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Parent Category: Products

Category: Amplifier Measurements

Created: 15 August 2022

Link: reviewed by Dennis Burger on SoundStage! Access on August 15, 2022

General Information

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

The Marantz 40n was conditioned for 1 hour at 1/8th full rated power (~9W into 8 ohms) before any measurements were taken. All measurements were taken with both channels driven, using a 120V/20A dedicated circuit, unless otherwise stated.

The 40n offers three line-level analog inputs (RCA), one pair (left/right) of moving-magnet (MM) phono inputs (RCA), one pair of fixed line-level outputs (RCA), one variable sub-out (RCA), a pair of main-in inputs (RCA), one digital coaxial (RCA) input, one optical (TosLink) input, one digital HDMI input, two speaker-level outputs and one headphone output over a 1/4″ TRS connector. The 40n also offers a Bluetooth input and streaming via ethernet or WiFi. The 40n also features two digital filters, labelled Filter 1 and Filter 2, as well as a Lock Range (Narrow/Medium/Wide) for digital inputs to minimize the effects of jitter. For the purposes of these measurements, the following inputs were evaluated: digital coaxial, analog line-level, and phono. Unless otherwise stated, Source Direct mode was engaged, which bypasses the balance and tone controls. Filter 1 and the default Wide Lock Range were used for the digital-input measurements, unless otherwise specified.

Most measurements were made with a 2Vrms line-level analog input, 5mVrms MM input, and 0dBFS digital input. The volume control is variable from 0 to 100. The following volume settings yielded approximately 10W into 8 ohms: 45 for analog line-level and digital inputs, and 64 for MM input. The signal-to-noise ratio (SNR) measurements were made with the same input signal values but with the volume set to achieve the rated output power of 70W (8 ohms). For comparison, on the line-level input, a SNR measurement was also made with the volume at maximum, where only 0.219Vrms was required to achieve 70W into 8 ohms.

Based on the accuracy and variability of the left/right volume channel matching (see table below), the 40n volume control is likely digitally controlled but operates in the analog domain. The 40n offers 4dB volume steps ranging from volume levels 1 to 6, 2dB steps from 6 to 17, 1dB steps from 17 to 46, and 0.5dB steps from 46 to 100. Overall range is -57.1dB to +40.7dB (line-level input, speaker output).

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

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Marantz for the 40n compared directly against our own. The published specifications are sourced from Marantz’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 extended to 500kHz, assume, unless otherwise stated, 10W into 8 ohms and a measurement input bandwidth of 10Hz to 90kHz, and the worst-case measured result between the left and right channels.

Our primary measurements revealed the following using the line-level analog input and digital coaxial input (unless specified, assume a 1kHz sine wave at 2Vrms or 0dBFS, 10W output, 8-ohm loading, 10Hz to 90kHz bandwidth):

For the continuous dynamic power test, the 40n was able to sustain 129W into 4 ohms (~2% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (12.9W) for 5 seconds, for 5 continuous minutes without inducing a fault or the initiation of a protective circuit. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the top of the 40n was warm to the touch, but did not cause discomfort.

Our primary measurements revealed the following using the phono-level input, MM configuration (unless specified, assume a 1kHz 5mVrms sinewave input, 10W output, 8-ohm loading, 10Hz to 90kHz bandwidth):

Our primary measurements revealed the following using the analog input at the headphone output (unless specified, assume a 1kHz sine wave, 2Vrms output, 300 ohms loading, 10Hz to 90kHz bandwidth):

Frequency response (8-ohm loading, line-level input)

In our frequency-response plots above, measured across the speaker outputs at 10W into 8 ohms, the 40n is perfectly flat within the audio band (20Hz to 20kHz). At the extremes the 40n is -0.1dB at 5Hz and -0.2dB at 100kHz, making “wide bandwidth audio amplifier,” as Marantz calls it, an apt descriptor for the 40n. 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 (8-ohm loading, line-level input, bass and treble controls)

Above is a frequency-response plot measured at the speaker-level outputs into 8 ohms, with the bass and treble controls set to maximum (blue/red plots) and minimum (purple/green plots). We see that for the bass and treble controls, roughly +/- 9.5dB of gain/cut is available, centered at 50Hz and 15kHz.

Frequency response (8-ohm loading, variable subwoofer output)

Above is a frequency response plot measured at the line-level sub-out, relative to 20Hz. We see that the corner frequency (-3dB) is at 80Hz, with a second-order (12dB/octave) slope, which are the default settings. The crossover frequency can also be set to 40, 60, 100, and 120Hz.

Phase response (8-ohm loading, line-level input)

Above are the phase-response plots from 20Hz to 20kHz for the line-level input, measured across the speaker outputs at 10W into 8 ohms. The 40n does not invert polarity and exhibits, at worst, about 30 degrees (at 20kHz) of phase shift within the audio band.

Frequency response vs. input type (analog and 16/44.1, 24/96, and 24/192 inputs with 8-ohm loading, left channel only)

The chart above shows the 40n’s frequency response as a function of input type, measured across the speaker outputs at 10W into 8 ohms. The green trace is the same analog input data from the previous graph (but limited to 80kHz). The blue trace is for a 16-bit/44.1kHz dithered digital input signal from 5Hz to 22kHz using the coaxial input, and the purple trace is for a 24/96 dithered digital input signal from 5Hz to 48kHz, while the pink trace is for a 24/192 dithered digital input signal from 5Hz to 96kHz. For all digital signals, Filter 1 was used. The 16/44.1 data exhibits steeper filtering, with a -3dB point at 18.8kHz. The 24/96 and 24/192 kHz data yielded -3dB points at 38.4kHz and 60.5kHz, respectively.

Frequency response vs. filter type (16/44.1 input with 8-ohm loading, left channel only)

The chart above shows the 40n’s frequency response as a function of filter type, measured across the speaker outputs at 10W into 8 ohms, for a 16/44.1 signal at the coaxial input. The blue trace is Filter 1, and the purple trace is Filter 2. We see that Filter 2 offers a more brick-wall type behavior than Filter 1. At 20 kHz, Filter 1 is at -4dB, while Filter 2 is at -1dB.

Frequency response (8-ohm loading, MM phono input)

The chart above shows the frequency response for the phono input (MM). 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). For the 40n, we see a maximum deviation within the audio band of about +0.4dB between 100Hz and 200Hz.

Phase response (MM input)

Above is the phase response plot from 20Hz to 20kHz for the phono input (MM), measured across the speaker outputs at 10W into 8 ohms. The 40n does not invert polarity. 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 5kHz.

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

The chart above shows the results of a linearity test for the coaxial digital input for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the line-level output of the 40n. 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. Both data were essentially perfect as of -110dBFS down to 0dBFS. At -120dBFS, the 16/44.1 data were only +3dB above reference, while the 24/96 data remained perfect, so we investigated down to . . .

. . . -140dBFS, where the 24/96 data were within +/-1dB of the 0dB reference. This is an exceptional digital linearity test result.

Impulse response (24/44.1 data)

The graph above shows the impulse responses for the 40n, fed to the coaxial digital input, measured at the line level output, 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. Filter 1 is in blue and displays a symmetrical function with minimal pre- and post-ringing, while Filter 2, in purple, which Marantz describes as having an “asymmetrical impulse response,” shows less pre-ringing and more post-ringing.

J-Test (coaxial) with Lock Range set to Wide

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level output of the 40n. J-Test was developed by Julian Dunn in the 1990s. It is a test signal—specifically, a -3dBFS undithered 12kHz square wave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz square wave is removed by the bandwidth limitation of the sampling frequency, we are left with a 12kHz sine wave (the main peak). In addition, an undithered 250Hz square wave 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 sine-wave jitter by the Audio Precision, to show how well the DAC rejects jitter.

The coaxial input exhibits effectively zero low-level peaks in the audio band above the -150 dBrA noise floor. This is a very good J-Test result, indicating that 40n DAC should yield strong jitter immunity.

J-Test (optical) with Lock Range set to Wide

The chart above shows the results of the J-Test test for the optical digital input measured at the line-level output of the 40n. Interestingly, the optical input yielded a worse J-test result than the coaxial input, with peaks within the audio band reaching -110dBrA near the main 12kHz peak.

J-Test (optical) with Lock Range set to Medium

The chart above shows the results of the J-Test test for the optical digital input measured at the line-level output of the 40n, with the Lock Range changed to Medium (Lock Range set to Narrow yielded the same result). Here we see the same ideal J-Test result as with the coaxial input with the Lock Range set to the default Wide position.

J-Test with 100ns of injected jitter (coaxial) with Lock Range set to Wide

Since the coaxial input performed very well on the above J-Test test, it was also tested for jitter immunity by injecting artificial sine-wave jitter at 2kHz, which would manifest as sidebands at 10kHz and 14kHz without any jitter rejection. Jitter immunity proved very good, with visible sidebands at the 100ns jitter level at a very low -130dBrA.

J-Test with 100ns of injected jitter (optical) with Lock Range set to Wide

The optical input was also tested for jitter immunity by injecting artificial sine-wave jitter at 2kHz, which would manifest as sidebands at 10kHz and 14kHz without any jitter rejection. Jitter immunity proved predictably worse than with the coaxial input, with visible sidebands at the 100ns jitter level at a much higher -85dBrA.

J-Test with 100ns of injected jitter (optical) with Lock Range set to Medium

The optical input was further tested for jitter immunity by injecting artificial sine-wave jitter at 2kHz, which would manifest as sidebands at 10kHz and 14kHz without any jitter rejection. Here the test was conducted with the Lock Range set to Medium and the same 100ns jitter level, and predictably, jitter rejection improved substantially. Sidebands are at a very low -140dBrA, barely visible above the noise floor.

J-Test with 500ns of injected jitter (coaxial) with Lock Range set to Wide

The coaxial input was further tested for jitter immunity by injecting artificial sine-wave jitter at 2kHz, which would manifest as sidebands at 10kHz and 14kHz without any jitter rejection, at a much higher 500ns level. Jitter immunity proved very good yet again, with sidebands down at a very low -120dBrA. Above this level of jitter, the 40n DAC lost sync with the signal. Obviously, the coaxial input offers better performance than the optical input. With the optical input, the 40n DAC could not sync up with the signal with high jittter levels of 500ns, regardless of the Lock Range setting.

Wideband FFT spectrum of white noise and 19.1kHz sine-wave tone (coaxial input, Filter 1)

The chart above shows a fast Fourier transform (FFT) of the 40n’s line-level output with white noise at -4dBFS (blue/red) and a 19.1 kHz sine wave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1 for Filter 1. The fairly gentle roll-off around 20kHz in the white-noise spectrum shows the behavior of the 40n’s reconstruction filter. There are two low-level aliased image peaks within the audio band, at around 6kHz at -120dBrA, and 13kHz at -115dBrA. The primary aliasing signal at 25kHz is at -15dBrA, while the second and third distortion harmonics (38.2, 57.3kHz) of the 19.1kHz tone, and other IMD products, are at -40dBrA and below. Ultrasonic noise levels rise above 50kHz and peak just over 65kHz, before decreasing into to the lowest levels at about 80kHz.

Wideband FFT spectrum of white noise and 19.1kHz sine-wave tone (coaxial input, Filter 2)

The chart above shows a fast Fourier transform (FFT) of the 40n’s line-level output with white noise at -4dBFS (blue/red) and a 19.1kHz sine wave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1 for Filter 2. The steep roll-off around 20kHz in the white-noise spectrum shows the behavior of the 40n’s reconstruction filter. Unlike Filter 1, there are no aliased image peaks within the audio band. The primary aliasing signal at 25kHz is at -75dBrA, while the second and third distortion harmonics (38.2, 57.3kHz) of the 19.1kHz tone are at -110dBrA and below. With this filter, there is a sharp rise in ultrasonic noise beginning at about 65kHz that peaks at just below 70kHz, but then steadily decreases, yet still extends past 95kHz, our measurement limit for this test.

RMS level vs. frequency vs. load impedance (1W, left channel only)

The chart above shows RMS level (relative to 0dBrA, which is 1W into 8 ohms or 2.83Vrms) as a function of frequency, for the analog line-level input swept from 5Hz to 100kHz. The blue plot is into an 8-ohm load, the purple is into a 4-ohm load, the pink plot is an actual speaker (Focal Chora 806, measurements can be found here), and the cyan plot is no load connected. The chart below . . .

. . . is the same but zoomed in to highlight differences. Here we find that maximum deviation between no load and a 4-ohm load is quite small, at 0.08dB. This is an indication of a relatively high damping factor, or low output impedance. With a real speaker, maximum deviations in RMS level were smaller, at about 0.06dB.

THD ratio (unweighted) vs. frequency vs. output power

The chart above shows THD ratios at the output into 8 ohms as a function of frequency for a sine-wave stimulus at the analog line-level input. The blue and red plots are for left and right at 1W output into 8 ohms, purple/green at 10W, and pink/orange at 66W. The power was varied using the volume control. The 10W data outperformed the 1W data by 3-4dB from 20Hz to 1kHz, hovering between 0.001% and 0.0006%. From 2kHz to 20kHz, both the 1W and 10W data performed similarly, with THD ratios ranging from 0.0008% up to 0.005%. The 66W THD values were higher, starting at 0.002% at 20Hz, and then rising steadily past 1kHz up to 0.03% at 20kHz.

THD ratio (unweighted) vs. frequency at 10W (MM phono input)

The chart above shows THD ratios as a function of frequency plots for the phono input measured across an 8-ohm load at 10W. The input sweep is EQ’d with an inverted RIAA curve. The THD values vary from around 0.005% (20Hz) down to 0.0006% (1kHz), then up to 0.004% (20kHz).

THD ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms

The chart above shows THD ratios measured at the output of the 40n as a function of output power for the analog line level-input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). The right channel into 4 ohms exhibited much higher THD ratios than the left. The test was repeated to rule out any anomalies, and the results were repeatable. While the left channel ranged from 0.005% (50mW) down to 0.001% (2W to 50W), the right channel ranged from 0.005% up to 0.02% between 2W and 50W. The 8-ohm data showed THD ratios from 0.002% (50mW) down to 0.0005% (5W to 20W). The “knees” were observed just past 60W for the 8-ohm data, and just past 100W for the 4-ohm data. The 1% THD marks were hit at 78W (8 ohms) and 127W (4 ohms).

THD+N ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms

The chart above shows THD+N ratios measured at the output of the 40n as a function of output power for the analog line level-input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). Aside from the right channel into 4 ohms (described above), overall, THD+N values for both loads were similar up to 50W. The 8-ohm data ranged from 0.02% down to just below 0.002%. The 8-ohm data outperformed the 4-ohm data by 3-4dB.

THD ratio (unweighted) vs. frequency at 8, 4, and 2 ohms (left channel only)

The chart above shows THD ratios measured at the output of the 40n as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage that yielded 10W at the output into 8 ohms (and roughly 20W into 4 ohms, and 40W into 2 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the 4-ohm load the purple trace, and the 2-ohm load the pink trace. The 8-ohm and 4-ohm track closely from 20Hz to 500Hz, between 0.001% and 0.0006%. Above 1kHz, the 4-ohm data yielded about 5dB more distortion, and measuring 0.005% at 20kHz. The 2-ohm data yielded much higher distortion, hovering between 0.02% and 0.03% from 20Hz to 20kHz.

THD ratio (unweighted) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows THD ratios measured at the output of the 40n as a function of frequency into an 8 ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). Between 500Hz and 20kHz, all three data sets have similar THD ratios, between 0.001% and 0.005%. Below 500Hz, there are signficant differences between the dummy load and the real speakers. While the dummy load yiedled the same constant 0.001% down to 20Hz, the three-way Paradigm was at 0.005%, and the two-way Focal at 0.05%.

IMD ratio (CCIF) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the 40n as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here the CCIF IMD method is used, where the primary frequency is swept from 20kHz (F1) down to 2.5kHz, and the secondary frequency (F2) is always 1kHz lower than the primary, with a 1:1 ratio. The CCIF IMD analysis results are the sum of the second (F1-F2 or 1kHz) and third-modulation products (F1+1kHz, F2-1kHz). The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three data sets track fairly closely, with both speaker data yielding slightly lower IMD ratios than the dummy load. The 8-ohm dummy load yielded a fairly constant 0.002%, while the three-way Paradigm yideld the lowest IMD ratios—as low as 0.0007%.

IMD ratio (SMPTE) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows IMD ratios measured at the output of the 40n as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here, the SMPTE IMD method is used, where the primary frequency (F1) is swept from 250Hz down to 40Hz, and the secondary frequency (F2) is held at 7kHz with a 4:1 ratio. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three data sets are close enough to be judged as identical, hovering around the 0.005% mark.

