NAD C 700 Integrated Amplifier-DAC

Link: reviewed by Gordon Brockhouse on SoundStage! Simplifi on December 1, 2021

General Information

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

The C 700 was conditioned for 1 hour at 1/8th full rated power (~10W 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 700 offers two analog inputs (RCA), two digital S/PDIF inputs (coaxial and optical), an HDMI input, an Ethernet connection for streaming, line-level subwoofer and pre-outs (RCA), and a pair of speaker level outputs. For the purposes of these measurements, the following inputs were evaluated: coaxial digital (RCA), and the analog line-level unbalanced (RCA) input.

Most measurements were made with a 1Vrms line-level analog input, and -0.6dBFS digital input. The volume control is variable from 0 to 100. The following volume settings yielded 10W into 8 ohms: 80 for analog line-level and 72 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 80W. For comparison, on the analog input, a SNR measurement was also made with the volume at maximum, where only 0.550Vrms was required to achieve 80W into 8ohms.

Based on the accuracy and repeatability at various volume levels of the left/right channel matching (see table below), the C 700 volume control is likely operating in the digital domain. Consequently, all analog signals are digitized at the C 700’s inputs so the unit may apply volume, bass management, tone controls, etc. The volume control offers a total range from -35dB to +33.5dB (speaker level outputs). Below about 20%, volume increments range from 2 to 2.5dB. Above 20%, 1dB, with the occasional 0.5dB increment.

Because the C 700 uses 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 C 700 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 is set at its maximum (DC to 1MHz), assume, unless otherwise stated, a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channels.

*Theoretical instantaneous power based on measured no-load 1%THD output

Our primary measurements revealed the following using the analog/coaxial input (unless specified, assume a 1kHz sinewave at 1Vrms or -0.6dBFS, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

For the continuous dynamic power test, the C 700 was able to sustain 100W (1% THD) into 4 ohms using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (10W) 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 C 700 stayed relatively cool to the touch.

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

In our measured frequency response chart above, the C 700 is nearly flat within the audioband (20Hz to 20kHz). At the extremes, the C 700 is -0.04dB down at 20Hz, and -0.05/0.1dB (left/right) at 20kHz. But the C 700 cannot be considered a high-bandwidth audio device as the -3dB point is just past 20kHz. In fact, the C 700 exhibits brick-wall-type behavior just past 20kHz, because it is likely sampling the input at 44.1kHz. 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 C 700’s frequency response (left channel only) as a function of input type. The green trace is the same analog input data from the previous graph. The red trace is for a 16-bit/44.1kHz dithered digital input signal from 5Hz to 22kHz, the purple trace is for a 24/96 dithered digital input signal from 5Hz to 48kHz, and finally green is 24/192 from 5Hz to 96kHz. The analog-in frequency response is identical to the 16/44.1 digital input, with brickwall-type filtering just above 20kHz. The behavior at low frequencies is the same across all input types: -0.15dB at 10Hz. The behavior at high frequencies for all digital input types is typical. The 24/96 data shows brickwall-type filtering right around 48kHz, while the 24/192 data shows a gentler slope with a -3dB point at 77kHz.

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

Above are two frequency response plots for the analog input, measured at 10W (8-ohm) at the speaker outputs, with the treble/balance controls set at both minimum and maximum. They show that the C 700 will provide a maximum gain/cut of approximately 6dB at 20Hz, and a maximum gain/cut of approximately 6dB at 10kHz.

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 at 120Hz. The C 700 DSP crossover uses 18dB/octaves, and the subwoofer output is flat down to 10Hz.

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

Above is the phase response plot from 20Hz to 20kHz for the analog input. The C 700 does not invert polarity and exhibits a little over 20 degrees of phase shift 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 700. 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 digital input types performed similarly, approaching the ideal 0dB relative level at -100 dBFS, then yielding perfect results to 0dBFS. At -120dBFS, the 16/44.1 input data overshot the ideal output signal amplitude by 1-2dB, while the 24/96 data overshot by only 1dB.

