NAD Masters M23 Stereo/Mono Amplifier Measurements

Link: reviewed by Roger Kanno on SoundStage! Hi-Fi on March 1, 2023

General information

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

The NAD Masters M23 was conditioned for 1 hour at 1/8th full rated power (~25W 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 M23 has both unbalanced (RCA) and balanced inputs (XLR), and two pairs of speaker-level outputs. We found small differences in terms of THD between the RCA and XLR inputs (see published specifications vs. our primary measurements below).

The M23 can be operated in stereo or bridged (mono) modes. There is also a gain switch with Low, Mid, and High settings. As expected, there were small differences in THD and noise levels between the gain settings, with the Low gain setting yielding the lowest (i.e., best) results and the High gain setting yielding the highest results (see primary measurements table below). Unless otherwise stated, the balanced inputs were used with the Mid gain setting for all measurements. With these settings, 580mVrms was required at the input to achieve the reference 10W into 8 ohms.

Because the M23 uses 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 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.

Published specifications vs. our primary measurements

The table below summarizes the measurements published by NAD for the Masters M23 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 to 500kHz, assume, unless otherwise stated, 10W into 8 ohms and a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channels.

Our primary measurements in stereo mode revealed the following using the line-level balanced analog input (unless specified, assume a 1kHz sinewave at 580mVrms, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth): 

For the continuous dynamic power test, the M23 was able to sustain 538W into 4 ohms (~1% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (53.8W) 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 M23 was only slightly warm to the touch.

Our primary measurements in mono mode revealed the following using the line-level balanced analog input (unless specified, assume a 1kHz sinewave at 290mVrms, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

Frequency response (8-ohm loading, stereo mode)

In our frequency- response plots above, measured across the speaker outputs at 10W into 8 ohms, the M23 is near flat within the audioband (0/+0.08dB, 20Hz/20kHz), which essentially corroborates NAD’s claim of 20Hz-20kHz at ±0.06dB. At the extremes, the M23 is at 0dB at 5Hz and -3dB at about 70kHz. 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.

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

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. The M23 does not invert polarity and exhibits, at worst, about 15 degrees (at 20kHz) of phase shift within the audioband.

RMS level vs. frequency vs. load impedance (1W, left channel only, stereo mode)

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, in stereo mode. 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 find that maximum deviation between no-load and a 4-ohm load is very small, at around 0.02dB. This is an indication of a very high damping factor, or low output impedance. With a real speaker, there are essentially no deviations between 20Hz and 1kHz, then a rise up to 0.08dB at 20kHz, due to the M23’s frequency response.

THD ratio (unweighted) vs. frequency vs. output power (stereo mode)

The chart above shows THD ratios at the output into 8 ohms as a function of frequency for a sinewave stimulus at the analog line-level input in stereo mode. The blue and red plots are for the left and right channels at 1W output into 8 ohms, purple/green at 10W, and pink/orange at 200W. The 10W data yielded the lowest THD figures, ranging from 0.0001% at 20Hz, down to nearly 0.00003% between 200Hz and 500Hz, then up to 0.0001% at 6kHz. These are extraordinarily low THD ratios, nearing the limits of the APx555 analyzer. At 1W, THD ratios ranged from 0.0001% from 20Hz to 1kHz, then down to 0.00005% at 6kHz.  A 200W, THD ratios were higher, from 0.0003% at 20Hz, down to 0.0001% from 100Hz to 400Hz, then up to 0.0005% at 4kHz.

THD ratio (unweighted) vs. frequency vs. output power (bridged mode)

The chart above shows THD ratios at the output into 8 ohms as a function of frequency for a sinewave stimulus at the analog line-level input in bridged mode. The blue plot is at 1W output into 8 ohms, purple at 10W, and pink at 700W. The 10W data yielded the lowest THD figures, ranging from 0.0001% at 20Hz, down to just above 0.00005% between 200Hz and 500Hz, then up to 0.0001% at 2-3kHz. At 1W, THD ratios ranged from 0.0002% from 20Hz to 1kHz, then down to 0.00008% at 6kHz.  A 700W, THD ratios were higher, from 0.01% from 20Hz to 1kHz, then up to 0.03% at 2-4kHz.

THD ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms (stereo mode)

The chart above shows THD ratios measured at the output of the M23 as a function of output power for the analog line-level input in stereo mode, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). The 8-ohm data ranged from about 0.002% at 50mW, down to 0.00005% from 5 to 100W, then up to the “knee” just past 200W. The 4-ohm data ranged from about 0.001% at 50mW, down to 0.00007% from 5 to 100W (for the left channel, the right channel showed 10dB higher THD ratios at 20W and 150W), then up to the “knee” around 350W. The 1% THD marks were hit at 278W (8 ohms) and 540W (4 ohms).

THD+N ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms (stereo mode)

The chart above shows THD+N ratios measured at the output of the M23 as a function of output power for the analog line level-input in stereo mode, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). THD+N values for the 8-ohm data ranged from 0.003% down to just below 0.0002% at 100W. The 4-ohm data yielded THD+N values 3-4dB higher.

THD ratio (unweighted) vs. frequency at 8, 4, and 2 ohms (left channel only, stereo mode)

The chart above shows THD ratios measured at the output of the M23 as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage that yields 20W at the output into 8 ohms (and roughly 40W into 4 ohms, and 80W into 2 ohms) for the balanced analog line-level input in stereo mode. The 8-ohm load is the blue trace, the 4-ohm load the purple trace, and the 2-ohm load the pink trace. The 8- and 4-ohm THD data are close, with the 4-ohm data yielding values about 5dB higher above 200Hz. The 8-ohm data ranged from 0.0001% at 20Hz, down to 0.00003% at 200-500Hz, then up to 0.0001% at 3-4kHz. The 2-ohm data ranged from 0.0001 to 0.0002% across the audioband.