FFT spectrum – 1kHz (line-level input)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the analog line-level input. We see that the signal’s second harmonic, at 2kHz, is at -105dBrA, or 0.0006%, while all subsequent harmonics are below -110dBrA, or 0.0003%. Power-supply-related noise peaks are evident, but at a very low -120dBrA, or 0.0001%, level and below.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the coaxial digital input, sampled at 16/44.1. Signal harmonics and power-supply-related noise peaks are very similar in level to the analog input FFT above, though between 40 and 50kHz, we can see the IMD products of the sample rate (44.1kHz) and the main signal at 43.1kHz and 45.1kHz.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the coaxial digital input, sampled at 24/96. Signal harmonics and power-supply-related noise peaks are very similar in level to the analog input FFT above, although without the 44.1kHz sample rate IMD products.

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

Shown above is the FFT for a 1kHz -90dBFS dithered 16/44.1 sine-wave stimulus at the coaxial digital input, measured at the output across an 8-ohm load. We see the 1kHz primary signal peak, at the correct amplitude, and very low-level power-supply related peaks at -120dBrA, or 0.0001%, and below.

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

Shown above is the FFT for a 1kHz -90dBFS dithered 24/96 input sine-wave stimulus at the coaxial digital input, measured at the output across an 8-ohm load. We see the 1kHz primary signal peak, at the correct amplitude, and very low-level power-supply related peaks at -120dBrA, or 0.0001%, and below.

FFT spectrum – 1kHz (MM phono input)

Shown above is the FFT for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the MM phono input. We see the second (2kHz), third (3kHz), and fifth (5kHz) signal harmonics at very low levels: -110 and -120dBrA, respectively, or 0.0003%, and 0.0001%. The most significant power-supply-related noise peaks can be seen at 60Hz at -90dBrA, or 0.003%. Higher-order power-supply related peaks can also be seen at lower amplitudes. This is a very clean MM phono FFT for an integrated amplifier.

FFT spectrum – 50Hz (line-level input)

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the output across an 8-ohm load at 10W for the analog 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 dominant (non-signal) peak is the second signal harmonic (100Hz) at -105dBrA, or 0.0006%. Power-supply-related peaks are at -120dBrA, or 0.0001%, and below.

FFT spectrum – 50Hz (MM phono input)

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the output across an 8-ohm load at 10W for the MM phono 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 from the 60Hz power-supply fundamental at -90dBrA, or 0.003%. Signal-related harmonics are all but non-existent above the noise floor.

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 sine-wave stimulus tone measured at the output across an 8-ohm load at 10W 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, 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 just below -110dBrA, or 0.0003%. This is a very clean IMD result.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sine-wave stimulus tone measured at the output across an 8-ohm load at 10W 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 -120dBRa, or 0.0001%, while the third-order modulation products, at 17kHz and 20kHz, are just below -110dBrA, or 0.0003%.

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

Shown above is an FFT of the intermodulation (IMD) products for an 18kHz + 19kHz summed sine-wave stimulus tone measured at the output across an 8-ohm load at 10W 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 -120/-115dBRa (left/right), or 0.0001/0.0002%, while the third-order modulation products, at 17kHz and 20kHz, are just above -110dBrA, or 0.0003%.

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 sine-wave stimulus tone measured at the output across an 8-ohm load at 10W for the MM phono input. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is just above the noise floor for the left channel only at -115dBRa, or 0.0002%, while the third-order modulation products, at 17kHz and 20kHz, are around -105dBrA, or 0.0006%.

Square-wave response (10kHz)

Above is the 10kHz square-wave response using the analog line-level input, at roughly 10W into 8 ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the 40n’s slew-rate performance. Rather, it should be seen as a qualitative representation of the 40n’s extended bandwidth. An ideal square wave can be represented as the sum of a sine wave 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 40n’s square-wave response is superb, showing no visible over/undershoot, or ringing near the corners. 

Damping factor vs. frequency (20Hz to 20kHz)

The final graph above is the damping factor as a function of frequency. We see a relatively constant, and high damping factor, and both channels tracking closely, around 230 from 20Hz to 20kHz.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Amplifier Measurements

Created: 01 July 2022

General Information

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

The Fiera4 was conditioned for 1 hour at 1/8th full rated power (~18W into 8 ohms) before any measurements were taken. All measurements were taken with both channels driven, using a 120V/20A dedicated circuit, unless otherwise stated.

The Fiera4 is a four-channel amplifier that can be configured for stereo (two-channel) applications. Outputs 1 and 3 can be configured in “BRIDGE” mode (as opposed to “STD” mode), where two channels are configured in parallel to output the same output voltage, but with more current handling, and thus higher power, in particular into lower impedances.

The Fiera4 offers four unbalanced (RCA) and four balanced (XLR) line-level analog inputs. Unless otherwise stated, the Fiera4 was operated in “BRIDGE” mode, where Output 1 was interpreted as the left channel and Output 3 the right channel, using the balanced inputs. To achieve the standard 10W into 8 ohms, 0.75Vrms was required at the input.

Because the Fiera4 is a digital amplifier technology that exhibits considerable noise above 20kHz (see FFTs below), our typical input bandwidth filter setting of 10Hz-90kHz was necessarily changed to 10Hz-22.4kHz for all speaker output measurements, except for frequency response and for FFTs. In addition, THD versus frequency sweeps were limited to 6kHz to adequately capture the second and third signal harmonics with the restricted bandwidth setting.

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Starke Sound for the Fiera4 compared directly against our own. The published specifications are sourced from Starke Sound’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 extended from DC to 500kHz, assume, unless otherwise stated, 10W into 8 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 line-level analog input (unless specified, assume a 1kHz sinewave at 0.75Vrms, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

For the continuous dynamic power test, the Fiera4 was able to sustain 299W into 4 ohms (>1% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (29.9W) for 5 seconds, for 5 minutes, without inducing any protection or shutdown circuits. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the top of the Fiera4 was only slightly warm to the touch.

Frequency response (8-ohm loading, line-level input)

In our measured frequency-response chart above, the Fiera4 is nearly flat within the audio band (20Hz to 20kHz). At the extremes, the Fiera4 is at -0.05dB at 20Hz and +1dB at 20kHz. Starke Sound claims a frequency response of 10Hz-20kHz (-1.2dB), which, although not stated, is likely into 4 ohms. It should be noted that the rise at high frequencies is typical for this type of digital amplifier technology, which exhibits a low damping factor (high output impedance) at high frequencies (see damping factor vs. frequency chart below). When frequency response is measured into a 4-ohm load (see RMS level vs. frequency vs load impedance chart below), there is a dip instead of a rise in the response at high frequencies. 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 (8-ohm loading, line-level input)

Above are the left- and right-channnel phase-response plots from 20Hz to 20kHz for the balanced line- level input, measured across the speaker outputs at 10W into 8 ohms. The Fiera4 does not invert polarity and exhibits, at worst, about 50 degrees (at 20Hz) of phase shift within the audio band.

RMS level vs. frequency vs. load impedance (1W, left channel only)

The chart above shows RMS level (relative to 0dBrA, which is 1W into 8ohms or 2.83Vrms) as a function of frequency, for the balanced line-level input swept from 5Hz to 100kHz. The blue plot is into an 8 ohm load, the purple is into a 4-ohm load, the pink plot is an actual speaker (Focal Chora 806, measurements can be found here), and the cyan plot is no load connected. The chart below . . .

. . . is the same but zoomed in to highlight differences. The most obvious feature in these plots is the significant deviations in response at different loads at high frequencies (above 5kHz or so). This is a characteristic of this type of digital amplifier technology, where there’s a rising output impedance (or low damping factor) at high frequencies. This can have the effect of either brightening or dulling the treble response in a system, depending on the characteristic impedance of the speaker being driven. In the flatter part of the curves (below 5kHz), we can see a maximum deviation within the audio band of about 0.2dB from 4 ohms to no load, which is an indication of a mid-level damping factor. The maximum variation in RMS level when a real speaker was used in the flat portion of the curve is a little less, deviating by about 0.1dB.

THD ratio (unweighted) vs. frequency vs. output power

The chart above shows THD ratios at the output into 8 ohms as a function of frequency for a sine-wave stimulus at the balanced line-level input. The blue and red plots are for left and right channels at 1W output into 8 ohms, purple/green at 10W, and pink/orange at 132W. The Fiera4 shows a trend of rising THD as the frequency increases, as well as higher THD ratios at higher output power levels. Between 500Hz and 6kHz, at 1W, THD ratios ranged from 0.003% to 0.02%, while at 10W, then at 132W, THD ratios were about 10dB higher at each increased power increment. From 20Hz to 300Hz, THD ratios at 1W and 10W were similar, hovering between 0.002 and 0.003%. At 132W, from 20Hz to 300Hz, THD ratios ranged from 0.005% to 0.015%, with the left channel outperforming the right channel at low frequencies by about 5dB.

THD ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms

The chart above shows THD ratios measured at the output of the Fiera4 as a function of output power for the balanced line-level input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). The 8-ohm THD ratios ranged from about 0.03% at 50mW down to about 0.002% at 100mW, followed by a steady rise to 0.04% at the “knee,” at roughly 130W. The 4-ohm data yielded THD ratios roughly 5dB higher. The “knee” in the 4-ohm data occurs around 230W. The 1% THD values are reached at 153W (8 ohms) and 288W (4 ohms).

THD+N ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms

The chart above shows THD+N ratios measured at the output of the Fiera4 as a function of output power for the balanced line-level input for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). Since the Fiera4 produces levels of THD that dominate relative to the noise, the THD+N plots look very similar to the THD versus output power plots above.

THD ratio (unweighted) vs. frequency at 8, 4, and 2 ohms (left channel only)

The chart above shows THD ratios measured at the output of the Fiera4 as a function of frequency into three different loads (8/4/2 ohms), for a constant input voltage that yields 20W at the output into 8 ohms (and roughly 40W into 4 ohms, and 80W into 2 ohms) for the balanced line-level input. We find increasing THD values from 8 to 4 to 2 ohms. Into 8 ohms, THD ratios range from 0.002% at 20Hz up to 0.08% at 6kHz. The 4-ohm THD ratios are 10dB higher through most of the frequency range (200Hz to 6kHz), and the 2-ohm data, about 8dB higher than the 4-ohm data. Even into 2 ohms, the Fiera4 manages THD ratios between 0.006% and 0.5% at 80W.

THD ratio (unweighted) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows THD ratios measured at the output of the Fiera4 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms), for the balanced line-level input. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). At high frequencies (5-6kHz), the dummy load and both speakers track closely with THD ratios between 0.01% and 0.03%. Below 1kHz, there are significant diffrences in THD ratios between the dummy load and real speakers, by over 30dB at 20Hz, where the two-way speaker THD ratio was 0.15% compared to the 0.002% measured across the dummy load. Between 50Hz and 1kHz, deviations between the dummy load and speakers were within 10dB or so. From this, we conclude that the Fiera4 seems to be sensitive to load variations in terms of THD.

IMD ratio (CCIF) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the Fiera4 as a function of frequency into an 8-ohm load and two different speakers, for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the balanced line-level input. Here the CCIF IMD method was used, where the primary frequency is swept from 20kHz (F1) down to 2.5kHz, and the secondary frequency (F2) is always 1kHz lower than the primary, with a 1:1 ratio. The CCIF IMD analysis results are the sum of the second (F1-F2 or 1kHz) and third modulation products (F1+1kHz, F2-1kHz). The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three plots are similar. The IMD ratios range from 0.006% at 2.5kHz up to 0.15% between 10kHz and 20kHz. The largest devations between the dummy load and speakers came at high freqenices, where the two-way speaker measured almost 10dB lower than the three-way speaker between 10kHz and 20kHz.

IMD ratio (SMPTE) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows IMD ratios measured at the output of the Fiera4 as a function of frequency into an 8-ohm load and two different speakers, for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the balanced line-level input. Here, the SMPTE IMD method was used, where the primary frequency (F1) is swept from 250Hz down to 40Hz, and the secondary frequency (F2) is held at 7kHz with a 4:1 ratio. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three results track closely, hovering between 0.01% and 0.02%.

FFT spectrum – 1kHz (line-level input)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the balanced line-level input. The signal harmonics are evident with the third (3kHz) harmonic dominating at -80dBrA, or 0.01%. The second (2kHz) and fifth (5kHz) harmonics are between -95dBrA and -100dBrA, or 0.002% and 0.001%. On the left side of the main signal peak, we find virtually no power-supply noise related harmonics. The right channel shows a -120dBrA, or 0.0001%, peak at 60Hz, and the left channel shows a -125dBrA, or 0.00006% peak at 120Hz. The low noise of the Fiera4 results in good signal-to-noise ratio performance (see primary table above).

FFT spectrum – 50Hz (line-level input)

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the output across an 8-ohm load at 10W for the balanced 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 third signal harmonic (150Hz) dominates at -95dBrA, or 0.002%, with other signal harmonics seen at -100dBrA and below. Once again, only a hint of power-supply noise related harmonics can be seen at 60Hz and 120Hz, just above the -130dBrA, or 0.00003%, noise floor. 

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

Shown above is an FFT of the intermodulation (IMD) products for an 18kHz + 19kHz summed sine-wave stimulus tone measured at the output across an 8-ohm load at 10W 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 -100/105dBrA (left/right), or 0.001/0.0006%. The third-order modulation products, at 17kHz and 20kHz, are much higher, at around -65dBrA, or 0.06%.

Square-wave response (10kHz)

Above is the 10kHz square-wave response using the balanced line-level input, at roughly 10W into 8 ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the Fiera4’s slew-rate performance. Rather, it should be seen as a qualitative representation of the Fiera4’s mid-level bandwidth. An ideal square wave can be represented as the sum of a sine wave 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 we can see the 450kHz switching oscillator frequency used in the digital amplifier section visibly modulating the waveform.

Square-wave response (10kHz) with 250kHz bandwidth

Above is the 10kHz square-wave response using the balanced input, at roughly 10W into 8 ohms, with a 250kHz restricted bandwidth to remove the modulation from the 450kHz oscillator. We see more evidence here, in the over and undershoot at the square-wave corners, of the Fiera4’s limited bandwidth.

FFT spectrum of 400kHz switching frequency relative to a 1kHz tone

The Fiera4’s class-D amplifier relies on a switching oscillator to convert the input signal to a pulse-width modulated (PWM) square wave (on/off) signal before sending the signal through a low-pass filter to generate an output signal. The Fiera4 oscillator switches at a rate of about 450kHz, and this graph plots a wide-bandwidth FFT spectrum of the amplifier’s output at 10W into 8 ohms as it’s fed a 1kHz sine wave. We can see that the 450kHz peak is quite evident, and at -30dBrA. There is also a peak at 900kHz (the second harmonic of the 450kHz peak), at -65dBrA. Those peaks—the fundamental and its second harmonic—are direct results of the switching oscillators in the Fiera4 amp modules. The noise around those very-high-frequency signals are in the signal, but all that noise is far above the audio band—and is therefore inaudible—and so high in frequency that any loudspeaker the amplifier is driving should filter it all out anyway.

Damping factor vs. frequency (20Hz to 20kHz)

The final graph above is the damping factor as a function of frequency. Both channels show a relatively constant damping factor from 20Hz to 600Hz, right around 92/101 (left/right). Above 600Hz, the damping factor decreases sharply, hitting a low of about 7 at 20kHz.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Amplifier Measurements

Created: 15 June 2022

Link: reviewed by Roger Kanno on SoundStage! Hi-Fi on June 15, 2022

General Information

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

The SHD Power was conditioned for 1 hour at 1/8th full rated power (~15W into 8 ohms) before any measurements were taken. All measurements were taken with both channels driven, using a 120V/20A dedicated circuit, unless otherwise stated.

The SHD Power is a highly customizable audio component. Beyond its function as an integrated amplifier-DAC, the SHD Power can be utilized as a stand-alone two-channel-in/four-channel-out digital processor with full bass management, parametric equalization, gain trim, and compression. Users can also individually assign any of the four outputs over four different presets. On top of all the output customization, Dirac Live room EQ can be applied to the system as a whole. The SHD Power can be configured using the miniDSP SHD control application (v1.15 was used for these measurements, installed on a Windows 11 laptop, controlling the SHD Power over USB).

The SHD Power has four digital inputs (2x L/R, and labelled Dirac 1 and Dirac 2 in the control app) accessed over S/PDIF (coaxial), S/PDIF TosLink (optical), AES/EBU (XLR), or USB. The SHD Power can also be accessed over LAN (ethernet) and used as streamer using the built-in Volumino platform. The SHD Power has four outputs (or two stereo L/R outputs labeled Output 1 through Output 4 in the control app). The speaker outputs are assigned Output 1 and Output 2; there are two analog sub-outs assigned Output 3 and Output 4; and there are two L/R digital outputs (AES-EBU over XLR), the first assigned Output 1 and Output 2, the second assigned Output 3 and Output 4. Any of the two inputs can be assigned, or “routed” to any output, with gain trim available from +12dB to -72dB in 0.1dB increments. This also includes the option to link outputs for mono configuration. Each output can be low- or high-pass filtered (LPF or HPF) with a customizable slope and cut-off frequency. In addition, a ten-band parametric EQ and a compressor can be applied to each output, and there are four presets available, making for a dizzying number of possibilities. For example, if one wanted to use the SHD Power as a bass-managed integrated amp in a 2.2 channel system, crossed over at 120Hz, for preset 1, one would:

• Select Config 1 in the control application
• Under Routing, route Output 1 (speaker L) to Dirac 1 (input L),
• Route Output 2 (speaker R) to Dirac 2 (input R),
• Route Output 3 (sub out L) to Dirac 1 (input L),
• Route, Output 4 (sub out R) to Dirac 2 (input R),
• Under Outputs, apply a 120Hz (with customizable slope) HPF to Outputs 1 and 2,
• Apply a 120Hz LPF to Outputs 3 and 4

After this, Dirac Live can be applied to the system as a whole using Dirac Live 3, a UMIK-1 microphone, and the SHD Power control application.

