Integrated Amp Reviews

I measured the Masters Series M10 using my Audio Precision SYS2722 system (see the January 2008 "As We See It"). As the NAD is a class-D design, I didn't run my usual preconditioning test of using the amplifier to drive a 1kHz tone at one-third power into 8 ohms. Nevertheless, before doing any testing I ran it for an hour at a moderate power level, to ensure that it was fully warmed up. Because class-D amplifiers emit relatively high levels of ultrasonic noise that would drive my analyzer's input into slew-rate limiting, all measurements were taken with Audio Precision's auxiliary AUX-0025 passive low-pass filter, which eliminates noise above 200kHz. Without the filter, there was 250mV of ultrasonic noise present at the loudspeaker terminals, which is typical for an nCore output stage.

The M10 runs warm. After an afternoon of testing, the top panel's temperature was 95.6°F/35.4°C. The System Info menu on the front-panel display indicated that the internal temperatures were "Left 60°C, Right 62°C."

Looking first at the analog inputs: With the NAD's volume control set to its maximum, the voltage gain at 1kHz into 8 ohms measured a relatively low 29.6dB from the speaker terminals and 4dB from the preamplifier outputs. The line inputs preserved absolute polarity (ie, were noninverting), and the input impedance was 16k ohms at low and middle frequencies, dropping to 4.5k ohms at 20kHz.

The preamplifier output impedance was a usefully low 92 ohms in the midrange above but increased to 135 ohms at the bottom of the audioband, probably due to the presence of an output coupling capacitor. The M10's output impedance at the speaker terminals was very low, at <0.01 ohms from 20Hz to 20kHz, the measured value including the series resistance of 6' of speaker cable. Consequently, the variation in frequency response with our standard simulated loudspeaker (fig.1, gray trace) is minimal. The traces in fig.1 cut off sharply above 20kHz, due to the M10 converting its analog inputs to digital with what appears to be a sample rate of 44.1kHz. Unlike NAD's more expensive M32, which allows the analog inputs to be digitized at 48, 96, or 192kHz, the M10's analog/ digital converter runs at just the one rate. The M10's reproduction of an analog 1kHz squarewave has the ringing on its leading and trailing edges that are typical of a linear phase reconstruction filter (fig.2) and a 10kHz squarewave is reproduced as a sinewave (fig.3), due to all the odd-order harmonics that would give the wave its square shape being removed by the A/D converter's antialiasing filter.

The M10's treble and bass controls offer a maximum boost of 6dB. As you can't have levels higher than 0dBFS in a digital system, switching on the tone controls with them set to do nothing reduces the signal level by the same 6dB. Fig.4 shows the effect of the NAD's bass and treble controls set to their +6dB and –6dB positions. The bass control offers a range of ±6dB below 70Hz, the treble control +5.5/–6.7dB at 20kHz. I created this graph using S/PDIF data sampled at 192kHz to show what happens above the audioband. It implies that the M10's DSP operates at 96kHz even when fed 192kHz data.

Channel separation was good rather than great, at >70dB in both directions below 1kHz but decreasing to 45dB at 20kHz. With the Audio Precision ultrasonic filter, the analog inputs shorted to ground, and the volume control set to the maximum, the wideband, unweighted signal/noise ratio (ref. 2.83V into 8 ohms) measured 55.8dB in the left channel and 60.3dB in the right. Restricting the measurement bandwidth to 22kHz increased the ratio to a respectable 75dB in both channels, and an A-weighting filter increased it further, to 78.2dB. Spectral analysis of the NAD's low-frequency noise floor (fig.5) revealed there to be a spurious tone present at 60Hz, this related to the AC power-line frequency but probably inaudible at –68dB ref. 2.83V into 8 ohms.

The M10 is specified as delivering a maximum continuous output power of 100Wpc into 8 ohms (20dBW). At our usual definition of clipping (ie, when the percentage of THD+noise in the amplifier's output reaches 1%), with continuous drive the M10 exceeded its specified power. It clipped at 155Wpc into 8 ohms (fig.6, 21.9dBW) and at 295Wpc into 4 ohms (fig.7, 21.7dBW). Distortion levels at moderate powers (fig.8) were low and dominated by noise, more so in the left channel (blue, cyan, and gray traces) than the right (red and magenta). Although the shape of the THD+N spuriae waveform at 30Wpc into 8 ohms (fig.9, bottom trace) is obscured by high-frequency noise, the distortion signature appears to be primarily third-harmonic in nature. This was confirmed by spectral analysis (fig.10), with the second harmonic 6dB lower in level and higher- order harmonics all lying below –100dB (0.001%). Intermodulation distortion was very low (fig.11), but as with the M32 the noise floor begins to rise above 15kHz.

To examine the M10's performance with digital input data, I used the Audio Precision's optical and coaxial S/PDIF outputs and data sent to the M10 via Wi-Fi from my Roon Nucleus+ server. There were no significant differences between the data sources. The coaxial input locked to datastreams with all sample rates up to 192kHz. The optical TosLink input, however, was restricted to 96kHz and below. The BluOS Wi-Fi connection worked with data sampled at up to 192kHz.

With digital input signals the M10's front-panel meters are calibrated in dBFS—ie, 1kHz at –12dBFS is indicated as "–12dB." A 1kHz digital signal at –12dBFS resulted in an output level of 15.8V into 8 ohms with the volume control set to the maximum, which suggests that the M10's gain architecture is well-organized. The NAD's impulse response with 44.1kHz data (fig.12) indicates that the reconstruction filter is a conventional linear-phase type, with time-symmetrical ringing to either side of the single sample at 0dBFS. With 44.1kHz-sampled white noise (fig.13, red and magenta traces), the M10's response rolled off sharply above 20kHz, reaching full stop-band suppression just above half the sample rate (vertical green line). An aliased image at 25kHz of a full-scale tone at 19.1kHz (blue and cyan traces) can't therefore be seen, though the noise floor starts to rise at ultrasonic frequencies. The distortion harmonics of the 19.1kHz tone are visible above the ultrasonic noise floor, the third harmonic being the highest in level at –60dB (0.1%).

When I examined the NAD's digital frequency response with coaxial S/ PDIF data at 44.1, 96, and 192kHz (fig.14), the response at the two lower rates dropped off sharply just below half the sample rate. The response with 192kHz data (blue and red traces) extended higher than with 96kHz data (cyan, magenta) but not by much. When I increased the bit depth from 16 to 24 with a dithered 1kHz tone at –90dBFS (fig.15), the noise floor dropped by almost 12dB, meaning that the M10 offers 18 bits' worth of resolution, 2 bits less than the M32. With undithered data representing a tone at exactly –90.31dBFS (fig.16), the three DC voltage levels described by the data were well resolved and the waveform was perfectly symmetrical. With undithered 24-bit data, the result was a clean sinewave (fig.17).

The NAD's rejection of word-clock jitter with 16-bit Wi-Fi data (fig.18) was superb, with all the odd-order harmonics of the LSB-level, low-frequency squarewave at the correct levels, indicated by the sloping green line in this graph. However, the left channel (blue trace) was a little noisier than the right (red).

Like its big brother, NAD's M10 packs a lot of well-engineered performance into its relatively small chassis.—John Atkinson