NAD M51 Direct Digital DAC


On the Bench

Distortion measurements were within an 80 kHz bandwidth unless otherwise stated. Signals were at - 5 dBFS unless otherwise noted. I used the XLR balanced outputs.

To summarize what I found, distortion was always at 0.003% - 0.004% THD+N, except for IMD, which was near the limits of the Audio Precision (less than 0.001% THD+N) when using the HDMI input.

With the Coax input, there are no visible harmonics with a 1 kHz test signal that was generated at 16 bit, 44.1 kHz sampling frequency.


Here is the digital clipping I mentioned. The input signal was at 0 dBFS, and the volume control was also at 0.0. It is very unlikely that a consumer will encounter a musical piece that hits 0 dBFS, but if it does, you can always set the volume control on the M51 to - 1 dBFS.


19 kHz and 20 kHz sine waves produced a B-A peak at 1 kHz that was 107 dB below the fundamentals. Notice also that there are no other peaks at the base of the fundamentals. This is very clean sound reproduction.


In this IMD spectrum, there are no visible peaks at the base of the 7 kHz peak (IMD tests use 60 Hz and 7 kHz sine waves at a 4:1 ratio). Again, clean, clean, clean. This is a graphical representation of what you will hear (or not hear) in terms of musical detail.


The measured Frequency Response at 16/44.1 was flat to 20 kHz.


Using 24/192 test signals, the noise band can be seen above about 35 kHz, which is far above the audible region. Again, though, THD+N was less than 0.004% with a 1 kHz sine wave input.


With the 19 kHz, 20 kHz pair, there is a slight rise in the noise floor below 4 kHz, but there is no B-A peak visible, so if there is one, it is lower than the noise floor.


The lack of any peaks at the base of the 7 kHz peak in this spectrum is practically a definition of clean detail in the sound.


The measured Frequency Response with 24/192 signal, using the Coax input, was flat to 32 kHz, and then rose 3 dB at 90 kHz.


OK, now for the HDMI input on the M51. This is the first DAC I know of that has this type of input, and did it perform!!

At 1 kHz, distortion was 0.003% as were all the other distortion tests using the HDMI input, except for IMD, which was at the limits of the Audio Precision test instrument. And, as I mentioned, after seeing these results, I used the HDMI output from the OPPO to the HDMI-1 input on the M51 for all my subsequent listening, some of which was while I was reading the newspaper. Good thing the music sounded so great, because the news was all bad.


Here is the same test, but with a 24/192 test signal. Again we see the noise floor rise above 35 kHz. A rise in noise at out-of-band frequencies is also seen with SACD, but it is a much larger rise than seen here with the M51.


19 kHz, 20 kHz, 16/44.1 sampling. No peaks around the base of the fundamentals, and no clear B-A peak that can be absolutely distinguished from the noise floor.


19 kHz, 20 kHz, 24/192 sampling. No peaks at the base of the fundamentals. No distinguishable B-A peak.


IMD, 16/44/1 sampling. Just the input signals and a noise floor. Kind of lonesome without any distortion peaks.


And with 24/192 sampling, pretty much the same thing.


Using the USB input on the M51, and 16/44.1 test signals, distortion is way down there. Here, at 1 kHz, THD+N is 0.002%.


With 19 kHz and 20 kHz, there is no visible B-A peak at 1 kHz. There are two peaks on either side of the fundamentals, but they are 97 dB below.


IMD through the USB input was less than 0.002%. Note that the USB input is asynchronous, and it will accept 24/192 music.