For the purposes of these measurements, the coaxial input was used, along with the speaker outputs and analog sub-outs.

Most measurements were made with a standard 0dBFS digital input. Because the SHD Power utilizes a digital amp with a poor damping factor a high frequencies, it also offers an 8/4-ohm switch (in the control software), to optimize frequency response for different speakers. Unless otherwise stated, the switch was left in the 8-ohm position.

Because the SHD Power uses digital-amplifier technology that exhibits considerable noise above 20kHz (see FFTs below), our typical input bandwidth filter setting of 10Hz-90kHz was necessarily changed to 10Hz-22.4kHz for all measurements, except for frequency response and for FFTs. In addition, THD versus frequency sweeps were limited to 6kHz to adequately capture the second and third signal harmonics with the restricted bandwidth setting.

We typically publish the plot of a 10kHz squarewave output for amplifiers. In the case of the SHD Power, a useable output could not be captured using the square-wave input from the APx555, and thus this graph is omitted herein.

Based on the accuracy and repeatable results at various volume levels of the left/right channel matching (see table below), and the lack of analog inputs, the SHD Power volume control is likely applied entirely in the digital domain. The volume control offers a total range of -127.5dB to 0dB on the display, in 0.5dB steps. With the factory defaults, a 0dBFS input will yield 120W into 8 ohms with the volume at maximum (0dB). However, extra gain (up to 12 dB) can be added to any individual preset and output through the miniDSP control software.

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

Published specifications vs. our primary measurements

The table below summarizes the measurements published by miniDSP for the SHD Power compared directly against our own. The published specifications are sourced from miniDSP’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 extended from 10Hz to 500kHz, assume, unless otherwise stated, 10W into 8 ohms and a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channels.

*auto-shutdown after a few seconds, below 1% THD threshold

Our primary measurements revealed the following using the digital coaxial input (unless specified, assume a 1kHz sinewave at 0dBFS, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

*auto-shutdown after a few seconds, below 1% THD threshold

**extra gain can be added via control app

Our typical continuous dynamic power test counld not be performed with the SHD Power due to its lack of analog inputs, and the Audio Precision's lack of a sinewave burst generator using the DAC generator.

Frequency response vs. input type (8-ohm loading)

The chart above shows the SHD Power’s frequency response as a function of input type measured across the speaker outputs at 10W into 8 ohms. The blue/red (left/right channels) traces are for a 16-bit/44.1kHz dithered digital input signal from 5Hz to 22kHz using the coaxial digital input, and the purple/green (left/right channels) traces are for a 24/96 dithered digital input signal from 5Hz to 48kHz, while the pink/cyan (left/right channels) traces are for a 24/192 dithered digital input signal from 5Hz to 96kHz.

The 24/96 and 24/192 responses overlap perfectly, indicating 24/192 data is likely downsampled to 24/96. All input data are at -0.15dB at 10Hz, corroborating miniDSP’s claim of 10Hz-30kHz, ±0.5dB. The 16/44.1 data exhibits brick-wall type filtering, with a -3dB point at 21.0kHz. The 24/96 and 24/192 kHz data yielded -3dB points at 45.2kHz. The ±0.5dB claim up to 30kHz is not corroborated, as there is a rise in the response just before 30kHz, peaking at +0.7dB. It should be noted that this response is intentional, because this type of digital amplifier technology exhibits a low damping factor (high output impedance) at high frequencies (see damping factor vs frequency chart below). When frequency response is measured into a 4-ohm load (see RMS level vs. frequency vs load impedance chart below), there is a dip instead of a rise in the response at high frequencies.

Frequency response (8-ohm loading, with bass management, 24/96 data)

Above are two frequency-response plots, measured at 10W (8-ohm) at the speaker outputs, and at the line-level subwoofer outputs, with the crossover set at 120Hz, for a 24/96 dithered input signal. The SHD Power crossover uses adjustable slopes. Here, 24dB/octave was chosen. The subwoofer output is essentially flat down to 10Hz.

Frequency response (4-ohm loading, 4-ohm setting, 24/96 data)

Above are the frequency-response plots measured at the speaker-level outputs into 4 ohms for a 24/96 dithered input, with the SHD Power speaker impedance setting at 4 ohms. There is a distinct difference between the left (blue) and right (red) channels at high frequencies. The left channel is at about -0.75dB at 20kHz, while the right channel is at roughly 0dB.

Frequency response (8-ohm loading, Roger Kanno Dirac Live Preset 1, 24/96)

Above are frequency-response plots measured at the speaker-level outputs into 8 ohms, for a 24/96 dithered input. These show the results with the Dirac Live filter engaged in Roger Kanno’s review, using his PSB Alpha T20 speakers.

Frequency response (8-ohm loading, Roger Kanno Dirac Live Preset 2, 24/96)

Above are frequency response plots measured at the speaker-level outputs into 8 ohms, for a 24/96 dithered input. These show the results with the Dirac Live filter engaged in Roger Kanno’s review, using his MartinLogan ESL 9 speakers.

Frequency response (8-ohm loading, Roger Kanno Dirac Live Preset 3, 24/96)

Above are frequency response plots measured at the speaker-level outputs into 8 ohms, for a 24/96 dithered input. These show the results with the Dirac Live filter engaged in Roger Kanno’s review, using his MartinLogan ESL 9 speakers and JL Audio E-Sub e112 subwoofers. Note that for this preset, Roger did not apply any bass management, hence the severe cut between 50 and 100Hz, where both the main speakers and subs are reproducing bass.

Phase response (8-ohm loading, 16/44.1 and 24/96 data)

Above are the phase response plots from 20Hz to 20kHz, measured across the speaker outputs at 10W into 8 ohms, for a 16/44,1 (blue/red) and 24/96 (purple/green) dithered digital input. The SHD Power does not invert polarity and exhibits, at worst, a little over 20 degrees (at 20Hz for the 16/44.1 data) of phase shift within the audio band.

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

The chart above shows the results of a linearity test for the coaxial digital input for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the line-level sub-outs of the SHD Power for a 1Vrms (at 0dBFS) output signal. 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. Both data were essentially perfect from 0dBFS to -100dBFS. At -120dBFS, the 16/44.1 data were about +2dB above reference, while the 24/96 data were still perfect. Because of this DAC’s exceptional linearity performance, a second measurement, which we don't usually do, was performed extending down to -140dBFS . . .

. . . which shows the 24/96 data remained within 1dB of flat—a great result. The 16/44.1 data results are as expected, showing significant deviations below -120dBFS. But it is worth highlighting that a linearity result that is flat down to -100dBFS for 16-bit input data is also exceptional.

Impulse response (24/44.1 data)

The chart 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 line-level sub-outs of the SHD Power. We can see that the SHD Power utilizes a typical sinc function reconstruction filter.

J-Test (coaxial)

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level sub-outs of the SHD Power. J-Test was developed by Julian Dunn in the 1990s. It is a test signal, specifically a -3dBFS undithered 12kHz square wave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz square wave 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 square wave 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 sine-wave jitter by the Audio Precision, to show how well the DAC rejects jitter.

The coaxial input exhibits low-level peaks throughout the audio-band, at -130dBrA and below. This is a good J-Test result, indicating that the SHD Power DAC likely has strong jitter immunity.

J-Test (optical)

The chart above shows the results of the J-Test test for the optical digital input, measured at the line-level sub-outs of the SHD Power. The optical input exhibits low-level peaks throughout the audio band, at -130dBrA and below. As with the coaxial input, this is a good J-Test result, indicating that the SHD Power DAC likely has strong jitter immunity.

J-Test with 500ns of injected jitter (coaxial)

Both the coaxial and optical inputs were also tested for jitter immunity by injecting artificial sine-wave jitter at 2kHz, which would manifest as sidebands at 10kHz and 14kHz without any jitter rejection. The results were similar for both inputs, so only the coaxial result is shown. Jitter immunity proved excellent, with no visible sidebands even at a high 500ns of jitter level. At 1000ns of jitter level, the SHD Power DAC lost sync with the input signal.

Wideband FFT spectrum of white noise and 19.1kHz sine-wave tone (coaxial input)

The plot above shows a fast Fourier transform (FFT) of the SHD Power’s line-level sub-outs with white noise at -4dBFS (blue/red) and a 19.1kHz sine wave at 0dBFS (1Vrms) fed to the coaxial digital input, sampled at 16/44.1. The sharp roll-off above 20kHz in the white-noise spectrum shows the implementation of a brick-wall-type reconstruction filter. There are no aliased image peaks in the audio band above the -135dBrA noise floor. The main 25kHz alias peak is highly suppressed at -130dBrA. The second and third distortion harmonics (38.2, 57.3kHz) of the 19.1kHz tone are higher, between -100dBrA and -110dBrA.

RMS level vs. frequency vs. load impedance (1W, left channel only)

The chart above shows RMS level (relative to 0dBrA, which is 1W into 8 ohms, or 2.83Vrms) as a function of frequency, for the coaxial input with a dithered 24/96 signal swept from 5Hz to 100kHz. The blue plot is into an 8-ohm load, the purple is into a 4-ohm load, the pink plot is an actual speaker (Focal Chora 806, measurements can be found here), and the cyan plot is no load connected. The chart below . . .

. . . is the same but is zoomed in to highlight differences. The most obvious feature in these plots is the significant deviations in response at different loads at high frequencies (above 5kHz or so). This is a characteristic of this type of digital-amplifier technology—there is a rising output impedance (or low damping factor) at high frequencies. This can have the effect of either brightening or dulling the treble response in a system, depending on the characteristic impedance of the speaker. It must be pointed out that, in the case of the SHD Power, this is not an issue, given that one of the primary reasons for purchasing this device is to compensate for in-room frequency response aberrations through the built-in parametric EQ and/or Dirac Live room correction systems. In the flatter part of curves (below 5kHz), we can see a maximum deviation within the audio band of about 0.15dB from 4 ohms to no load, which is an indication of a mid-level damping factor. The maximum variation in RMS level when a real speaker was used in the flat portion of the curve is a little less, deviating by about 0.1dB.

THD ratio (unweighted) vs. frequency vs. output power (24/96 data)

The chart above shows THD ratios at the output into 8 ohms as a function of frequency for a 24/96 dithered sine-wave stimulus at the coaxial input. The blue and red plots are for left and right channels at 1W output into 8 ohms, purple/green at 10W, and pink/orange, just below the rated 120W. The power was varied using the volume control. The 1 and 10W are very similar, ranging from about 0.001% from 20Hz through to 1kHz, then up to 0.005-0.01% at 6kHz. The 120W data yielded higher results, but still admirably low, at about 0.005% from 20Hz to 600Hz, then up to 0.05% at 6kHz. At low frequencies, the left channel outperformed the right channel at 120W by as much as 5dB.

THD ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms (24/96 data)

The chart above shows THD ratios measured at the output of the SHD Power as a function of output power for a 24/96 dithered sine-wave stimulus at the coaxial input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). The 8-ohm data generally outperformed the 4-ohm data by almost 10dB, from 1W to the “knee” at just past 100W. At low output power, THD ratios were as high as 0.05% to 0.2% at 50mW. From 1W to roughly 100W, THD ratios for the 8-ohm data ranged from 0.001% to 0.005%, and 0.002% to 0.01% for the 4-ohm data. The 1% THD mark was reached for the 8-ohm data at 130W. We could not reach the 1% THD level with a 4-ohm load without tripping the protection circuit. The highest sustainable power output we measured was 149W, but only for a few seconds before the unit shut down.

THD+N ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms (24/96 data)

The chart above shows THD+N ratios measured at the output of the SHD Power as a function of output power for a 24/96 dithered sine-wave stimulus at the coaxial input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). The 8-ohm data generally outperformed the 4-ohm data by 5-10dB, from 10W to the “knee” at just past 100W. At low output power, THD+N ratios were as high as 0.1% to 0.2% at 50mW. From 1W to roughly 100W, THD+N ratios for the 8-ohm data ranged from 0.005% to as low as 0.002% (at 20-30W), and 0.005% to 0.01% for the 4-ohm data.

THD ratio (unweighted) vs. frequency at 8, 4, and 2 ohms (left channel only, 24/96 data)

The chart above shows THD ratios measured at the output of the SHD Power as a function of frequency into three different loads (8/4/2 ohms), for a 24/96 dithered sine-wave stimulus at the coaxial input that yields 10W at the output into 8 ohms (and roughly 20W into 4 ohms, and 40W into 2 ohms). The 8-ohm load is the blue trace, the 4-ohm load the purple trace, and the 2-ohm load the pink trace. We find increasing THD values from 8 to 4 to 2 ohms. Into 8 ohms, THD ratios are at or just below 0.001% from 20Hz to 500Hz and up to 0.008% at 4kHz. The 4-ohm THD ratios are between 3 and 10dB higher through most of the frequency range, and the 2-ohm data, about 10dB higher than the 4-ohm data. Even into 2 ohms, the SHD Power manages THD ratios between 0.003% and 0.05% at 40W.

THD ratio (unweighted) vs. frequency into 8 ohms and real speakers (left channel only, 24/96 data)

The chart above shows THD ratios measured at the output of the SHD Power as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for a 24/96 dithered sine-wave stimulus at the coaxial input. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). At high frequencies (5-6kHz), the dummy load and three-way speaker track closely, while the two-way speaker yielded lower results by a smuch as 15dB at 3-4kHz. Below 1kHz, there are significant diffrences in THD ratios between the dummy load and real speakers, by as much as 40dB at 20Hz, where the two-way speaker THD ratio was 0.1% comapred to the 0.001% measured across the dummy load. The amplifier section of the SHD Power seems to be sensitive to load variations in terms of THD.

IMD ratio (CCIF) vs. frequency into 8 ohms and real speakers (left channel only, 24/96 data)

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the SHD Power as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for a 24/96 dithered sine-wave stimulus at the coaxial input. Here the CCIF IMD method is used, where the primary frequency is swept from 20kHz (F1) down to 2.5kHz, and the secondary frequency (F2) is always 1kHz lower than the primary, with a 1:1 ratio. The CCIF IMD analysis results are the sum of the second (F1-F2 or 1kHz) and third modulation products (F1+1kHz, F2-1kHz). The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). Here again we see variations between the dummy load, which yielded the lowest IMD results between 0.002% at 2.5kHz and 0.025% at 20kHz, and the real speakers, where the 3-way speaker was 15dB higher at 4-5kHz. The two-way speaker yielded IMD results between the dummy load and three-way speaker.

IMD ratio (SMPTE) vs. frequency into 8 ohms and real speakers (left channel only, 24/96 data)

The chart above shows IMD ratios measured at the output of the SHD Power as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for a 24/96 dithered sine-wave stimulus at the coaxial input. Here, the SMPTE IMD method is used, where the primary frequency (F1) is swept from 250Hz down to 40Hz, and the secondary frequency (F2) is held at 7kHz with a 4:1 ratio. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. Here all three data sets are fairly close together, between 0.005% and 0.01% from 40Hz to 250Hz. The three-way speaker did yield results about 5dB higher than the 2-way speaker and dummy load between 80Hz and 250Hz.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the coaxial digital input, sampled at 16/44.1. Here we find the second (2kHz) and third (3kHz) harmonic of the signal at just below -100/-105dBrA (left/right), or 0.001/0.0006%. Higher order signal harmonics are visible at -110dBrA, or 0.0003%, and below. The noise-related peaks on the left side of the signal peak are virtually non-existent.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the coaxial digital input, sampled at 24/96. The FFT spectrum is nearly identical within the audio band to the 16/44.1 FFT above, but for a slightly lower noise floor due to the 24-bit depth.

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 sine-wave stimulus at the coaxial digital input, measured at the output across an 8-ohm load. We see the 1kHz primary signal peak, at the correct amplitude, and signal harmonics are non-existent above the 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 sine-wave stimulus at the coaxial digital input, measured at the output across an 8-ohm load. The FFT spectrum is nearly identical within the audio band to the 16/44.1 FFT above.