Impulse response (16/44.1 and 24/96 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 700. We can see that the C 700 utilizes a typical sinc function reconstruction filter.

J-Test (coaxial input)

The plot above shows the results of the J-Test Test for the coaxial digital input measured at the line-level pre-outs of the C 700. J-Test was developed by Julian Dunn the 1990s. It is a test signal—specifically a -3dBFS undithered 12kHz squarewave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz squarewave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sinewave. In addition, an undithered 250Hz squarewave at -144dBFS is mixed with the signal. This test file causes the 22 least significant bits to constantly toggle, which produces strong jitter spectral components at the 250Hz rate and its odd harmonics. The test file shows how susceptible the DAC and delivery interface are to jitter, which would manifest as peaks above the noise floor at 500Hz intervals (e.g., 250Hz, 750Hz, 1250Hz, etc.). Note that the alternating peaks are in the test file itself, but at levels of -144dBFS and below.  The test file can also be used in conjunction with artificially injected sinewave jitter by the Audio Precision, to show how well the DAC rejects jitter.

We see very small peaks in the audioband at -120to 130dBFS. This is a very good J-Test result, and an indication that the C 700 DAC has good jitter immunity. When sinewave jitter was injected at 2kHz, which would manifest as sidebands at 10kHz and 14kHz without any jitter rejection, no further peaks, or worsening of existing peaks, were observed up to about 200ns of jitter level, beyond which the C 700 DAC lost sync with the signal.

J-Test (optical input)

The plot above shows the results of the J-Test test for the optical digital input measured at the line-level pre-outs of the C 700. We see very small peaks in the audioband at -120to 130dBFS. This is a very good J-Test result once again, and an indication that the C 700 DAC has good jitter immunity. Like the coaxial input, when sinewave jitter was injected at 2kHz, which would manifest as sidebands at 10kHz and 14kHz without any jitter rejection, no further peaks, or worsening of existing peaks, were observed up to about 200ns of jitter level. Beyond 200ns, the C 700 DAC lost sync with the 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 C 700’s line-level pre-outs with white noise at -4dBFS (blue/red), and a 19.1kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1. The 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 -95dBrA.

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 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. Zoomed in, we can see a maximum deviation within the audioband of less than 0.2dB 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 with a real speaker was about the same, deviating by about 0.1dB within the flat portion of the curve (100Hz to 5kHz), with the lowest RMS level, which would correspond 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 (20Hz to 6kHz) for a sinewave stimulus at the analog 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 68W. The power was varied using the volume control. All three THD plots are relatively flat. The 1W data exhibited the lowest THD values, with values varying around 0.003%. The 10W data shows THD values around 0.006%. At 68W, THD values were around 0.05%.

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 700 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). Although there are fluctuations before the “knee,” both the 4-ohm and 8-ohm data are close to the same, ranging from 0.002% to 0.05%. The “knee” in the 8-ohm data occurs just past 70W, hitting the 1% THD mark at 80W. For the 4-ohm data, the “knee” occurs just below 90W, hitting the 1% THD mark around 100W.

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 700 as a function of output power for the analog line level-input, for an 8-ohm load (blue/red for left/right) and a 4-ohm load (purple/green for left/right). There’s a distinct 5dB jump in THD+N (also visible in the THD plot above, but to a lesser degree) when the output voltage is around 1Vrms (i.e., 0.15W into 8 ohms, 0.3W into 4 ohms). This behavior was repeatable over multiple measurement trials. Overall, THD+N values before the “knee” ranged from around 0.01% (3 to 20W into 8 ohms and 10 to 30W into 4 ohms) to 0.05/0.03% (8/4 ohms at the knee).