THD ratio (unweighted) vs. frequency at 8, 4, and 2 ohms (bridged mode)

The chart above shows THD ratios measured at the output of the M23 as a function of frequency into two different loads (8/4ohms - the 2-ohm load tripped the M23’s protection circuit within about two to three seconds) for a constant input voltage that yields 100W at the output into 8 ohms (and roughly 200W into 4 ohms) for the analog line-level input in bridged mode. The 8-ohm load is the blue trace, and the 4-ohm load the purple trace. We see increasing levels (5-10dB) of THD from 8 to 4 ohms between about 50Hz and 6kHz, with the 8-ohm data ranging from 0.00006% to 0.0003%. When a stereo amplifier’s bridged mode applies the same signal (with one phase inverted) to each channel, and connects the speaker load across the outputs of both channels, the effective speaker impedance seen by the amplifier is halved. It is therefore not surprising that the M23 is not stable into 2-ohm loads when bridged.

THD ratio (unweighted) vs. frequency into 8 ohms and real speakers (left channel only, stereo mode)

The chart above shows THD ratios measured at the output of the M23 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 balanced analog line-level input in stereo mode. 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 8-ohm plot is fairly flat and between 0.0001% and 0.00003% from 20Hz to 6kHz. Between 300Hz and 6kHz, the THD ratios when real speakers were used as loads are identical to the dummy load. The two-way speaker THD results were as high as 0.003% at 20Hz. Between 40Hz and 200Hz, the speaker THD results were roughly 5dB higher than that of the dummy load. This is an impressive result, showing that the M23 can maintain it’s ultra-low distrotion into real-world loads.

IMD ratio (CCIF) vs. frequency into 8 ohms and real speakers (left channel only, stereo mode)

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the M23 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 IMD plots are within 5dB of one another, hovering between 0.0003% and 0.0001%.

IMD ratio (SMPTE) vs. frequency into 8 ohms and real speakers (left channel only, stereo mode)

The chart above shows IMD ratios measured at the output of the M23 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 in stereo mode. 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 data sets are close enough to be judged as identical, hovering around the 0.001% level.

FFT spectrum – 1kHz (line-level input, stereo mode)

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 in stereo mode. We see that the signal’s second (2kHz) and third (3kHz) harmonic are slightly below -130dBrA, or 0.00003%. The power-supply-related noise peak at the fundamental (60Hz) frequency is evident at the -140dBrA, or 0.00001%, level. A rise in the noise floor can be seen above 20kHz, indicative of this type of digital amplifier technology. This is an exceptionally clean FFT result.

FFT spectrum – 1kHz (line-level input, stereo mode)

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 in bridged mode. We see that the signal’s second (2kHz) and third (3kHz) harmonic are slightly below and above -130dBrA, respectively, or 0.00003%. The power-supply-related noise peak at the fundamental (60Hz) frequency is evident at the -140dBrA, or 0.00001%, level. The M23 exhibits only slightly more THD in bridged mode compared to stereo mode.

FFT spectrum – 50Hz (line-level input, stereo mode)

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 line-level input in stereo mode. 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 most dominant (non-signal) peaks are the second (100Hz) and third (150Hz) signal harmonic at roughly -125dBrA, or 0.00006%, and -135/-150dBrA (left/right channels), or 0.00002/0.000003%. The power-supply-related noise peaks at the fundamental (60Hz) frequency is evident at the -135dBrA, or 0.00002%, level.

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

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 analog line-level input in stereo mode. 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 -125dBrA, or 0.00006%, and the third-order modulation products, at 17kHz and 20kHz, are at roughly -115dBrA, or 0.0002%.

Squarewave response (10kHz, stereo mode)

Above is the 10kHz squarewave response using the balanced 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 M23’s slew-rate performance. Rather, it should be seen as a qualitative representation of the M23’s mid-tier bandwidth. An ideal squarewave 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. In the case of the M23, however, what dominates the plateaus of the squarewave in the top graph is a 500kHz sinewave, the frequency at which the switching oscillator in this class-D amp is operating (see FFT below).

Squarewave response (10kHz, restricted 250kHz bandwidth, stereo mode)

Above is the 10kHz squarewave response using the analog input, at roughly 10W into 8 ohms, with a 250kHz restricted bandwidth to remove the modulation from the 500kHz oscillator. Here we find a relatively clean squarewave, with some overshoot in the corners.

FFT spectrum (1MHz bandwidth, stereo mode)

The M23’s amplifier 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 M23 oscillator switches at a rate of about 500kHz, 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 500kHz peak is quite evident, and at -40dBrA. There is also a peak at 1MHz (the second harmonic of the 500kHz peak), at -70dBrA. Those two peaks—the fundamental and its second harmonic—are direct results of the switching oscillators in the M23 amp modules. The noise around those very-high-frequency signals are in the signal, but all that noise is far above the audio band—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, stereo mode)

The graph above is the damping factor as a function of frequency in stereo mode. We see both channels with very high damping factors, ranging from around the 3500/5000 (left/right) mark at 20Hz, down to 2000/2200 (left/right) at 20kHz.

Damping factor vs. frequency (20Hz to 20kHz, bridged mode)

The final graph above is the damping factor as a function of frequency in bridged mode. We see a very high damping factor, ranging around the 4700 mark at 20Hz, down to 700 at 20kHz. The damping factor is less across the audioband than in stereo mode, but that is expected in bridged operation.

Diego Estan
Electronics Measurement Specialist