FFT spectrum – 50Hz (digital input, 24/96 data at 0dBFS)

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the output across an 8-ohm load at 10W for a 24/96 dithered sine-wave stimulus at the coaxial 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 second (100Hz) harmonic dominates at -100/-105dBrA (left/right), or 0.001/0.0006%, but even and odd signal harmonics at lower levels (below -120dBrA, or 0.0001%) can be seen throughout.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sine-wave stimulus tone measured at the output across an 8-ohm load at 10W 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 105/110dBrA (left/right channels), or 0.0006/0.0003%.  The third-order modulation products, at 17kHz and 20kHz, are around -75dBrA, or 0.02%. We also see the main aliased peaks at 25.1kHz and 26.1kHz around -70dBrA due to the 44.1kHz sample rate (e.g., 44.1kHz-19kHz = 25.1kHz).

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sine-wave stimulus tone measured at the output across an 8-ohm load at 10W for the digital coaxial input at 24/96. The FFT spectrum is nearly identical within the audio band to the 16/44.1 FFT above.

FFT spectrum of 400kHz switching frequency relative to a 1kHz tone (24/96 data)

The SHD Power’s amplifier relies on a switching oscillator to convert the input signal to a pulse-width modulated (PWM) square wave (on/off) signal before sending the signal through a low-pass filter to generate an output signal. The SHD Power’s oscillator switches at a rate of about 450kHz, and this graph plots a wide bandwidth FFT spectrum of the amplifier’s output at 10W into 8 ohms as it’s fed a 1kHz dithered 24/96 sine wave. We can see that the 400kHz peak is quite evident, and at -30dBrA. There is also a peak at 900kHz (the second of the 400kHz peak), at -60dBrA. Those three peaks—the fundamental and its harmonics—are direct results of the switching oscillators in the SHD Power’s amp modules. The noise around those very-high-frequency signals are in the signal, but all that noise is far above the audio band—and therefore inaudible—and so high in frequency that any loudspeaker the amplifier is driving should filter it all out anyway.

Damping factor vs. frequency (20Hz to 20kHz)

The final graph above is the damping factor as a function of frequency. Both channels track closely and show a constant damping factor right around 110 from 20Hz to about 600Hz. Above this point, the damping factor decreases sharply, hitting a low of about 11 at 20kHz.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Amplifier Measurements

Created: 01 June 2022

Link: reviewed by Dan Kong on SoundStage! Hi-Fi on June 1, 2022

General Information

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

The Atoll Electronique IN200 Signature integrated amplifier was conditioned for 1 hour at 1/8th full rated power (~15W into 8 ohms) before any measurements were taken. All measurements were taken with both channels driven, using a 120V/20A dedicated circuit, unless otherwise stated.

The IN200 Signature offers five unbalanced (RCA) line-level analog inputs, four unbalanced (RCA) line-level analog outputs (Tape Out, By-pass and two pre-outs), and one pair of speaker-level outputs. On the front of the unit is a 1/4″ TRS headphone output.

Most measurements were made with a standard 2Vrms line-level analog input. The signal-to-noise ratio (SNR) measurements were made with the same input signal values, but with the volume set to achieve 1% THD at the output (8 ohms). For comparison, an SNR measurement was also made with the volume at maximum, where only 0.42Vrms was required to achieve 104W into 8ohms.

Based on the accurate and non-repeatable results at various volume levels of the left/right channel matching (see table below), the IN200 Signature volume control is likely digitally controlled but still operating in the analog domain. The volume control offers a total range of 0 to 80 on the display, which measured from -38dB (position 1) to +37dB between the line-level analog input and the speaker outputs, in increments of 2dB from positions 1 through 15, 1dB from 16 to 47, and 0.5dB from 48 to 80.

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

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Atoll for the IN200 Signature compared directly against our own. The published specifications are sourced from Atoll’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 extended from DC to 500kHz, assume, unless otherwise stated, 10W into 8ohms and a measurement input bandwidth of 10Hz to 90kHz, and the worst-case measured result between the left and right channel.

Our primary measurements revealed the following using the line-level analog input (unless specified, assume a 1kHz sinewave at 2Vrms, 10W output, 8-ohm loading, 10Hz to 90kHz bandwidth):

For the continuous dynamic power test, the IN200 Signature was able to sustain 178W into 4 ohms (~4% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (17.8W) for 5 seconds, for 5 minutes without inducing any protection or shut down circuits. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the top of the IN200 was quite warm to the touch.

Our primary measurements revealed the following using the analog input at the headphone output (unless specified, assume a 1kHz sine wave, 2Vrms output, 300 ohms loading, 10Hz to 90kHz bandwidth):

Frequency response (8-ohm loading, line-level input)

In our measured frequency-response chart above, the IN200 Signature is nearly flat within the audio band (20Hz to 20kHz). At the extremes the IN200 is at 0dB at 20Hz, and -0.4/0.3dB (left/right) at 20kHz. Atoll claims a frequency response of 5Hz to 100kHz, however, they do not specify deviations relative to 1kHz. At these extremes, the IN200 measured -0.1dB and -12/-11dB (left/right channels). 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 (8-ohm loading, line-level input)

Above are the left- and right-channel phase-response plots from 20Hz to 20kHz for the line-level input, measured across the speaker outputs at 10W into 8 ohms. The IN200 Signature does not invert polarity and exhibits, at worst, about 30 degrees (at 20Hz) of phase shift within the audio band.

RMS level vs. frequency vs. load impedance (1W, left channel only)

The chart above shows RMS level (relative to 0dBrA, which is 1W into 8 ohms or 2.83Vrms) as a function of frequency, for the analog line-level input swept from 5Hz to 100kHz. The blue plot is into an 8-ohm load, the purple is into a 4-ohm load, the pink plot is an actual speaker (Focal Chora 806, measurements can be found here), and the cyan plot is no load connected. The chart below . . .

. . . is the same but zoomed in to highlight differences. Here we can see a maximum deviation within the audio band of about 0.15dB from 4 ohms to no load, which is an indication of a mid-level damping factor, or average output impedance. The maximum variation in RMS level when a real speaker was used is less, deviating by about 0.13dB within the flat portion of the curve (20Hz to 5kHz). Note that the dip in RMS level at higher frequencies is a result of the frequency response of the IN200 signature, and not a damping factor issue, as all four plots show the same dip, at roughly the same rate.

THD ratio (unweighted) vs. frequency vs. output power

The chart above shows THD ratios at the output into 8 ohms as a function of frequency for a sine-wave stimulus at the analog line-level input. The blue and red plots are for the left and right channels at 1W output into 8 ohms, purple/green at 10W, and pink/orange at 95W. The power was varied using the volume control. The IN200 yields consistently high THD ratios across a wide range of power output levels. Above about 500Hz, THD is roughly the same at all power levels, ranging from about 0.2% up to 3-5%. Below 500Hz, the 1W data shows the lowest THD values, at 0.05% at 20Hz. The 10W data is just above at about 0.7% at 20Hz, while the 95W data is at 0.2% (30Hz).

THD ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms

The chart above shows THD ratios measured at the output of the IN200 Signature as a function of output power for the analog line-level input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). The 8-ohm THD ratios ranged from about 0.5% at 50mW down to about 0.2-0.3% from 30W to the “knee,” which is at roughly 80W. The 4-ohm data yielded THD ratios roughly 5dB higher, except for the right channel below 1W, which tracked the 8-ohm data very closely. The “knee” in the 4-ohm data occurs around 120W. The 1% THD values are reached at about 105W (8-ohm) and 170W (4-ohm). These measurements do not corroborate Atoll’s specified power ratings for the IN200 Signature of 120W and 200W into 8/4 ohms. It is possible that the European version, which would use 220VAC mains, does achieve this power rating. Note: our mains supply is a dedicated 120VAC/20A circuit, and it never dipped below 123VAC during these measurements.

THD+N ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms

The chart above shows THD+N ratios measured at the output of the IN200 Signature as a function of output power for the line level-input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). Since the IN200 produces high levels of THD, these dominate when measuring THD+N; therefore, the THD+N plots look very similar to the THD versus output power plots above.

THD ratio (unweighted) vs. frequency at 8, 4, and 2 ohms (left channel only)

The chart above shows THD ratios measured at the output of the IN200 Signature as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage that yields 10W at the output into 8 ohms (and roughly 20W into 4 ohms, and 40W into 2 ohms) for the analog line-level input. We find increasing THD values from 8 to 4 to 2 ohms; however, the differences are fairly small (2-3dB). Into 2 ohms, THD ratios range from 0.1% at 40Hz, up to almost 5% at 20kHz.

THD ratio (unweighted) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows THD ratios measured at the output of the IN200 Signature as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three plots track relatively closely, ranging from 0.05% at low freqeuncies, up to 5% at 20kHz.

IMD ratio (CCIF) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the IN200 Signature as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here the CCIF IMD method is used, where the primary frequency is swept from 20kHz (F1) down to 2.5kHz, and the secondary frequency (F2) is always 1kHz lower than the primary, with a 1:1 ratio. The CCIF IMD analysis results are the sum of the second (F1-F2 or 1kHz) and third modulation products (F1+1kHz, F2-1kHz). The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a 3-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three plots are similar. Just like the measured THD ratios, IMD ratios are high, ranging from about 0.3% up to almost 5% at 20kHz.

IMD ratio (SMPTE) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows IMD ratios measured at the output of the IN200 Signature as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here, the SMPTE IMD method is used, where the primary frequency (F1) is swept from 250Hz down to 40Hz, and the secondary frequency (F2) is held at 7kHz with a 4:1 ratio. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). The 8-ohm dummy load and 2-way speaker tracked almost perfectly, with a constant 0.2% IMD. The three-way speaker yielded IMD results about 10dB higher, hovering around 1%.

FFT spectrum – 1kHz (line-level input)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the analog line-level input. The signal harmonics are evident and very high in amplitude.  The odd harmonics (3/5/7/9kHz, etc.) are higher than the even harmonics (2/4/6/8kHz, etc.) at -60dBrA and below, or 0.1%, compared to the even harmonics at -75dBrA and below, or 0.02%. The exception is the second harmonic (2kHz) at just below -60dBrA. On the left side of the main signal peak, we find noise peaks at the fundamental (60Hz) and all subsequent harmonics at -90dBrA, or 0.003%, and below. At the second harmonic (120Hz) noise peak and above, the left channel yielded higher peaks than the right, by roughly 10dB.

FFT spectrum – 50Hz (line-level input)

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the output across an 8-ohm load at 10W for the analog 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 second harmonic (100Hz) dominates at -60dBrA, or 0.1%, while even and odd signal and power-supply noise-related harmonics at lower levels (-90dBrA or 0.003% and below), as well as their resultant IMD products, can be seen throughout.

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 sine-wave stimulus tone measured at the output across an 8-ohm load at 10W 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, 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 -70dBrA (left/right), or 0.03%. The third-order modulation products, at 17kHz and 20kHz, are quite high at around -50dBrA, or 0.3%.

Square-wave response (10kHz)

Above is the 10kHz square-wave response using the analog line-level input, at roughly 10W into 8 ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the IN200’s slew-rate performance. Rather, it should be seen as a qualitative representation of the IN200’s mid-level bandwidth. An ideal square wave can be represented as the sum of a sine wave 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, as well as softening of the edges. Here we can see an average looking square-wave reproduction, with softened corners but little to no ringing.

Damping factor vs. frequency (20Hz to 20kHz)

The final graph above is the damping factor as a function of frequency. Both channels show a relatively constant damping factor from 20Hz to 500Hz, right around 116. Above 500Hz, the right channel outperforms the left, with a damping factor as high as 130 at 15kHz, compared to the left channel’s 92 at 20kHz.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Amplifier Measurements

Created: 01 May 2022

Link: reviewed by Dennis Burger on SoundStage! Access on May 1, 2022

General Information

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

The A12MKII was conditioned for 1 hour at 1/8th full rated power (~8W into 8 ohms) before any measurements were taken. All measurements were taken with both channels driven, using a 120V/20A dedicated circuit, unless otherwise stated.

The A12MKII offers three unbalanced line-level analog inputs (RCA), one unbalanced moving-magnet (MM) phono input (RCA), one S/PDIF coaxial input (RCA), one S/PDIF optical input (TosLink), one USB digital input, a Bluetooth receiver, one pair of line-level pre-outs (RCA), and two stets (A and B) of speaker level outputs. On the front panel is a 1/8” TRS headphone output. For the purposes of these measurements, the following inputs were evaluated: coaxial digital, plus the analog line-level and MM unbalanced inputs.

Most measurements were made with a standard 2Vrms line-level analog input, 5mVrms MM input, and 0dBFS digital input. For the analog inputs, the tone-control bypass function was enabled, except for the chart showing the effects of the tone controls. The signal-to-noise ratio (SNR) measurements were made with the same input signal values but with the volume set to achieve the rated output power of 60W (8 ohms). For comparison, on the line-level input, an SNR measurement was also made with the volume at maximum, where only 0.94Vrms was required to achieve 60W into 8 ohms.

Based on the high accuracy and non-repeatable results at various volume levels of the left/right channel matching (see table below), the A12MKII volume control is likely operating in the analog domain, but is digital controlled. The volume control offers a total range of 0 to 96 on the display, which measured from -53.3dB (position 1) to +27.4dB between the line-level analog input and the speaker outputs, in increments of 6 to 2dB below 6, 1dB from 6 to 80, then 0.5dB steps from 80 to 96. One oddity that was observed was that between volume steps 7 to 40, every second volume increment did not change the output voltage.

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

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Rotel for the A12MKII compared directly against our own. The published specifications are sourced from Rotel’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 extended from DC to 500kHz, assume, unless otherwise stated, 10W into 8 ohms and a measurement input bandwidth of 10Hz to 90kHz, and the worst-case measured result between the left and right channels.

Our primary measurements revealed the following using the line-level analog input and digital coaxial input (unless specified, assume a 1kHz sine wave at 2Vrms or 0dBFS, 10W output, 8-ohm loading, 10Hz to 90kHz bandwidth):

For the continuous dynamic power test, the A12MKII was able to sustain 133W into 4 ohms (~1% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (13.3W) for 5 seconds, for about 4 minutes (out of a 5- minute test) before the protection circuit shut down the unit. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the top of the A12MKII was quite warm to the touch. It should be noted that this test was conducted after a few hours of testing with an average of 10W at the output.

Our primary measurements revealed the following using the phono-level input, MM configuration (unless specified, assume a 1kHz 5mVrms sine wave input, 10W output, 8-ohm loading, 10Hz to 90kHz bandwidth):

Our primary measurements revealed the following using the analog input at the headphone output (unless specified, assume a 1kHz sine wave, 2Vrms output, 300 ohms loading, 10Hz to 90kHz bandwidth):

Frequency response (8-ohm loading, line-level input)

In our measured frequency response chart above, the A12MKII is nearly flat within the audio band (20Hz to 20kHz). At the extremes the A12MKII is about 0.25dB down at 20Hz, and 0dB at 20kHz. The A12MKII appears to be AC coupled (i.e., not flat down to DC), contradicting Rotel’s frequency response claim of 10Hz-100kHz, 0±0.5dB. At the high-frequency extremes, however, Rotel’s claim is verified as we are within 0.5dB of flat, even 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.

Frequency response (8-ohm loading, line-level input, bass and treble controls)

Above are frequency response plots measured at the speaker-level outputs into 8 ohms, with the bass and treble controls set to maximum (blue/red plots) and minimum (purple/green plots). We see that for the bass and treble controls, roughly +/- 12dB and +/- 9dB, respectively, of gain/cut are available at 20Hz and 20kHz.

Phase response (8-ohm loading, line-level input)

Above are the phase response plots for the left and right channels from 20Hz to 20kHz for the line-level input, measured across the speaker outputs at 10W into 8 ohms. The A12MKII does not invert polarity and exhibits, at worst, less than 20 degrees (at 20Hz) of phase shift within the audio band.

Frequency response vs. input type (8-ohm loading, left channel only)

The chart above shows the A12MkII’s frequency response as a function of input type measured across the speaker outputs at 10W into 8 ohms. The green trace is the same analog input data from the previous graph. The blue trace is for a 16bit/44.1kHz dithered digital input signal from 5Hz to 22kHz using the coaxial digital input, and the purple trace is for a 24/96 dithered digital input signal from 5Hz to 48kHz, while the pink trace is for a 24/192 dithered digital input signal from 5Hz to 96kHz. The analog input shows slightly flatter response at low frequencies, with the digital input -2dB at 10Hz and -0.5dB at 20Hz. The 16/44.1 data exhibits brick-wall-type filtering, with a -3dB point at 21.0kHz. The 24/96 and 24/192 kHz data yielded -3dB points at 46.2kHz and 91.9kHz, respectively, reflecting the higher sampling frequencies and, therefore, increased bandwidth.

Frequency response (8-ohm loading, MM phono input)

The chart above shows the frequency response for the MM phono input. What is being displayed is the deviation from the RIAA curve, where the input signal sweep is EQ’d with an inverted RIAA curve supplied by Audio Precision analyzer (i.e., zero deviation would yield a flat line at 0dB). We see a maximum deviation of about +0.5 at 40Hz and 20kHz, from 20Hz to 20kHz. The worst-case channel deviation is between about 10kHz to 20kHz, at about 0.2dB. 

