitude, and no signal- or noise-related harmonic peaks above the -140dBrA noise floor.

FFT spectrum – 1kHz (digital input, 24/96 data at -90dBFS)

fft spectrum 1khz 2496 90dbfs

Shown above is the FFT for a 1kHz -90dBFS dithered 24/96 input sinewave stimulus at the coaxial digital input, measured at the balanced outputs. We see the 1kHz primary signal peak, at the correct amplitude, and signal-related harmonic peaks at 2kHz (left, -135dBrA, or 0.00002%) and 4kHz (-140dBrA, or 0.00001%). Other signal-related harmonics can be see but at the extremely low -150dBrA, or 0.000003%, level.

FFT spectrum – 1kHz (MM phono input)

fft spectrum 1khz phono mm

Shown above is the FFT for a 1kHz input sinewave stimulus, measured at the balanced outputs across for the phono input, configured for MM (default 40dB of gain in phono stage). The dominant signal-related harmonic can be seen at 2kHz, at -115dBrA, or 0.0002%. Power-supply-related noise peaks can be seen at the -100dBrA, or 0.001%, level at 60Hz and 180Hz.

FFT spectrum – 1kHz (MC phono input)

fft spectrum 1khz phono mc

Shown above is the FFT for a 1kHz input sinewave stimulus, measured at the balanced outputs across for the phono input, configured for MC (default 60dB of gain in phono stage). There are no visible signal-related harmonic peaks above the -110 to -120dBrA noise floor. Power-supply-related noise peaks can be seen at the -80dBrA, or 0.01%, level at 60Hz and 180Hz.

FFT spectrum – 50Hz (line-level input)

fft spectrum 50hz

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the balanced outputs for the balanced line-level input. The X axis is zoomed in from 40 Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. The most predominant (non-signal) peaks are that of the signal’s second harmonic (100Hz) at -120dBrA, or 0.0001%, and the third signal harmonic (150Hz) nearing -110dBrA, or 0.0003%. Power-supply-related peaks can be seen at 60Hz (-140dBrA or 0.00001%) and 120Hz (-150dBrA or 0.000003%), but as discussed above, these are inherent to the Audio Precision sinewave generator.

FFT spectrum – 50Hz (MM phono input)

fft spectrum 50hz phono mm

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the balanced outputs for the phono input configured for MM. The most predominant (non-signal) peaks are that of the signal’s second harmonic (100Hz) at -110dBrA, or 0.0003%, and the primary (60Hz) and third (180Hz) power-supply-noise harmonics at -100dBrA, or 0.001%.

FFT spectrum – 50Hz (MC phono input)

fft spectrum 50hz phono mc

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the balanced outputs for the phono input configured for MC. The most predominant (non-signal) peaks are that of the signal’s second harmonic (100Hz)  at -90dBrA, or 0.003%, and the primary (60Hz) and third (180Hz) power-supply noise-harmonics at -80dBrA, or 0.01%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, line-level input)

intermodulation distortion fft 18khz 19khz summed stimulus

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs for the balanced line-level input. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 10W (0dBrA) into 8 ohms at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -125dBRA, or 0.00006%, while the third-order modulation products, at 17kHz and 20kHz, are at -120dBrA, or 0.0001%. This is a very clean IMD result.

Intermodulation distortion FFT (line-level input, APx 32 tone)

fft spectrum 32 tone

Shown above is the FFT of the balanced outputs of the 791 with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 2Vrms. The intermodulation products—i.e., the “grass” between the test tones—are distortion products from the amplifier and are around the extremely low -150dBrA, or 0.000003%, level.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, coaxial digital input, 16/44.1)

intermodulation distortion fft 18khz 19khz summed stimulus 1644-1

Shown above is an FFT of the intermodulation (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs for the digital coaxial input at 16/44.1. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -130dBrA, or 0.00003%, while the third-order modulation products, at 17kHz and 20kHz, are at around the same level. We also see the main aliased peaks at 25.1kHz and 26.1kHz at around -100dBrA.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, coaxial digital input, 24/96)

intermodulation distortion fft 18khz 19khz summed stimulus 2496

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs for the digital coaxial input at 24/96. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -130dBrA, or 0.00003%, while the third-order modulation products, at 17kHz and 20kHz, are at around the same level.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, MM phono input)

intermodulation distortion FFT 18kHz 19kHz summed stimulus phono mm

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs across for the phono input configured for MM. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -110dBrA, or 0.0003%, while the third-order modulation products, at 17kHz and 20kHz are at around -130dBrA, or 0.00003%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, MC phono input)

intermodulation distortion FFT 18kHz 19kHz summed stimulus phono mc

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced outputs across for the phono input configured for MC. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -100dBrA, or 0.001%, but only visible for the left channel, while the third-order modulation products, at 17kHz and 20kHz, are at around -105dBrA, or 0.0006%, but only visible at 20kHz.

Square-wave response (10kHz)

square wave response 10kHz

Above is the 10kHz squarewave response using the analog line-level input, at roughly 2Vrms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the 791’s slew-rate performance. Rather, it should be seen as a qualitative representation of its extended bandwidth. An ideal squarewave can be represented as the sum of a sinewave and an infinite series of its odd-order harmonics (e.g., 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot, and softening of the edges. The 791 reproduction of the 10kHz squarewave is very clean, with only extremely mild softening in the edges.

Diego Estan
Electronics Measurement Specialist

  1. Angela-Gilbert Yeung C312 Preamplifier
  2. Hegel Music Systems P30A Preamplifier

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