NAD C 399 Integrated Amplifier-DAC Measurements

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