Phase response (MM input)

Above is the phase-response plot from 20Hz to 20kHz for the MM phono input, measured across the speaker outputs at 10W into 8 ohms. The A12MKII does not invert polarity. 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 5kHz, and +20 degrees at 20Hz.

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

The chart above shows the results of a linearity test for the coaxial digital input for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the line-level pre-outs of the A12MKII for a 1Vrms (at 0dBFS) output signal. 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. Both data were essentially perfect as of -100dBFS down to 0dBFS. At -120dBFS, the 16/44.1 data were about +3.5dB above reference, while the 24/96 data were within +1.5/2.5dB ((left/right) of reference. This is an acceptable linearity test result.

Impulse response (24/44.1 data)

The chart 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 line-level pre-outs of the A12MKII. We can see that the A12MKII utilizes a typical sinc function reconstruction filter.

J-Test (coaxial)

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level pre-outs of the A12MKII. J-Test was developed by Julian Dunn in the 1990s. It is a test signal, specifically a -3dBFS undithered 12kHz square wave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz square wave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sine wave (the main peak). In addition, an undithered 250Hz square wave 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 sine-wave jitter by the Audio Precision, to show how well the DAC rejects jitter.

The coaxial input exhibits significant peaks in the audio band, peaking near the 12kHz primary signal, at -95dBrA and below. This is a poor J-Test result, indicating that the A12MKII DAC likely has poor jitter immunity.

J-Test (optical)

The chart above shows the results of the J-Test test for the optical digital input measured at the line- level pre-outs of the A12MKII. The optical input exhibits significant peaks in the audio band, peaking near the 12kHz primary signal, at -85dBrA and below. This, as with the coaxial-input test above, is a poor J-Test result, indicating that the A12MKII DAC likely has poor jitter immunity.

J-Test with 10ns of injected jitter (coaxial)

Both the coaxial and optical inputs were also tested for jitter immunity by injecting artificial sine-wave jitter at 2kHz, which would manifest as sidebands at 10kHz and 14kHz without any jitter rejection. Jitter immunity proved poor, with visible sidebands at only 10ns of jitter level. Clear sidebands can be seen at nearly -70dBrA. The optical input jitter result was very similar to the coaxial input result shown above. This, again, is a poor J-Test result.

Wideband FFT spectrum of white noise and 19.1kHz sine-wave tone (coaxial input)

The plot above shows a fast Fourier transform (FFT) of the A12MKII’s line-level pre-outs with white noise at -4dBFS (blue/red), and a 19.1 kHz sine wave at 0dBFS (1Vrms) fed to the coaxial digital input, sampled at 16/44.1. The sharp roll-off above 20kHz in the white-noise spectrum shows the implementation of a brick-wall-type reconstruction filter. There are no aliased image peaks in the audio band above the -120dBrA noise floor. The main 25kHz alias peak is at -60dBrA. The second, third, and fourth distortion harmonics (38.2, 57.3, 76.4kHz) of the 19.1kHz tone are between -75dBrA and -105dBrA.

RMS level vs. frequency vs. load impedance (1W, left channel only)

The chart above shows RMS level (relative to 0dBrA, which is 1W into 8 ohms, or 2.83Vrms) as a function of frequency, for the analog line-level input swept from 5Hz to 100kHz. The blue plot is into an 8-ohm load, the purple is into a 4-ohm load, the pink plot is an actual speaker (Focal Chora 806, measurements can be found here), and the cyan plot is no load connected. The chart below . . .

. . . is the same, but zoomed in to highlight differences. Here we can see a maximum deviation within the audio band of about 0.08dB from 4 ohms to no load, which is an indication of a relatively high damping factor, or low output impedance. The maximum variation in RMS level when a real speaker was used is even less, deviating by about 0.06dB within the flat portion of the curve (100Hz to 20kHz). Note that the dip in RMS level at lower frequencies is a result of the frequency response of the A12MKII, and not a damping-factor issue, as all four plots show the same dip, at roughly the same rate.

THD ratio (unweighted) vs. frequency vs. output power

The chart above shows THD ratios at the output into 8 ohms as a function of frequency for a sine-wave stimulus at the analog line-level input. The blue and red plots are for the left and right channels at 1W output into 8 ohms, purple/green at 10W, and pink/orange at the rated 60W. The power was varied using the volume control. The A12MKII manages to maintain consistent THD ratios across a wide range of power output levels, and easily makes the Rotel spec of <0.018% THD from 20Hz to about 10kHz, from 1W to 60W. The disparity in the plots is actually a channel disparity, with the right channel outperforming the left channel by as much as almost 10dB, with the right channel dipping as low as 0.003% from 50Hz to 2kHz across all power levels. We wanted to investigate whether the disparity was in the amp or preamp section, so . . . 

. . . we plotted THD ratios with a 1Vrms input (instead of 2Vrms), increasing the volume by 6dB to achieve the same 10W output into 8 ohms. Above is a chart that shows these THD ratios as a function of frequency for a 1Vrms sine-wave stimulus at the analog line-level input. Here we find better tracking between the left and right channels, and interestingly, below 2kHz, the left channel outperforming the right channel by about 5dB—the opposite result compared to a 2Vrms input. We can conclude from this that at least part of the reason for the THD channel disparity with a 2Vrms input is due to distortion in the preamp section.

THD ratio (unweighted) vs. frequency at 10W (MM input)

The chart above shows THD ratios as a function of frequency plots for the MM phono input measured across an 8-ohm load at 10W. The input sweep is EQ’d with an inverted RIAA curve. The THD values for the MM configuration vary from around 0.1% (20/30Hz) down to 0.001% (1kHz) for the left channel, then up to 0.03% at 20kHz. The right channel showed more constant THD ratios of roughly 0.01% from 50Hz to 10kHz.

THD ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms

The chart above shows THD ratios measured at the output of the A12MKII as a function of output power for the analog line-level input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). Both 8- and 4-ohm data sets, for the left channel, track fairly closely, with THD ratios from about 0.01% down to 0.002% at 2-3W, then back up to 0.01% at the “knees”—roughly 70W for the 8-ohm load, and about 120W for the 4-ohm load. The right channel performed worse than the left channel by as much as 10-12dB, with the exception of a crossover point just over 20W into 8 ohms, where the right channel begins to outperform the left channel with a nearly 10dB advantage at the knee. As discussed above, this may be due to distortions in the preamp section, as at the knee, we are approaching 2Vrms at the input. The 1% THD values are reached at about 91W (8 ohms) and 142W (4 ohms).

THD+N ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms

The chart above shows THD+N ratios measured at the output of the A12MKII as a function of output power for the line-level input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). Overall, THD+N values for both loads were similar but slightly lower for the 8-ohm load, ranging from about 0.1%, down to 0.005%. The exception was the right channel into 4 ohms, which performed worse by almost 10dB from 10 to 50W.

THD ratio (unweighted) vs. frequency at 8, 4, and 2 ohms (right channel only)

The chart above shows THD ratios measured at the output of the A12MKII as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage that yields 5W at the output into 8 ohms (and roughly 10W into 4 ohms, and 20W into 2 ohms) for the analog line-level input. The right channel was chosen because with these conditions (2Vrms in, 5W into 8 ohms), the right channel clearly outperformed the left channel. The 8-ohm load is the blue trace, the 4-ohm load the purple trace, and the 2-ohm load the pink trace. We find increasing THD values from 8 to 4 to 2 ohms. Into 8 ohms, THD ratios are as low as 0.003% from 50Hz to 3kHz. The 4-ohm THD ratios are more than 10dB higher through most of the frequency range, while the 2-ohm data is about 6dB higher than the 4-ohm data. Basically, THD increases as impedance descreases.

THD ratio (unweighted) vs. frequency into 8 ohms and real speakers (right channel only)

The chart above shows THD ratios measured at the output of the A12MKII as a function of frequency into an 8-ohm load and two different speakers, for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). At high frequencies (5-20kHz), all three plots show similar THD ratios from 0.005% to about 0.015%. Through the upper bass and midband, however, THD ratios were higher with real speakers, hovering around 0.01%, more than 10dB higher than the 0.003% measured with a dummy load. At very low frequencies, THD ratios were the highest with the two-way speaker, measuring 0.05%, a full 20dB higher than the 0.005% measured in the dummy load.

IMD ratio (CCIF) vs. frequency into 8 ohms and real speakers (right channel only)

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the A12MKII as a function of frequency into an 8-ohm load and two different speakers, for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here the CCIF IMD method was used, where the primary frequency is swept from 20kHz (F1) down to 2.5kHz, and the secondary frequency (F2) is always 1kHz lower than the primary, with a 1:1 ratio. The CCIF IMD analysis results are the sum of the second (F1-F2 or 1kHz) and third modulation products (F1+1kHz, F2-1kHz). The 8-ohm load is the blue trace, the purple plot is the same two-way speaker as above (Focal Chora 806, measurements can be found here), and the pink plot is the same three-way speaker as above (Paradigm Founder Series 100F, measurements can be found here). In the lower frequencies (4kHz and below), all three results are similar, with relatively constant IMD ratios from as low as 0.002% to 0.008% at 2.5kHz. At higher frequencies, IMD ratios were highest with a real speaker, as high as 0.01% at 20kHz for the three-way speaker, which is 5dB higher than with the dummy load, and 15dB higher than with two-way speaker, which yielded, in general, the lowest IMD values.

IMD ratio (SMPTE) vs. frequency into 8 ohms and real speakers (right channel only)

The chart above shows IMD ratios measured at the output of the A12MKII as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here, the SMPTE IMD method was used, where the primary frequency (F1) is swept from 250Hz down to 40Hz, and the secondary frequency (F2) is held at 7kHz with a 4:1 ratio. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 8-ohm load is the blue trace, the purple plot is the same two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is the same three-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three results are nearly identical, with flat IMD results just around 0.02%.

FFT spectrum – 1kHz (line-level input)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the analog line-level input. We find the left channel dominating at the second harmonic (2kHz) at -80dBrA, or 0.01%, compared to the right channel at below -90dBrA, or 0.003%. At the fourth harmonic (4kHz), the right channel dominates at about -95dBrA, or 0.002%, compared to the left channel at -110dBrA, or 0.0003%. At the third (3kHz) and fifth harmonics (5kHz), both channels yielded peaks at roughly -105dBrA, or 0.0006%. On the left side of the main signal peak, we find a small peak at 60Hz due to power-supply noise at about -115dBrA, or 0.0002%, for the left channel, and more dominant higher-order peaks at 120Hz, and especially 240Hz (fourth harmonic) at -105dBrA, or 0.0006%. Power-supply and signal-related harmonic peaks can be seen right out to 100kHz.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the coaxial digital input, sampled at 16/44.1. Here we find the right channel dominating at the second harmonic (2kHz) at -80dBrA, or 0.01%, compared to the left channel at below -100dBrA, or 0.001%. At the third (3kHz) harmonic, both channels yielded peaks at roughly -85dBrA, or 0.006%. The noise-related peaks on the left side of the signal peak are very similar to the line-level FFT above.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the coaxial digital input, sampled at 24/96. The FFT spectrum is nearly identical within the audio band to the 16/44.1 FFT above.

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 sine-wave stimulus at the coaxial digital input, measured at the output across an 8-ohm load. We see the 1kHz primary signal peak, at the correct amplitude, and signal harmonics are non-existent above the noise floor. The fourth (240Hz) and sixth (360Hz) power-supply related harmonics are visible at roughly -105dBrA, or 0.0006%.

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

Shown above is the FFT for a 1kHz -90dBFS dithered 24/96 input sine-wave stimulus at the coaxial digital input, measured at the output across an 8-ohm load. The FFT spectrum is nearly identical within the audio band to the 16/44.1 FFT above.

FFT spectrum – 1kHz (MM phono input)

Shown above is the FFT for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the MM phono input. We see the right channel dominating the even-order harmonics (2, 4, 6kHz), as high as -80dBrA, or 0.01%, at 2kHz. We see the primary (60Hz) power-supply-related peak at just under -60dBrA, or 0.1%, and subsequent power-supply-related peaks (120, 180, 240Hz, etc.) extending beyond the 1kHz signal peak, at -80dBrA (at 180Hz), or 0.01%, and below.

FFT spectrum – 50Hz (line-level input)

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the output across an 8-ohm load at 10W for the analog 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 second harmonic (100Hz) dominates at -85/-95dBrA (left/right), or 0.006/0.002%, but even and odd signal harmonics at lower levels can be seen throughout. We also see the 60Hz power-supply-related peak at -120dBrA, or 0.0001%, with higher-order peaks at 180Hz and 240Hz, between -100 and -110dBrA, or 0.001 and 0.0003%.

FFT spectrum – 50Hz (MM phono input)

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the output across an 8-ohm load at 10W for the MM phono 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) peak is from the 60Hz power-supply fundamental at -60dBrA, or 0.1%. The second-order signal peak at 100Hz is dominated by the right channel at -80dBrA, or 0.01%, while the third-order noise peak (180Hz) is at the same level, but for both channels.

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 sine-wave stimulus tone measured at the output across an 8-ohm load at 10W 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, 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 -90/-95dBrA (left/right), or 0.003/0.002%. The third-order modulation products, at 17kHz and 20kHz, are around -100dBrA, or 0.001%.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sine-wave stimulus tone measured at the output across an 8-ohm load at 10W 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 100/-85dBrA (left/right), or 0.001/0.006%.  The third-order modulation products, at 17kHz and 20kHz, are around -85dBrA, or 0.006%. We also see the main aliased peaks at 25.1kHz and 26.1kHz around -70dBrA, due to the 44.1kHz sample rate (e.g., 44.1kHz-19kHz = 25.1kHz).

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 sine-wave stimulus tone measured at the output across an 8-ohm load at 10W for the digital coaxial input at 24/96. The FFT spectrum is nearly identical within the audio band to the 16/44.1 FFT above.

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 sine-wave stimulus tone measured at the output across an 8-ohm load at 10W for the MM phono input. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at around -100/-80dBRa (left/right), or 0.001/0.01%, while the third-order modulation products, at 17kHz and 20kHz, are around -105dBrA, or 0.0006%.

Square-wave response (10kHz)

Above is the 10kHz square-wave response using the analog line-level input, at roughly 10W into 8 ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the A12MKII’s slew-rate performance. Rather, it should be seen as a qualitative representation of the A12MKII’s extended bandwidth. An ideal square wave can be represented as the sum of a sine wave 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 we can see an exceptionally clean square-wave reproduction, with sharp corners and little-to-no ringing, indicating a high bandwidth.

Damping factor vs. frequency (20Hz to 20kHz)

The final graph above is the damping factor as a function of frequency. Both channels show a relatively constant damping factor from 20Hz to 20kHz, right around 215. This is very close to Rotel’s claim of 220.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Amplifier Measurements

Created: 15 April 2022

Link: reviewed by Jason Thorpe on SoundStage! Hi-Fi on April 15, 2023

General information

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

The Hegel Music Systems H30A was conditioned for 1 hour at 1/8th full rated power (~60W into 8 ohms) before any measurements were taken. All measurements were taken with both channels driven, using a 120V/20A dedicated circuit, unless otherwise stated.

The Hegel H30A has both unbalanced (RCA) and balanced (XLR) inputs, and a pair of speaker level outputs. We found no appreciable differences in term of THD and noise between the RCA and XLR inputs. The H30A can be operated in stereo or mono mode, for which the latter uses a third unbalanced or balanced input. Hegel states that the H30A is designed as a mono power amplifier, but that it can also be used as a stereo amplifier. As such, essentially all measurements have been performed in both stereo (two-channel) and mono (single-channel) modes. Unless otherwise stated, the balanced inputs were used for all measurements.

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Hegel for the H30A compared directly against our own. The published specifications are sourced from Hegel’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 extended to 500kHz, assume, unless otherwise stated, 10W into 8 ohms and a measurement input bandwidth of 10Hz to 90kHz, and the worst-case measured result between the left and right channels.

* Hegel measures damping factor directly at the output stage, whereas we measure at the amp’s output terminals.

Our primary measurements in stereo mode revealed the following using the line-level balanced analog input (unless specified, assume a 1kHz sinewave at 223mVrms, 10W output, 8-ohm loading, 10Hz to 90kHz bandwidth):

For the continuous dynamic power test, the H30A was able to sustain 553W into 4 ohms (~1.5% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (55.3W) for 5 seconds, for 5 continuous minutes without inducing a fault or the initiation of a protective circuit. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the top of the H30A was only warm to the touch, without causing any discomfort.

Our primary measurements in mono mode revealed the following using the line-level balanced analog input (unless specified, assume a 1kHz sinewave at 223mVrms, 10W output, 8-ohm loading, 10Hz to 90kHz bandwidth):

Frequency response (8-ohm loading, stereo mode)

In our frequency-response plots (relative to 1kHz) above, measured across the speaker outputs at 10W into 8 ohms, the H30A is near flat within the audioband (-0.2/0dB, 20Hz/20kHz). At the extremes, the H30A is at -1.5dB at 5Hz and +-1.2dB 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 (8-ohm loading, line-level input, stereo mode)

Above are the phase response plots from 20Hz to 20kHz for the line level input, measured across the speaker outputs at 10W into 8 ohms. The H30A does not invert polarity and exhibits, at worst, about 30 degrees (at 20kHz) of phase shift within the audioband.

