NAD Masters Series M22 power amplifier Measurements

Sidebar 3: Measurements

I performed a full set of measurements on the Masters Series M22, using my Audio Precision SYS2722 system (see the January 2008 "As We See It"). As the M22 has a class-D output stage, for almost all measurements I used an Audio Precision AUX-0025 passive low-pass filter ahead of the analyzer, which eliminates noise above 200kHz.

The M22's voltage gain into 8 ohms measured 28.9dB from both its balanced and unbalanced inputs, and both inputs preserved absolute polarity (ie, were non-inverting). The unbalanced input impedance measured 23k ohms at 20Hz, dropping to 18k ohms at 1kHz and 10k ohms at 20kHz. The balanced input impedance was a consistent 75k ohms at low and middle frequencies, dropping inconsequentially to 70k ohms at the top of the audioband.

The output impedance was very low for an amplifier with a class-D output stage: 0.05 ohm at 20Hz, 0.03 ohm at 1kHz, and 0.07 ohm at 20kHz (all figures include the series resistance of 10' of speaker cable). As a result, the modulation of the amplifier's frequency response by the Ohm's law interaction between this impedance and that of our standard simulated loudspeaker was just ±0.05dB (fig.1, gray trace), and the audioband response didn't change as a pure resistive load dropped from 8 ohms (blue and red traces) to 2 ohms (green). This graph was taken without the AP low-pass filter; a sharply defined peak is visible at 60kHz and is most developed into 8 ohms. This ultrasonic peak results in a small degree of critically damped overshoot on the leading edges of a 10kHz squarewave (fig.2).

Fig.1 NAD M22, balanced frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (left channel blue, right, red), 4 ohms (left cyan, right magenta), 2 ohms (green) (0.5dB/vertical div.).

Fig.2 NAD M22, small-signal, 10kHz squarewave into 8 ohms.

Channel separation was superb, at >100dB in both directions below 2kHz, and still 88dB at 20kHz. Without the AP low-pass filter and no audio signal, around 330mV of RF noise was present at the M22's output, with a center frequency of 446kHz (footnote 1). The unweighted signal/noise ratio, ref.2.83V into 8 ohms and measured with the NAD's unbalanced input shorted to ground, was 72.9dB left and 70.0dB right, these figures improving respectively to 99.6 and 99.1dB when the measurement bandwidth was restricted to the audioband, and to 102.4 and 101.9dB when A-weighted. This is superb performance, but the slightly lower ratios in the right channel are due to a very small amount of power-supply contamination (fig.3). As all these spuriae are 120dB below the level of a 1W signal, they are inconsequential.

Fig.3 NAD M22, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms (linear frequency scale).

Fig.4 plots how the percentage of total harmonic distortion plus noise in the M22's output changed with power into 8 ohms with both channels driven. NAD usually specifies maximum power in terms of short-term delivery, but for the M22 quotes a power of >300Wpc into 8 ohms (>24.77dBW) at 0.1% THD+N with both channels driven. Fig.4 indicates that the THD+N reaches 1% (our usual definition of clipping) at 340Wpc into 8 ohms (25.3dBW). Fig.5 shows that the M22 clips at 400Wpc into 4 ohms with both channels continuously driven (23dBW), while with just one channel driven (fig.6), the amplifier clips at 570W (24.55dBW). Even into 2 ohms, the NAD M22 managed to output 700W (22.4dBW) with just one channel driven.

Fig.4 NAD M22, both channels driven, distortion (%) vs 1kHz continuous output power into 8 ohms.

Fig.5 NAD M22, both channels driven, distortion (%) vs 1kHz continuous output power into 4 ohms.

Fig.6 NAD M22, one channel driven, distortion (%) vs 1kHz continuous output power into 4 ohms.

With the M22's very low level of noise, I had to test how its THD+N percentage changed with frequency at a very high level, 28.3V (equivalent to 100W into 8 ohms, 200W into 4 ohms, or 400W into 2 ohms), to be sure I was looking at true distortion. Even so, the distortion remained very low and uniform with frequency, with a small rise in the top octave (fig.7). That the distortion waveform at this power level was primarily the subjectively innocuous third harmonic (fig.8) was confirmed by spectral analysis (fig.9). Testing the M22 at very high power with an equal mix of 19 and 20kHz tones (fig.10) showed that the second-order intermodulation product at 1kHz lay close to –110dB (0.0003%), with the higher-order products at 18 and 21kHz higher in the left channel (blue trace) than in the right (red). Even so, at –82dB (0.012%), these are of no concern.

Fig.7 NAD M22, THD+N (%) vs frequency at 28V into: 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (left, gray).

Fig.8 NAD M22, 1kHz waveform at 100W into 8 ohms, 0.0039% THD+N (blue); distortion and noise waveform with fundamental notched out (red, not to scale).

Fig.9 NAD M22, spectrum of 50Hz sinewave, DC–1kHz, at 200W into 4 ohms (linear frequency scale).

Fig.10 NAD M22, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 200W peak into 4 ohms (linear frequency scale).

NAD's Masters Series M22 amplifier measures extraordinarily well. It is the very model of a modern class-D amplifier!—John Atkinson



Footnote 1: Unlike the Spec RPA-W7EX Real-Sound, which is also reviewed this month and also uses a class-D output stage, the M22 didn't interfere with FM radio reception in my test lab.
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COMMENTS
CharlieG's picture

...and I sent it back for a replacement. Maybe my finger didn't have the magic touch on that standby button either! But here's another easy fix: if your front end has a 12v trigger output, connecting it to the trigger input on the back of the M22 will also disable that pesky switch, and allow you to toggle the amp between on/standby by simply turning the front end on/off. Convenient!

Kal Rubinson's picture

That's an excellent idea.

My son-in-law is doing exactly that with his M27. Unfortunately, I cannot.

pablolie's picture

I agree on the touch button on the M22. Terrible idea. The delay means one pushes it repeatedly, and both the user and (seemingly) the M22 end up confused. I did connect the 12V but now in the end I have just left it in auto-standby mode. The M22 is very reliable in detecting music streams and it turns itself back on (with a 3 sec delay or so after it clicks), and why not save some power when (honestly) I can't tell a difference in sound between cold and "warm" state (you truly have to crank up the volume to feel something resembling truly warm temp).

Anyhow it is a phenomenal amp.

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