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 700 as a function of load (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 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 levels of THD from 8 to 4 to 2 ohms, with about a 5-3dB increase when halving of the load. Overall, even with a 2-ohm load at roughly 20W, THD values were fairly flat within the audioband at between 0.01 and 0.02%.

FFT spectrum – 1kHz (line-level input)

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. We see that the signal’s second harmonic, at 2kHz, is just above (left) and below (right) -90dBrA, or 0.003%, and the third harmonic is at roughly the same level; the remaining signal harmonics are below -110dBrA, or 0.0003%. Below 1kHz, we do not see traditional peaks from linear power supplies (60/120Hz) because of the switching power supply. All other noise related peaks are below -90dBrA, or 0.003%. It appears that the analog signal is digitized with a 44.1kHz sample rate, as peaks can be seen at 44.1kHz, as well as the IMD products with the main signal at 43.1 and 45.1kHz.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the coaxial digital input, sampled at 16/44.1. We see close to the same signal harmonic at 3kHz as with the analog input, but lower at 2kHz, at -95/-115dBrA (left/right), or 0.002/0.0002%. Noise peaks remain below -90dBRa, or 0.003%.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the coaxial digital input, sampled at 24/96. We see essentially the same signal harmonic profile as with 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 sinewave 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 and third harmonics for the right channel are predominant at -100dBrA, or 0.001%. Left channel signal harmonics are essentially non-existent.

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

Shown above is the FFT for a 1kHz -90dBFS dithered 24/96 input sinewave stimulus at the coaxial digital input, measured at the output across an 8-ohm load. We see the 1kHz primary signal peak, at the correct amplitude, and the second, third, and fourth harmonic for the right channel are predominant at -100 to -110dBrA, or 0.001 to 0.0003%. Left channel signal harmonics are essentially non-existent.

FFT spectrum – 50Hz (line-level input)

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 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 the signal second (100Hz) and third (150Hz) harmonics at around -90dBrA, or 0.003%, with other signal harmonics seen below this level. The worst-case noise peak, which may actually be an IMD product between the signal and the oscillator in the class-D amp, is just below 500kHz at just above -100dBrA, or 0.001%.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the analog 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 -90dBrA, or 0.003%, while the third-order modulation products, at 17kHz and 20kHz, are at the same level. We also see the main aliased peaks at 25.1kHz and 26.1kHz around -65dBrA, indicating that the C 700 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 distortion (IMD) products for an 18kHz + 19kHz summed sinewave 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 -90dBrA, or 0.003%, while the third-order modulation products, at 17kHz and 20kHz, are at the same level.

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

Shown above is an FFT of the intermodulation (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the coaxial optical input at 24/96. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -90dBrA, or 0.003%, while the third-order modulation products, at 17kHz and 20kHz, are at the same level.

Square-wave response (10kHz)

Above is the 10kHz square-wave response using the analog 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 700’s slew-rate performance. Rather, it should be seen as a qualitative representation of its very limited bandwidth. An ideal square wave 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. Due to the C 700’s very limited bandwidth, only the square wave’s fundamental (10kHz) sinewave 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 input, at roughly 10W into 8 ohms, with a 250kHz restricted bandwidth to remove the modulation from the 400kHz oscillator. We see more evidence here, in the overshoot and undershoot at the square-wave corners, of the C 700’s limited bandwidth with an analog input.

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

The C 700’s class-D amplifier technology relies on a switching oscillator to convert the input signal to a pulse-width modulated (PWM) squarewave (on/off) signal before sending the signal through a low-pass filter to generate an output signal. The C 700 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 sinewave. We can see that the 400kHz peak is quite evident, and at -40dBrA. There is also a peak at 800kHz and 1200kHz (the second and third harmonics of the 400kHz peak), at -65/-90dBrA. Those three peaks—the fundamental and its second/third harmonics—are direct results of the switching oscillators in the C 700 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 chart 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 120 from 20Hz to 20kHz.

Diego Estan
Electronics Measurement Specialist