RMS level vs. frequency vs. load impedance (1W, left channel only, stereo mode)

The chart above shows RMS level (relative to 0dBrA, which is 1W into 8 ohms or 2.83Vrms) as a function of frequency, for the analog line-level input swept from 5Hz to 100kHz, in stereo mode. The blue plot is into an 8-ohm load, the purple is into a 4-ohm load, the pink plot is an actual speaker (Focal Chora 806, measurements can be found here), and the cyan plot is no load connected. The chart below . . .

. . . is the same but zoomed in to highlight differences. Here we find that maximum deviation between no load and a 4-ohm load is very small, at around 0.02dB. This is an indication of a very high damping factor, or low output impedance. With a real speaker, maximum deviations in RMS level were roughly the same.

RMS level vs. frequency vs. load impedance (1W, left channel only, mono mode)

The chart above shows RMS level (relative to 0dBrA, which is 1W into 8 ohms or 2.83Vrms) as a function of frequency, for the analog line-level input swept from 5Hz to 100kHz, in mono mode. The blue plot is into an 8-ohm load, the purple is into a 4-ohm load, the pink plot is an actual speaker (Focal Chora 806, measurements can be found here), and the cyan plot is no load connected. The chart below . . .

. . . is the same but zoomed in to highlight differences. Here we find that maximum deviation between no load and a 4-ohm load is roughly double in mono mode compared to stereo, at around 0.04dB. This is normal for a bridged amplifier, as the output impedance roughly doubles because each speaker output terminal is wired to an amplifier output (but out of phase). In a conventional amplifier, only the positive speaker output terminal is connected to the amplifier output, while the negative speaker output terminal is connected to ground.  Nonetheless, the output impedance is still very low in mono mode by any conventional standard. With a real speaker, maximum deviations in RMS level were roughly the same, at 0.04dB.

THD ratio (unweighted) vs. frequency vs. output power (stereo mode)

The chart above shows THD ratios at the output into 8 ohms as a function of frequency for a sinewave stimulus at the analog line-level input in stereo mode. The blue and red plots are for left and right channels at 1W output into 8 ohms, purple/green at 10W, and pink/orange at 200W. We find fairly consistent THD ratios at 1W and 10W, from 0.001-0.003% at 20Hz, up to 0.005-0.008% at 20kHz. At 200W, THD ratios were higher, from 0.006% from 20Hz to 2kHz, up to nearly 0.02% at 20kHz.

THD ratio (unweighted) vs. frequency vs. output power (mono mode)

The chart above shows THD ratios at the output into 8 ohms as a function of frequency for a sinewave stimulus at the analog line-level input in mono mode. The blue plot is at 1W output into 8 ohms, purple at 10W, and pink at 600W. THD ratios at 1W were the lowest, from 0.002-0.001% between 20Hz and 5kHz, then up to 0.006% at 20kHz. At 10W, THD values were roughly 5dB higher. At 600W, THD ratios were still commendably low, at 0.003-0.004% from 20Hz to 2kHz, then up to 0.006% near 20kHz.

THD ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms (stereo mode)

The chart above shows THD ratios measured at the output of the H30A as a function of output power for the analog line-level input in stereo mode, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). The 8-ohm data ranged from about 0.003% at 50mW, down to 0.0015% from 0.5 to 50W, then up to the “knee” just shy of 300W. The 4-ohm data yielded THD ratios 5-10dB higher through the flat portion of the curve up to 200W, and hit the “knee” at nearly 500W. The 1% THD marks were hit at 296W (8 ohms) and 525W (4 ohms).

THD+N ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms (stereo mode)

The chart above shows THD+N ratios measured at the output of the H30A as a function of output power for the analog line-level input in stereo mode, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). THD+N values for the 8-ohm data ranged from 0.05% down to 0.002% at 50W. The 4-ohm data yielded THD+N values 3-4dB higher, except at the lowest point, where 0.002% was also reached, but at around 150W.

THD ratio (unweighted) vs. output power at 1kHz into 8 ohms (mono mode)

The chart above shows THD ratios measured at the output of the H30A as a function of output power for the analog line level-input in mono mode into 8 ohms. THD values were at 0.003% at 50mW, down to 0.001% at 1-3W then up to 0.002% up to 500W, then to the “knee” between 800 and 900W. The 1% THD mark was hit at 1060W (8-ohm).

THD+N ratio (unweighted) vs. output power at 1kHz into 8 ohms (mono mode)

The chart above shows THD+N ratios measured at the output of the H30A as a function of output power for the analog line level-input in mono mode into 8 ohms. THD+N values were at 0.05% at 50mW, down to as low as 0.002% between 100 and 500W.

THD ratio (unweighted) vs. frequency at 8, 4, and 2 ohms (left channel only, stereo mode)

The chart above shows THD ratios measured at the output of the HA30 as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage that yielded 100W at the output into 8 ohms (and roughly 200W into 4 ohms, and 400W into 2 ohms) for the analog line-level input in stereo mode. The 8-ohm load is the blue trace, the 4-ohm load the purple trace, and the 2-ohm load the pink trace. We see increasing levels (5dB) of THD from 8 to 4 to 2 ohms at 3kHz and above. Below 1kHz, are three THD data sets are fairly close, with THD ratios ranging from 0.002% to 0.005%.

THD ratio (unweighted) vs. frequency at 8, 4, and 2 ohms (mono mode)

The chart above shows THD ratios measured at the output of the H30A as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage that yielded 200W at the output into 8 ohms (and roughly 400W into 4 ohms, and 800W into 2 ohms) for the analog line-level input in mono mode. The 8-ohm load is the blue trace, the 4-ohm load the purple trace, and the 2-ohm load the pink trace. We see increasing levels (5-10dB) of THD from 8 to 4 to 2 ohms between about 300Hz and 3kHz, with the 8-ohm data as low as 0.001-0.002%. At the frequency extremes, THD ratios were quite similar: 0.002% at 20Hz, and 0.007% near 20kHz.

THD ratio (unweighted) vs. frequency into 8 ohms and real speakers (left channel only, stereo mode)

The chart above shows THD ratios measured at the output of the H30A as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the balanced analog line-level input in stereo mode. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). The 8-ohm plot is fairly flat and between 0.001% and 0.002% from 20Hz to 5kHz, but the two speaker plots vary considerably. The two-way speaker ranges from 0.02% at 20Hz, to as low as 0.0005% at 2kHz, then back up to 0.01% at 20kHz. The three-way speaker THD plot ranges from 0.003% at 100Hz, down to as low as 0.0007% at 4kHz, then up to 0.01% at 20kHz.

THD ratio (unweighted) vs. frequency into 8 ohms and real speakers (mono mode)

The chart above shows THD ratios measured at the output of the H30A as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the balanced analog line-level input in mono mode. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). The 8-ohm plot ranges from 0.002% at 20Hz down to 0.0008-0.0009% between 100Hz and 3kHz, then up to 0.006% at 20kHz. The two speaker plots vary considerably more. The two-way speaker ranges from 0.03% at 20Hz, to as low as 0.0006% at 2-3kHz, then back up to 0.002% at 10kHz. The three-way speaker THD plot ranges from 0.004% at 100Hz, down to as low as 0.0007% at 2kHz, then up to 0.015% at 20kHz.

IMD ratio (CCIF) vs. frequency into 8 ohms and real speakers (left channel only, stereo mode)

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the H30A as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here the CCIF IMD method is used, where the primary frequency is swept from 20kHz (F1) down to 2.5kHz, and the secondary frequency (F2) is always 1kHz lower than the primary, with a 1:1 ratio. The CCIF IMD analysis results are the sum of the second (F1-F2 or 1kHz) and third modulation products (F1+1kHz, F2-1kHz). The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). The 8-ohm IMD data is fairly flat, between 0.002% and 0.003%. The two-way speaker data ranges from 0.001% at 2.5kHz up to 0.003% at 20kHz. The three-way speaker data ranges from 0.001% at 2.5kHz up to 0.007% at 20kHz.

IMD ratio (CCIF) vs. frequency into 8 ohms and real speakers (mono mode)

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the H30A as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input in mono mode. Here the CCIF IMD method is used, where the primary frequency is swept from 20kHz (F1) down to 2.5kHz, and the secondary frequency (F2) is always 1kHz lower than the primary, with a 1:1 ratio. The CCIF IMD analysis results are the sum of the second (F1-F2 or 1kHz) and third modulation products (F1+1kHz, F2-1kHz). The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). The 8-ohm IMD data is fairly flat, between 0.002% and 0.003%. The two-way speaker data ranges from 0.001% at 2.5kHz up to 0.005% at 20kHz. The three-way speaker data ranges from 0.001% at 5.5kHz up to 0.01% at 20kHz.

IMD ratio (SMPTE) vs. frequency into 8 ohms and real speakers (left channel only, stereo mode)

The chart above shows IMD ratios measured at the output of the H30A as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input in stereo mode. Here, the SMPTE IMD method is used, where the primary frequency (F1) is swept from 250Hz down to 40Hz, and the secondary frequency (F2) is held at 7kHz with a 4:1 ratio. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three data sets are close enough to be judged as nearly identical, hovering around the 0.006-0.008% level.

IMD ratio (SMPTE) vs. frequency into 8 ohms and real speakers (mono mode)

The chart above shows IMD ratios measured at the output of the H30A as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input in mono mode. Here, the SMPTE IMD method is used, where the primary frequency (F1) is swept from 250Hz down to 40Hz, and the secondary frequency (F2) is held at 7kHz with a 4:1 ratio. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three data sets are close enough to be judged as nearly identical, hovering around the 0.006-0.008% level.

FFT spectrum – 1kHz (line-level input, stereo mode)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the analog line-level input in stereo mode. We see that the signal’s second (2kHz) and third (3kHz) harmonic, are at roughly -100dBrA, or 0.001%. The subsequent harmonics (4/5/6/7/8kHz) are visible but at lower and descending levels below the -110dBrA, or 0.0003% mark. Power supply related noise peaks at the fundamental (60Hz) frequency are evident at -130/-115dBrA (left/right), or 0.00003/0.0002%, as well as both even and odd harmonics at a low -120dBrA, or 0.001%, and below.

FFT spectrum – 1kHz (line-level input, mono mode)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the analog line-level input in mono mode. We see that the signal’s even harmonics are lower than is seen in stereo mode. For example, the 2kHz peak here is at -115dBrA, or 0.0002%, versus -100dBrA in stereo mode. Power-supply-related noise peaks are roughly the same as is seen in the stereo FFT above.

FFT spectrum – 50Hz (line-level input, stereo mode)

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output across an 8-ohm load at 10W for the analog line-level input in stereo mode. 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 dominant (non-signal) peaks are the second (100Hz) and third (150Hz) signal harmonic at roughly -100dBrA (right), or 0.001%. Power-supply-related harmonics are generally below the -120dBrA, or 0.0001% level.

FFT spectrum – 50Hz (line-level input, mono mode)

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output across an 8-ohm load at 10W for the analog line-level input in mono mode. 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. Here again we see that the signal’s even harmonics are lower than is seen in stereo mode. For example, the 200Hz (fourth harmonic) peak here is nearly at -120dBrA, or 0.0001%, versus -110dBrA (left channel) in stereo mode. Power-supply-related noise peaks are roughly the same as is seen in the stereo FFT above.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the analog line-level input in stereo mode. 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 -100dBrA, or 0.001%, and the third-order modulation products, at 17kHz and 20kHz, are at the same level.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the analog line-level input in mono mode. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -110dBrA, or 0.0003%, which is lower than is seen in stereo mode, while the third-order modulation products, at 17kHz and 20kHz, are at -95dBrA, or 0.002%, which is higher than is seen in stereo mode.

Squarewave response (10kHz, stereo mode)

Above is the 10kHz squarewave response using the balanced analog line-level input, at roughly 10W into 8 ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the H30A’s slew-rate performance. Rather, it should be seen as a qualitative representation of the H30A’s 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 H30A’s squarewave response is superb, showing no visible over/undershoot, or ringing near the sharp corners.

Damping factor vs. frequency (20Hz to 20kHz, stereo mode)

The graph above is the damping factor as a function of frequency in stereo mode. We see both channels tracking closely and a very high damping factor, ranging around the 800 mark between 40Hz and 1kHz, then down to 300 at 20kHz.

Damping factor vs. frequency (20Hz to 20kHz, mono mode)

The final graph above is the damping factor as a function of frequency in mono mode. We see roughly the same plot as above in stereo mode, but at half the values, due to each speaker output terminal being connected to an amplifier output (bridged mode), each with its own output impedance.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Amplifier Measurements

Created: 01 April 2022

Link: reviewed by Dennis Burger on SoundStage! Access on April 1, 2022

General Information

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

The C 399 was conditioned for 1 hour at 1/8th full rated power (~22W into 8 ohms) before any measurements were taken. All measurements were taken with both channels driven, using a 120V/20A dedicated circuit, unless otherwise stated.

The C 399 offers two unbalanced line-level analog inputs (RCA), one unbalanced moving-magnet (MM) phono input (RCA), two coaxial (RCA) and two optical S/PDIF digital inputs, one HDMI digital input, Bluetooth support, two line-level subwoofer outputs (RCA), two line-level pre-outs (RCA), two pairs of speaker level outputs, and, lastly, one ethernet input (RJ45) for streaming in the optional MDC module, which was included in this sample. On the front of the unit is a 1/4″ TRS headphone output. For the purposes of these measurements, the digital coaxial, analog line-level, and MM inputs were evaluated.

Most measurements were made with a 1Vrms line-level analog input, 5mVrms MM input, and 0dBFS digital input. For the analog inputs, the Analog Bypass function was enabled, so the signals would not be digitzied. For comparison, however, THD+N at 1kHz was measured with the Analog Bypass enabled (0.001%) and disabled (0.0023%). In addition, FFT and frequency response comparisons were made (see graphs below) between Analog Bypass settings.

The following volume settings yielded approximately 10W into 8 ohms: -14.5dB for analog line-level, -4.5dB for MM input, and -20.5dB for digital. The signal-to-noise ratio (SNR) measurements were made with the same input signal values, but with the volume set to achieve the rated output power of 180W (8 ohms). For comparison, on the line-level input, a SNR measurement was also made with the volume at maximum, where only 0.196Vrms was required to achieve 180W into 8 ohms.

Based on the accuracy and non-repeatable results (i.e., they varied slighty over successive measurements) at various volume levels of the left/right channel matching (see table below), the C 399 volume control is likely digitally controlled in the analog domain. The volume control offers a total range from -80dB to +12dB on the C 399 display, which measured from -46dB to +46dB between the line-level analog input and the speaker outputs, in 0.5dB increments.

Because the C 399 is a digital amplifier technology that exhibits considerable noise above 20kHz (see FFTs below), our typical input bandwidth filter setting of 10Hz-90kHz was necessarily changed to 10Hz-22.4kHz for all speaker output measurements, except for frequency response and for FFTs. In addition, THD versus frequency sweeps were limited to 6kHz to adequately capture the second and third signal harmonics with the restricted bandwidth setting.

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

Published specifications vs. our primary measurements

The table below summarizes the measurements published by NAD for the C 399 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. With the exception of frequency response, where the Audio Precision bandwidth was extended from DC to 250kHz, assume, unless otherwise stated, 10W into 8 ohms and a measurement input bandwidth of 10Hz to 22.4kHz for the speaker outputs, and 10Hz to 90kHz for the line-level and headphone outputs, and the worst-case measured result between the left and right channels. All analog input measurements were taken with Analog Bypass engaged, as is specified by NAD.

Our primary measurements revealed the following using the line-level analog input and digital coaxial input (unless specified, assume a 1kHz sine wave at 1Vrms or 0dBFS, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

For the continuous dynamic power test, the C 399 was able to sustain 230W into 4 ohms (~1% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (19.5W) for 5 seconds, for 5 continuous minutes without inducing a fault or the initiation of a protective circuit. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the top of the C 399 was only slightly warm to the touch.

