Simaudio Moon 791 Streaming Preamplifier Measurements

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

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.

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):

*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):

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):

Frequency response (line-level input)

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)

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)

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)

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)

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)

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)

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.

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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)

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