Friday, September 22, 2017

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All amplifier measurements are performed independently by BHK Labs. All measurement data and graphical information displayed below are the property of the SoundStage! Network and Schneider Publishing Inc. Reproduction in any format is not permitted.

Note: Measurements of the NAD M32’s left channel were taken at its AES digital and Line 1 analog inputs, at a line voltage of 120V AC, both channels driven. The M32 is a Direct Digital switching amplifier, a technology developed by NAD. As usual, I had to use (except as noted) Audio Precision’s AUS-0025 external low-pass filter, to keep the out-of-band noise of the tested amplifier from corrupting the measurements taken with AP’s SYS-2722 measuring system.

Power output

  • Estimated power output at 1% THD+N: 193.0W @ 8 ohms, 190.0W @ 4 ohms
  • Estimated power output at 10% THD+N: 200.0W @ 8 ohms, 193.0W @ 4 ohms

Additional data

  • Input/output polarity (digital and analog inputs): noninverting
  • AC-line current draw: 35.0W, 0.456A, 0.63PF
  • Gain: output voltage divided by input voltage, 8-ohm load (Lch/Rch)
    • Analog unbalanced inputs (volume full up at +10dB): 64.85X, 36.2dB / 64.06X, 36.1dB
    • Digital input (-20dBFS input with volume at 0dB): 4.103V / 4.057V
  • Input sensitivity for 1W output into 8 ohms (Lch/Rch)
    • Analog unbalanced inputs: 43.6mV / 44.1mV
  • Output impedance @ 50Hz: 0.1 ohm
  • Input impedance @ 1kHz
    • Line 1 input: 9.92k ohms
  • Output noise, Line 1 analog input, volume at 10dB (Lch/Rch)
    • Wideband: 40.8mV/39.3mV, -36.9dBW/-37.1dBW
    • A weighted: 0.36mV/0.36mV, -77.9dBW/-77.9dBW
  • Output noise, Line 1 analog input, volume at -4dB (Lch/Rch)
    • Wideband: 42.4mV/40.3mV, -36.5dBW/-36.9dBW
    • A weighted: 0.113mV/0.113mV, -88.0dBW/-88.0dBW
  • Output noise, Line 1 analog input, volume at -20dB (Lch/Rch)
    • Wideband: 40.3mV/39.5mV, -36.9dBW/-37.1dBW
    • A weighted: 0.086mV/0.086mV, -90.3dBW/-90.3dBW
  • Output noise, AES/EBU digital input, volume at 10dB (Lch/Rch)
    • Wideband: 40.5mV/39.6mV, -36.9dBW/-37.1dBW
    • A weighted: 0.082mV/0.082mV, -90.8dBW/-90.8dBW
  • Output noise, AES/EBU digital input, volume at -4dB (Lch/Rch)
    • Wideband: 40.7mV/39.6mV, -36.9dBW/-37.1dBW
    • A weighted: 0.086mV/0.082mV, -90.3dBW/-90.8dBW
  • Output noise, AES/EBU digital input, volume at -20dB (Lch/Rch)
    • Wideband: 40.5mV/38.5mV, -36.9dBW/-37.3dBW
    • A weighted: 0.086mV/0.082mV, -90.3dBW/-90.8dBW

Measurements summary

Volume-control tracking, tested using a 1kHz test tone, was within less than 0.1dB in the range of volume settings of +10dB (max) to -40dB.

Chart 1A shows the frequency response of the M32 with varying loads and with my usual vertical scale. This was done by feeding the Line 1 analog inputs an internal sampling rate of 96kHz. As can be seen, the curves of the open circuit and the 8-ohm, 4-ohm, and NHT dummy-speaker loads deviate considerably from flatness in the high frequencies. NAD has a speaker-impedance setting that’s supposed to correct for the effects of variable loading on the output LCR low-pass filter in the amp. I tested it in both settings, and there was a subtle difference, but it really did nothing to improve the out-of-band response with loading. With a digital input, I plotted the output response at sample rates of 44.1, 96, and 192kHz. Again, an out-of-band rise in response appears, and with response beyond that of Chart 1B. This rise in out-of-band response could have an effect on the sound of high-frequency music information, as opposed to a flat response.

The frequency-response curve in Chart 1C is that of the RIAA equalization error of the analog phono input. This is quite good for this measurement.

Chart 2 illustrates how the M32’s total harmonic distortion plus noise (THD+N) vs. power varies for 1kHz and SMPTE intermodulation test signals and amplifier output load of 8 and 4 ohms. The amount of distortion is reasonably low in this design, and through much of the power range is dominated by noise rather than distortion per se. This test, measured at the analog input, yielded very similar results.

The M32’s THD+N as a function of frequency, at several different power levels, is plotted in Chart 3. Because this amplifier generates quite a bit of out-of-band noise, using my usual 80kHz low-pass filter to make it possible for me to measure the harmonics of 20kHz would have caused the measurements to be far too dominated by noise. So for this chart I reduced the measurement bandwidth to 22kHz, though this shows an increase in distortion in the last two octaves of the audioband. Nonetheless, the readings are still largely dominated by noise, and show the distortion’s tendency to rise at higher frequencies.

The M32’s damping factor vs. frequency is plotted in Chart 4. As is typical of many solid-state power amplifiers, the damping factor begins to fall off above about 1kHz.

Chart 5 plots the spectrum of the M32’s harmonic distortion and noise residue when fed a 10W, 1kHz test signal. The line harmonics are visible but low in magnitude. The signal harmonics are numerous and complex, with even and odd harmonics out to 20kHz.

A comment on the noise measurements above with both analog and digital data: The analog noise is higher at the higher volume settings due to the noise of the M32’s analog electronics feeding the A/D converter. In the digital data, the noise is independent of the volume setting.

Chart 1A - Frequency response of output voltage as a function of output loading

Chart 1A

Red line = open circuit
Magenta line = 8-ohm load
Blue line = 4-ohm load
Cyan line = NHT dummy-speaker load

Chart 1B - Frequency response of output voltage as a function of sample rate

Chart 1B

Red line = 44.1kHz
Magenta line = 96kHZ
Blue line = 192kHz

Chart 1C - Frequency response of output voltage as a function of phono RIAA equalization error

Chart 1C

Red line = left channel
Magenta line = right channel

Chart 2 - Distortion as a function of power output and output loading

Chart 2

(Line up at 100W to determine lines)
Top line = 4-ohm SMPTE IM distortion
Second line = 8-ohm SMPTE IM distortion
Third line = 4-ohm THD+N
Bottom line = 8-ohm THD+N

Chart 3 - Distortion as a function of power output and frequency

Chart 3

(8-ohm loading)
Red line = 1W
Magenta line = 10W
Blue line = 30W
Cyan line = 70W
Yellow line = 180W

Chart 4 - Damping factor vs. frequency and volume control

Chart 4

Damping factor = output impedance divided into 8

Chart 5 - Distortion and noise spectrum

Chart 5

1kHz signal at 10W into an 8-ohm load

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