Our primary measurements revealed the following using the phono-level input, MM configuration (unless specified, assume a 1kHz 5mVrms sine-wave input, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

Our primary measurements revealed the following using the analog input at the headphone output (unless specified, assume a 1kHz sine wave, 2Vrms output, 300 ohms loading, 10Hz to 90kHz bandwidth):

Frequency response (8-ohm loading, line-level input)

In our measured frequency response chart above, the C 399 is essentially perfectly flat within the audioband (20Hz to 20kHz), with both the Analog Bypass enabled (blue and red traces) and with Analog Bypass disabled (purple and green traces). At the extremes the C 399 is about 0.02dB down at 20Hz, and 0.02dB up at 20kHz. With Analog Bypass disabled, the incoming signal is digitized and sampled at 48kHz, which results in brick-wall-type filtering right around 24kHz. With Analog Bypass enabled, the incoming analog signal is not digitized, and we see a smooth high frequency rolloff, with a -3dB point around 90kHz. At low frequencies, there is also a small difference in rolloff between Analog Bypass enabled (-0.25dB at 5Hz) and Analog Bypass disabled (-0.5dB at 5Hz).  In the chart above and most of the charts 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 (8-ohm loading, line-level input, bass and treble controls)

Above are frequency response plots measured at the speaker-level outputs into 8 ohms, with the bass and treble controls set to maximum (blue/red plots) and minimum (purple/green plots). We see that for the bass and treble controls, roughly +/- 7dB and +/- 6dB respectively of gain/cut are available at 20Hz and 20kHz. Note: the tone controls are available with Analog Bypass enabled, meaning they are operating in the analog domain.

Phase response (8-ohm loading, line-level input)

Above are the phase response plots from 20Hz to 20kHz for the line-level input, measured across the speaker outputs at 10W into 8 ohms, with Analog Bypass enabled. The C 399 does not invert polarity and exhibits, at worst, less than 20 degrees (at 20kHz) of phase shift within the audioband.

Frequency response vs. input type (8-ohm loading, left channel only)

The chart above shows the C 399’s frequency response as a function of input type measured across the speaker outputs at 10W into 8 ohms. The green trace is the same 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 digital input, and the purple trace is for a 24/96 dithered digital input signal from 5Hz to 48kHz, while the pink trace (perfectly tracking the green trace) is for a 24/192 dithered digital input signal from 5Hz to 96kHz. The 16/44.1 data exhibits brick-wall-type filtering, with a -3dB at 21.0kHz. The 24/96 and 24/192 kHz data yielded -3dB points at 45.9kHz and 88.3kHz respectively. The analog data, with Analog Bypass enabled, looks nearly identical to the 24/192 digital data.

Frequency response with subwoofer-crossover engaged (120Hz, 8-ohm loading)

Above are two frequency response plots for the analog input, measured at 10W (8 ohms) at the speaker outputs, and at the line-level subwoofer output, with the crossover set at 120Hz. The C 399 DSP crossover uses 18dB/octave (third-order) slopes. Note: with Analog Bypass enabled, bass management is not operational, so the above plots were measured with Analog Bypass disabled, meaning that it is operating in the digital domain.

Frequency response (8-ohm loading, MM phono input)

The chart above shows the frequency response for the MM phono input. We see a maximum deviation of about -0.1/+0.15dB (20Hz/10kHz) from 20Hz to 20kHz. The worst-case channel deviation is between about 5kHz to 20kHz, at about 0.1dB. It’s important to know that 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 a very good phono frequency- response test result, since there’s close adherence to the RIAA curve.

Phase response (MM input)

Above is the phase response plot from 20Hz to 20kHz for the MM phono input, measured across the speaker outputs at 10W into 8 ohms. The C 399 does not invert polarity. 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 5kHz, and +20 degrees at 20Hz.

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

The chart above shows the results of a linearity test for the coaxial digital input for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the line-level pre-outs of the C 399 for a 1Vrms (at 0dBFS) output signal. 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. Both data were essentially perfect as of -110dBFS down to 0dBFS. At -120dBFS, the 16/44.1 data were about +2/4dB (left/right) above reference, while the 24/96 data were within +/-0.5dB of reference. This is a good linearity test result.  

Impulse response (24/44.1 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 line-level pre-outs of the C 399. We can see that the C 399 utilizes a typical sinc function reconstruction filter.

J-Test (coaxial)

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level pre-outs of the C 399. The J-Test test was developed by Julian Dunn in the 1990s. It is a test signal, specifically a -3dBFS undithered 12kHz square wave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz square wave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sine wave (the main peak). In addition, an undithered 250Hz square wave 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 sine-wave jitter by the Audio Precision, to show how well the DAC rejects jitter.

The coaxial input exhibits low-level peaks in the audioband, near the 12kHz primary signal peak, at -130dBrA and below. This is a reasonably good J-Test result, indicating that the C 399 DAC should yield good jitter immunity.

J-Test (optical)

The chart above shows the results of the J-Test test for the optical digital input measured at the line-level pre-outs of the C 399. The optical input exhibits low-level peaks in the audioband, near the 12kHz primary signal peak, at -130dBrA and below. This result is very similar to the coaxial input result above.

J-Test with 100ns of injected jitter (coaxial)

Both the coaxial and optical inputs were also tested for jitter immunity by injecting artificial sine-wave jitter at 2kHz, which would manifest as sidebands at 10kHz and 14kHz without any jitter rejection. Jitter immunity proved exceptional, with no visible sidebands at the 100ns jitter level. The C 399 DAC did lose sync with the signal when jitter was increased beyond 500ns or so. The optical input jitter result was very similar to the coaxial input result.

Wideband FFT spectrum of white noise and 19.1kHz sine-wave tone (coaxial input)

The plot above shows a fast Fourier transform (FFT) of the C 399’s line-level pre-outs with white noise at -4dBFS (blue/red), and a 19.1kHz sine wave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1. The sharp rolloff above 20kHz in the white-noise spectrum shows the implementation of a brick-wall-type reconstruction filter. There are no aliased image peaks in the audioband above the -135dBrA noise floor. The main 25kHz alias peak is near -75dBrA. The second, third, and fourth distortion harmonics (38.2, 57.3, 76.4kHz) of the 19.1kHz tone are between -75dBrA and -80dBrA.

RMS level vs. frequency vs. load impedance (1W, left channel only)

The chart above shows RMS level (relative to 0dBrA, which is 1W into 8 ohms, or 2.83Vrms) as a function of frequency, for the analog line-level input swept from 5Hz to 100kHz. The blue plot is into an 8-ohm load, the purple is into a 4-ohm load, the pink plot is an actual speaker (Focal Chora 806, measurements can be found here), and the cyan plot is no load connected. The chart below . . .

. . . is the same but zoomed in to highlight differences. Here we can see a maximum deviation within the audioband of a little more than 0.03dB from 4 ohms to no load, which is an indication of a very high damping factor, or very low output impedance. The maximum variation in RMS level when a real speaker was used is even less, deviating by just under 0.01dB within the flat portion of the curve (100Hz to 2kHz). Note that the rise in RMS level at higher frequencies is a result of the frequency response of the C 399, not a damping-factor issue, as all four plots show the same rise, at roughly the same rate.

THD ratio (unweighted) vs. frequency vs. output power

The chart above shows THD ratios at the output into 8 ohms as a function of frequency for a sine-wave stimulus at the analog line-level input. The blue and red plots are for left and right channels at 1W output into 8 ohms, purple/green at 10W, and pink/orange at the rated 180W. The power was varied using the volume control. At 1W and 10W, THD ratios were relatively flat and very low at around 0.0005%, up to 1kHz. Above 1kHz, there is a rise in the 10W data, up to 0.002% at 6kHz. The 180W THD values are higher but still quite low, ranging from just over and under 0.002% at 20-3kHz, then up to 0.005% at 6kHz. The C 399 manages to maintain low THD across a wide range of power output levels, and easily makes the NAD spec of <0.02%THD at 180W, from 20Hz to 6kHz.

THD ratio (unweighted) vs. frequency at 10W (MM input)

The chart above shows THD ratio as a function of frequency plots for the MM phono input measured across an 8-ohm load at 10W. 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 0.0006% (1.5-2kHz), then up to 0.0025% at 6kHz.

THD ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms

The chart above shows THD ratios measured at the output of the C 399 as a function of output power for the analog line-level input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). Both data sets track fairly closely, even the “knees” are close together. With an 8-ohm load, the “knee” occurs at about 180W, while the 4-ohm “knee” occurs at around 200W. From low to high power levels, THD ratios are very low and in the 0.005 to 0.0003% range. The 1% THD values are reached at about 209W (8 ohms) and 239W (4 ohms).

THD+N ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms

The chart above shows THD+N ratios measured at the output of the C 399 as a function of output power for the line-level input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). Overall, THD+N values for both loads were similar, ranging from 0.01%, down to 0.0005%, with the 8-ohm data outperforming the 4-ohm data by about 3-5dB.

THD ratio (unweighted) vs. frequency at 8, 4, and 2 ohms (left channel only)

The chart above shows THD ratios measured at the output of the C 399 as a function of frequency into three different loads (8/4/2 ohms), for a constant input voltage that yields 10W at the output into 8 ohms (and roughly 20W into 4 ohms, and 40W into 2 ohms), for the analog line-level input. The 8-ohm load is the blue trace, the 4-ohm load the purple trace, and the 2-ohm load the pink trace. We find roughly the same THD values of 0.0005% to 0.002% from 20Hz to 6Hz for the 8-ohm and 4-ohm data. For the 2-ohm data, THD ratios are also fairly constant from 20Hz to 6kHz, but are higher at around 0.001-0.003%. This is a good result, and shows the C 399 is stable, and yields low distortion, even into a 2-ohm load.

THD ratio (unweighted) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows THD ratios measured at the output of the C 399 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). The dummy load and two-way Paradigm speaker yielded very similar and constant 0.0005 to 0.001% THD ratios from 20Hz to 6kHz. There were greater deviations with the two-way Focal at low frequencies, reaching 0.005% at 20Hz.

IMD ratio (CCIF) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the C 399 as a function of frequency into an 8-ohm load and two different speakers, for a constant output voltage of 2.83Vrms (1W into 8 ohms), for the analog line-level input. Here the CCIF IMD method is used, where the primary frequency is swept from 20kHz (F1) down to 2.5kHz, and the secondary frequency (F2) is always 1kHz lower than the primary, with a 1:1 ratio. The CCIF IMD analysis results are the sum of the second (F1-F2 or 1kHz) and third modulation products (F1+1kHz, F2-1kHz). The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three results are similar, with relatively constant IMD ratios from as low as 0.0005% to 0.001% at 20kHz.

IMD ratio (SMPTE) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows IMD ratios measured at the output of the C 399 as a function of frequency into an 8-ohm load and two different speakers, for a constant output voltage of 2.83Vrms (1W into 8 ohms), for the analog line-level input. Here, the SMPTE IMD method is used, where the primary frequency (F1) is swept from 250Hz down to 40Hz, and the secondary frequency (F2) is held at 7kHz with a 4:1 ratio. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a two-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three results are nearly identical, with flat IMD results just under 0.003%.

FFT spectrum – 1kHz (line-level input, Analog Bypass enabled)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the analog line-level input. We see that the signal’s second and third harmonics, at 2 and 3kHz, are around -110dBrA, or 0.0003%. The subsequent signal harmonics are below -120dBrA, or 0.0001%. There are absolutely no noise related peaks on the left side of the main 1kHz peak, above the very low -130 to -140dBrA noise floor. There is also a rise in the noise above 20kHz, characteristic of digital amplifiers.

FFT spectrum – 1kHz (line-level input, Analog Bypass disabled)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the analog line-level input, with the Analog Bypass function disabled (i.e., the analog signal is being digitized after input). The main differences between this FFT and the one above with analog bypass enabled are a higher second harmonic (2kHz) level at just over -100dBrA, or 0.001%; a higher noise floor, at -120 to -130dBrA; and the evidence of 48kHz sampling, with IMD peaks at 47 and 49kHz (i.e., 48kHz +/- the 1kHz signal).

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the coaxial digital input, sampled at 16/44.1. The second (2kHz) and third (3kHz) signal harmonics are between -100 and 110dBrA, or 0.001 and 0.0003%. There are no noise-related peaks above the -130dBrA noise floor.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the coaxial digital input, sampled at 24/96. The second (2kHz) and third (3kHz) signal harmonics are between -100 and 110dBrA, or 0.001 and 0.0003%. There are no noise-related peaks above the characteristically lower (due to 24-bit depth) noise floor at -130 to -140dBrA.

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 sine-wave stimulus at the coaxial digital input, measured at the output across an 8-ohm load. We see the 1kHz primary signal peak, at the correct amplitude, and signal harmonics at 2/3/4kHz for the right channel only, at -105 to -120dBrA, or 0.0006 to 0.0001%. The noise floor on the right channel is higher, at just above -140dBrA, than the left channel, at just above -150dBrA. But even the right channel’s noise-floor level is still very low.

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

Shown above is the FFT for a 1kHz -90dBFS dithered 24/96 input sine-wave stimulus at the coaxial digital input, measured at the output across an 8-ohm load. We see the 1kHz primary signal peak, at the correct amplitude, and the second (2kHz) signal harmonic at -125dBrA, or 0.00006%. The noise floor on the right channel is higher, at -140dBrA, than the left, at -150dBrA.

FFT spectrum – 1kHz (MM phono input)

Shown above is the FFT for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the MM phono input. We see the second (2kHz), third (3kHz), fourth (4kHz), and fifth (5kHz) signal harmonics dominating ranging from -100 down to -120dBrA respectively, or 0.001% down to 0.0001%. We see the primary (60Hz) power-supply-related peak at just over -80dBrA, or 0.01%, and subsequent odd-order power-supply related peaks (180, 300, 420Hz, etc.) extending beyond the 1kHz signal peak, at -80 to -120dBrA, or 0.01 to 0.0001%.

FFT spectrum – 50Hz (line-level input)

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the output across an 8-ohm load at 10W for the analog 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 second (100Hz) and third (150Hz) signal harmonic are at -110dBrA, or 0.0003%. We also see the 60Hz power-supply-related peak, and subsequent harmonics, just above the noise floor at -135 to -140dBrA, or 0.00002 to 0.00001%.

FFT spectrum – 50Hz (MM phono input)

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the output across an 8-ohm load at 10W for the MM phono 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 from the 60Hz power- supply fundamental and its third harmonic at -80dBrA, or 0.01%. Further odd-order power-supply-related peaks can be seen at lower amplitudes, while no signal harmonics are visible above the noise-floor.

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 sine-wave stimulus tone measured at the output across an 8-ohm load at 10W 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, 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 -110/-115dBrA (left/right channels), or 0.0003/0.0002%. The third-order modulation products, at 17kHz and 20kHz, are around -105dBrA, or 0.0006%.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sine-wave stimulus tone measured at the output across an 8-ohm load at 10W 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 -110/-115dBrA (left/right), or 0.0003/0.0002%.  The third-order modulation products, at 17kHz and 20kHz, are around -105dBrA, or 0.0006%. We also see the main aliased peaks at 25.1kHz and 26.1kHz around -80dBrA due to the 44.1kHz sample rate (e.g., 44.1kHz-19kHz = 25.1kHz).

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 sine-wave stimulus tone measured at the output across an 8-ohm load at 10W for the digital coaxial input at 24/96. We find the same second- and third-order modulation products as seen in the 16/44.1 FFT above.

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 sine-wave stimulus tone measured at the output across an 8-ohm load at 10W for the MM phono input. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at around -105dBRa, or 0.0006%, while the third-order modulation products, at 17kHz and 20kHz, are around -100dBrA, or 0.001%.

Square-wave response (10kHz)

Above is the 10kHz square-wave response using the analog line-level input, at roughly 10W into 8 ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the C 399’s slew-rate performance. Rather, it should be seen as a qualitative representation of the C 399’s restricted bandwidth. An ideal square wave can be represented as the sum of a sine wave 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 we can see the 400kHz switching oscillator frequency used in the digital amplifier section visibly modulating the waveform.

Square-wave response (10kHz, restricted 250kHz bandwidth)

Above is the 10kHz square-wave response using the analog line-level input, at roughly 10W into 8 ohms, this time with a 250kHz input bandwidth on the analyzer to filter out the 400kHz switching frequency. We see more evidence here, in the overshoot/undershoot and soft corners of the square wave, of the C 399’s mid-level bandwidth with an analog input.

FFT spectrum (1MHz bandwidth)

The C 399’s class-D amplifier relies on a switching oscillator to convert the input signal to a pulse-width modulated (PWM) square wave (on/off) signal before sending the signal through a low-pass filter to generate an output signal. The C 399 oscillator switches at a rate of about 400kHz, and this graph plots a wide-bandwidth FFT spectrum of the amplifier’s output at 10W into 8 ohms as it’s fed a 1kHz sine wave. We can see that the 400kHz peak is quite evident, and at -35dBrA. There are also two peaks at 800kHz and 1.2MHz (the second and third harmonic of the 400kHz peak), at -60 and -110dBrA. Those peaks—the fundamental and its harmonics—are direct results of the switching oscillators in the C 399 amp modules. The noise around those very-high-frequency signals are in the signal, but all that noise is far above the audioband—and therefore inaudible—and so high in frequency that any loudspeaker the amplifier is driving should filter it all out anyway.

Damping factor vs. frequency (20Hz to 20kHz)

The final graph above is the damping factor as a function of frequency. Both channels show a relatively constant damping factor from 30Hz to 10kHz, ranging from 1731/1911 (left/right), down to 1361/1464 (left/right). The damping factor for the left and right channels is higher at the frequency extremes, reaching 2195 and 2351 for the right channel at 20Hz and 20kHz. The C 399 possesses an exceptionally high damping factor, meaning a very low output impedance.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Amplifier Measurements

Created: 01 March 2022

Link: reviewed by James on SoundStage! Xperience on March 1, 2022

General Information

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

The NAD Masters M10 V2 was conditioned for 1 hour at 1/8th full rated power (~12W into 8 ohms) before any measurements were taken. All measurements were taken with both channels driven, using a 120V/20A dedicated circuit, unless otherwise stated.

The M10 V2 offers two unbalanced analog inputs (RCA), one S/PDIDF coaxial (RCA) input, one optical S/PDIF digital input, one Ethernet (RJ45) digital input, Bluetooth support, two line-level subwoofer outputs (RCA), two line-level pre-outs (RCA), and a pair of speaker-level outputs. For the purposes of these measurements, the following inputs were evaluated unless indicated otherwise: digital coaxial and the analog line-level unbalanced input.

Based on the accuracy and repeatability at various volume levels of the left/right channel matching (see table below), the M10 V2 volume control is likely operating in the digital domain. Consequently, all analog signals are digitized at the M10’s inputs so the unit may apply volume adjustment, bass management, and tone controls. The volume control offers a total range from 0% (-64dB) to 100% (+36dB) in 1dB increments.

Most measurements were made with 1Vrms line-level analog or 0dBFS digital input signals. The following volume settings yielded approximately 10W into 8 ohms: 76% for analog line-level and 67% for digital. The signal-to-noise ratio (SNR) measurements were made with the same input signal values but with the volume set to achieve the rated output power of 100Wpc (8 ohms). For comparison, on the line-level input, a SNR measurement was also made with the volume at maximum, where only 0.450Vrms was required to achieve 100W into 8 ohms.

Because the M10 V2 uses a switching-amplifier technology that exhibits considerable noise above 20kHz (see FFTs below), our typical input bandwidth filter setting of 10Hz-90kHz was necessarily changed to 10Hz-22.4kHz for all measurements, except for frequency response and for FFTs. In addition, THD versus frequency sweeps were limited to 6kHz to adequately capture the second and third signal harmonics with the restricted bandwidth setting.

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

Published specifications vs. our primary measurements

The table below summarizes the measurements published by NAD for the M10 V2 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. With the exception of frequency response, where the Audio Precision bandwidth was extended from DC to 250kHz, assume, unless otherwise stated, 10W into 8 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 line-level analog input and digital coaxial input (unless specified, assume a 1kHz sinewave at 1Vrms or 0dBFS, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

For the continuous dynamic power test, the M10 V2 was able to sustain 195W into 4 ohms (~1% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (19.5W) for 5 seconds, for 5 continuous minutes without inducing a fault or the initiation of a protective circuit. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the top of the M10 V2 was warm to the touch, but did not cause discomfort to the touch.

Frequency response (8-ohm loading, line-level input)

In our measured frequency-response chart above, the M10 V2 is nearly flat within the audioband (20Hz to 20kHz). At the frequency extremes, the M10 V2 is down 0.16dB at 20Hz and down 0.54dB at 20kHz. The M10 V2 cannot be considered a high-bandwidth audio device as the -3dB point is just past 20kHz. In fact, the M10 V2 exhibits brick-wall-type filtering just past 20kHz for the analog input, because it is digitized using a 44.1kHz 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 vs. input type (8-ohm loading, left channel only)

The chart above shows the M10 V2’s frequency response as a function of input type measured across the speaker outputs at 10W into 8 ohms. The green trace is the same analog input data from the previous graph. The blue trace (perfectly tracking the green 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 the pink trace is for a 24/192 dithered digital input signal from 5Hz to 96kHz. The 16/44.1 data exhibits brick-wall-type filtering, with a -3dB at 20.9kHz. The 24/96 and 24/192 kHz data yielded -3dB points at 45.4kHz and 67.5kHz respectively. The analog data looks nearly identical to the 16/44.1 digital data, which is evidence for the M10 V2 sampling incoming analog signals at 44.1kHz.

Frequency response (8-ohm loading, line-level input, bass and treble controls)

Above are frequency-response plots measured at the speaker-level outputs into 8 ohms, with the bass and treble controls set to maximum (blue/red plots) and minimum (purple/green plots). We see that for the bass and treble controls, roughly +/-6dB of gain/cut is available.

Frequency response (subwoofer output engaged, 120Hz crossover)

Above are two frequency-response plots for the analog input, measured at 10W (8-ohm) at the speaker outputs, and at the line-level subwoofer output, with the crossover set to 120Hz. From the rolloff characteristics, we can see that the M10 V2 DSP crossover uses 18dB/octave slopes.

Phase response (8-ohm loading, line-level input)

Above are the left- and right-channel phase response plots from 20Hz to 20kHz for the line-level input, measured across the speaker outputs at 10W into 8 ohms. The M10 V2 does not invert polarity and exhibits, at worst, less than 20 degrees (at 20kHz) of phase shift within the audioband.

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

The chart above shows the results of a linearity test for the coaxial digital input for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the line-level output of the M10 V2. To produce this chart, 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. Both data were essentially perfect as of -100dBFS down to 0dBFS. At -120dBFS, the 16/44.1 data were only about +2dB above reference, while the 24/96 data were within +/-1dB of reference. This is a good linearity test result.  

Impulse response (16/44.1 and 24/96 data)

The chart 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 line-level pre-outs of the M10 V2. We can see that the M10 V2 utilizes a typical sinc function reconstruction filter.

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 M10 V2. The J-Test was developed by Julian Dunn in the 1990s. It is a test signal—specifically a -3dBFS undithered 12kHz square wave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz square wave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sine wave (the main peak). In addition, an undithered 250Hz square wave 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 sine-wave jitter by the Audio Precision, to show how well the DAC rejects jitter.

The coaxial input exhibits low-level peaks in the audioband, at -125dBrA and below. This is a good J-Test result, indicating that the M10 V2 DAC should yield good jitter immunity.

J-Test (optical input)

The chart above shows the results of the J-Test test for the optical digital input measured at the line- level output of the M10 V2. The optical input exhibits low-level peaks in the audioband, at -125dBrA and below. This result is very similar compared to the coaxial input, although with some visible higher amplitude peaks around the 12kHz fundamental.

J-Test (coaxial input) 100ns of injected jitter

Both the coaxial and optical inputs were also tested for jitter immunity by injecting artificial sine-wave jitter at 2kHz, which would manifest as sidebands at 10kHz and 14kHz without any jitter rejection. Jitter immunity proved exceptional through both inputs, with no visible sidebands at the 100ns jitter level (only the coaxial is shown above, since the optical performed similarly). The M10 V2 DAC did lose sync with the signal when jitter was increased beyond 200ns or so.

Wideband FFT spectrum of white noise and 19.1kHz sine-wave tone (coaxial input)

The plot above shows a fast Fourier transform (FFT) of the M10 V2’s line-level pre-outs with white noise at -4dBFS (blue/red), as well as a 19.1kHz sine wave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1. The sharp roll-off above 20kHz in the white-noise spectrum shows the implementation of a brick-wall-type reconstruction filter. There are effectively no aliased image peaks in the audioband above the -130dBrA noise floor. The main 25kHz alias peak is near -75dBrA. The second, third, and fourth distortion harmonics (38.2, 57.3, 76.4kHz) of the 19.1kHz tone are between -75dBrA and -105dBrA.

RMS level vs. frequency vs. load impedance (1W, left channel only)

The chart above shows RMS level (relative to 0dBrA, which is 1W into 8ohms or 2.83Vrms) as a function of frequency, for the analog line-level input swept from 10Hz to 100kHz. The blue plot is into an 8-ohm load, the purple is into a 4-ohm load, the pink plot is an actual speaker (Focal Chora 806, measurements can be found here), and the cyan plot is no load connected. The chart below . . .

. . . is the same but zoomed in to highlight differences. Here we can see a maximum deviation within the audioband of a little more than 0.1dB from 4 ohms to no load, which is an indication of a relatively high damping factor, or low output impedance. The maximum variation in RMS level when a real speaker was used is even less, deviating by just under 0.1dB within the flat portion of the curve (100Hz to 5kHz), with the lowest RMS level, which would correspond à to the lowest impedance point for the load, exhibited around 200Hz, and the highest RMS level, which would correspond to the highest impedance point for the load, at around 2kHz.

THD ratio (unweighted) vs. frequency vs. output power

The chart above shows THD ratios at the output into 8 ohms as a function of frequency for a sine-wave stimulus at the analog line-level input. The blue and red plots are for left and right channels at 1W output into 8 ohms, purple/green at 10W, and pink/orange at 100W. The power was varied using the volume control. At 1W, THD ratios were relatively flat at around 0.003%. At 10W, THD values were slightly lower, hovering above and below 0.002%; however, the right channel outperformed the left by about 2-3dB. The 100W THD values are still quite low, ranging from just over 0.005% at 20-50Hz, down to 0.003% at 1 to 4kHz, then up to 0.007% at 6kHz. The M10 V2 manages to maintain low THD across a wide range of power-output levels.

THD ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms

The chart above shows THD ratios measured at the output of the M10 V2 as a function of output power for the analog line-level input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). Both data sets track fairly closely until the “knees,” which are at about 100W (8-ohm) and 200W (4-ohm). From low to high power levels, THD ratios are in the 0.005 to 0.001% range. The 1% THD values are reached at about 150W (8 ohms) and 285W (4 ohms).

THD+N ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms

The chart above shows THD+N ratios measured at the output of the M10 V2 as a function of output power for the line-level input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). Overall, THD+N values for both loads were similar up to 100W, ranging from 0.05%, down to 0.005%, with the 8-ohm data outperforming the 4-ohm data by about 3-5dB.

THD ratio (unweighted) vs. frequency at 8, 4, and 2 ohms (left channel only)

The chart above shows THD ratios measured at the output of the M10 V2 as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage that yields 10W at the output into 8 ohms (and roughly 20W into 4 ohms, and 40W into 2 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the 4-ohm load the purple trace, and the 2-ohm load the pink trace. We find roughly the same THD values of 0.002% to 0.003% from 20Hz to 6Hz for the 8- and 4-ohm data. For the 2-ohm data, THD ratios are also fairly constant from 20Hz to 6kHz, but are higher at around 0.003-0.005%.

THD ratio (unweighted) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows THD ratios measured at the output of the M10 V2 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), while the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). The dummy load and three-way Paradigm speaker yielded very similar and constant 0.003 to 0.005% THD ratios from 20Hz to 6kHz. There were greater deviations with the two-way Focal, ranging from as low as 0.0015% at 200Hz, to as high as 0.06% at 20Hz. From 500Hz to 6kHz, all three plots are virtually identical and flat, at 0.003% THD.

IMD ratio (CCIF) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the M10 V2 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here the CCIF IMD method is used, where the primary frequency is swept from 20kHz (F1) down to 2.5kHz, and the secondary frequency (F2) is always 1kHz lower than the primary, with a 1:1 ratio. The CCIF IMD analysis results are the sum of the second (F1-F2, or 1kHz) and third modulation products (F1+1kHz, F2-1kHz). The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three results are similar, with relatively constant IMD ratios at around 0.003%. There is a peak in the IMD results around 13-14kHz, where both the dummy load and two-way Focal yielded a 0.005% IMD, while the two-way Paradigm yielded just over 0.01%.

IMD ratio (SMPTE) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows IMD ratios measured at the output of the M10 V2 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here, the SMPTE IMD method was used, where the primary frequency (F1) is swept from 250Hz down to 40Hz, and the secondary frequency (F2) is held at 7kHz with a 4:1 ratio. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three results are nearly identical, with flat IMD results just over 0.01%.

FFT spectrum – 1kHz (line-level input)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the analog line-level input. We see that the signal’s second harmonic, at 2kHz, is at around -95dBrA (left channel slightly higher than right), or 0.002%, while the odd harmonic at 3kHz is much lower at -110dBrA, or 0.0003%. There are various noise related peaks on the left side of the main 1kHz peak, including a 20Hz peak at -100dBrA, or 0.001%, and the 60Hz power-supply related peak at -110dBrA, or 0.0003%. There is also a rise in the noise above 20kHz, characteristic of digital amplifiers.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the coaxial digital input, sampled at 16/44.1. Signal harmonics are very similar to the analog input FFT above, although the second harmonic (2kHz) here is slightly lower, and the third harmonic (3kHz) is slightly higher. The noise-related peaks are fewer here, with the dominant peak at 60Hz yielding a level of -110dBrA, or 0.0003%.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the coaxial digital input, sampled at 24/96. Signal harmonics are very similar to the 16/44.1 FFT above. Noise related peaks are at or below -110dBrA, or 0.0003%.

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 sine-wave stimulus at the coaxial digital input, measured at the output across an 8-ohm load. We see the 1kHz primary signal peak, at the correct amplitude, and other peaks, unrelated to signal harmonics, as high as -100dBrA (left channel), or 0.001%, at just shy of 4kHz.

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

Shown above is the FFT for a 1kHz -90dBFS dithered 24/96 input sine-wave stimulus at the coaxial digital input, measured at the output across an 8-ohm load. We see the 1kHz primary signal peak, at the correct amplitude, and other peaks, unrelated to signal harmonics, as high as -100dBrA (left channel), or 0.001%, at just shy of 4kHz. The noise floor on the left channel is slightly higher than the righ channel, by about 5dB.

FFT spectrum – 50Hz (line-level input)

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the output across an 8-ohm load at 10W for the analog 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 second signal harmonic (100Hz) is at -90dBrA, or 0.003%, while all other peaks are at or below -110dBrA, or 0.0003%, including the 60Hz power-supply-related peak.

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 sine-wave stimulus tone measured at the output across an 8-ohm load at 10W 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, 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%, for the left channel, and -100dBRa, or 0.001%, for the right channel. The third-order modulation products, at 17kHz and 20kHz, are around -105dBrA, or 0.0006%. We also see the main aliased peaks at 25.1kHz and 26.1kHz around -85dBrA, indicating again that the M10 V2 ADC is digitizing the incoming analog signal at 44.1kHz (i.e., 44.1kHz-19kHz = 25.1kHz).

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

Shown above is an FFT of the intermodulation (IMD) products for an 18kHz + 19kHz summed sine-wave stimulus tone measured at the output across an 8-ohm load at 10W 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 -105dBrA, or 0.0006%, for the left channel, and -95dBRa, or 0.002%, for the right channel. The third-order modulation products, at 17kHz and 20kHz, are around -105dBrA to -110dBrA, or 0.0003% to 0.0006%.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sine-wave stimulus tone measured at the output across an 8-ohm load at 10W for the digital coaxial input at 24/96. We find the same second- and third-order modulation products as seen in the 16/44.1 FFT above.

Square-wave response (10kHz)

Above is the 10kHz square-wave response using the analog line-level input, at roughly 10W into 8 ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the M10 V2’s slew-rate performance. Rather, it should be seen as a qualitative representation of the M10 V2’s low bandwidth. An ideal square wave can be represented as the sum of a sine wave 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. Due to M10 V2’s very limited bandwidth, only the square wave’s fundamental (10kHz) sine wave is reproduced here. In addition, we can see the 400kHz switching-oscillator frequency used in the digital-amplifier section visibly modulating the waveform.

Square-wave response (1kHz) — 250kHz bandwidth

Above is the 1kHz square-wave response using the analog line-level input, at roughly 10W into 8 ohms, this time with a 250kHz input bandwidth on the analyzer to filter out the 400kHz switching frequency. We see more evidence here, in the overshoot and undershoot at the square-wave corners, of the M10 V2’s limited bandwidth with an analog input.

FFT spectrum of 400kHz switching frequency relative to a 1kHz tone

The M10 V2’s class-D amplifier relies on a switching oscillator to convert the input signal to a pulse-width modulated (PWM) square wave (on/off) signal before sending the signal through a low-pass filter to generate an output signal. The M10 V2 oscillator switches at a rate of about 400kHz, and this graph plots a wide-bandwidth FFT spectrum of the amplifier’s output at 10W into 8 ohms as it’s fed a 1kHz sine wave. We can see that the 400kHz peak is quite evident, and at -40dBrA. There is also a peak at 800kHz (the second harmonic of the 400kHz peak) at -65dBrA. Those two peaks—the fundamental and its second harmonic—are direct results of the switching oscillators in the M10 V2 amp modules. Also seen are the 43.1/44.1/45.1kHz peaks due to the ADC sampling the incoming signal at 44.1kHz. The noise around those very-high-frequency signals are in the signal, but all that noise is far above the audioband—and therefore inaudible—and so high in frequency that any loudspeaker the amplifier is driving should filter it all out anyway.

Damping factor vs. frequency (20Hz to 20kHz)

The final graph above is the damping factor as a function of frequency. Both channels show a relatively constant damping factor from 20Hz to 20kHz. The damping factor for the left and right channels are right around 200 from 20Hz to 6-7kHz, then rise up to about 240 at 20kHz.

Diego Estan
Electronics Measurement Specialist