NAD M51 Direct Digital D/A converter Measurements

Sidebar 3: Measurements

I used Stereophile's loan sample of the top-of-the-line Audio Precision SYS2722 system to measure the NAD M51 (see and the January 2008 "As We See It"; for some tests, I also used my vintage Audio Precision System One Dual Domain.

Both the M51's S/PDIF and USB inputs successfully locked to data with sample rates of up to 192kHz, including 88.2 and 176.4kHz. Although the TosLink format is specified to work only up to 96kHz, the M51's TosLink input did lock to 192kHz data. The Mac's USB Prober utility revealed the M51's product ID to be "NAD USB Audio 2.0" and that the USB receiver was operating in isochronous asynchronous mode (which means that the DAC, not the computer, controls the D/A conversion timing).

The volume control operated in accurate 1dB steps. With the output level set to 0dB, the M51's maximum output level at 1kHz in fixed output mode was 4.75V from the balanced XLR jacks and 2.375V from the unbalanced RCA jacks, the latter 1.5dB higher than the CD standard's 2V, as JI found in his auditioning. The output impedance at low and middle frequencies was 187 ohms from the XLRs, 141 ohms from the RCAs, these figures respectively increasing to 213 and 153 ohms at 20kHz. I had set the output polarity to Positive with the Menu button; even so, the NAD's output from both its XLRs and RCAs inverted polarity (fig.1). This graph also indicates that the NAD's reconstruction filter is a time-symmetrical FIR type.

Fig.1 NAD M51, impulse response (4ms time window).

Fig.2 shows the M51's frequency response with data sampled at 44.1kHz (cyan and magenta traces), 96kHz (blue and green), and 192kHz (gray and red). The output above the audioband rises before plunging to just below half the sample rate. At 192kHz it peaks by 2.5dB at 80kHz; while I doubt this will have any audible effect, it is unusual. Channel separation (not shown) was superb at >125dB in both directions below 1kHz, and still 113dB at 20kHz.

Fig.2 NAD M51, frequency response at –12dBFS into 100k ohms with data sampled at: 44.1kHz (left channel cyan, right magenta), 96kHz (left blue, right green), 192kHz (left gray, right red) (0.5dB/vertical div.).

The top pair of traces in fig.3 shows a 1/3-octave spectral analysis of the NAD's outputs while it decoded dithered 16-bit S/PDIF data representing a 1kHz tone at –90dBFS. The traces peak at exactly –90dBFS, suggesting low linearity error, and the noise floor is free from AC- or harmonic-related spuriae. These traces actually show the spectrum of the dither noise used to encode the data. Extending the bit depth to 24 gives the middle pair of traces in fig.3: The noise floor has dropped by up to 30dB, which implies resolution of 21 bits! The M51 is among the highest-resolution DACs I have measured, and readily resolved a tone at –120dBFS (bottom pair of traces).

Fig.3 NAD M51, 1/3-octave spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS, with: 16-bit data (top), 24-bit data (middle), dithered 1kHz tone at –120dBFS with 24-bit data (bottom) (right channel dashed).

To remain consistent with the measurements of DAC resolution I have performed since 1989, I used a swept-bandpass technique to generate the traces in fig.3. Repeating the analysis with a modern FFT technique gave a similar picture (fig.4), confirming that there are no harmonic-distortion or power-supply–related spuriae present with 24-bit data. Rerunning the analysis with USB data confirmed that the M51 preserves the 24-bit resolution through this input, though now a trace of third-harmonic distortion, at a mind-numbingly low –120dB, is evident (fig.5).

Fig.4 NAD M51, FFT-derived spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS, with: 16-bit data (left channel cyan, right magenta), 24-bit data (left blue, right red).

Fig.5 NAD M51, FFT-derived spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS, with 24-bit data vis USB (left blue, right red).

With its very low noise floor and very high resolution, the M51's reproduction of an undithered 16-bit tone at exactly –90.31dBFS was essentially perfect (fig.6), with a symmetrical waveform and the Gibbs Phenomenon "ringing" on the waveform tops well defined. With 24-bit data, the M51 produced a superbly defined sinewave (fig.7).

Fig.6 NAD M51, waveform of undithered 1kHz sinewave at –90.31dBFS, 16-bit data (left channel blue, right red).

Fig.7 NAD M51, waveform of undithered 1kHz sinewave at –90.31dBFS, 24-bit data (left channel blue, right red).

The level of the M51's noise floor varied with signal level to a greater extent than the norm. The bottom two pairs of traces in fig.8 were taken with 1kHz tones at –40 and –90dBFS; the noise floor is not affected by the change in signal level. However, with a 1kHz tone at 0dBFS, the noise floor rises by up to 30dB, depending on frequency. (While there is a small effect on the level of the noise, due to the Audio Precision's autoranging input circuitry, this is an order of magnitude less than what can be seen in this graph.) I repeated the noise analysis with a 1kHz tone at 0, –3, and –10dBFS (fig.9). This graph reveals that the largest modulation of the noise floor occurs when the signal rises above –3dBFS. It would appear that to obtain that superb resolution of low-level information, NAD's engineers have sacrificed some dynamic range for the very highest-level signals, which will be relatively rare in all music other than hypercompressed pop.

Fig.8 NAD M51, spectrum of 1kHz sinewave, DC–1kHz, at: 0dBFS (left channel cyan, right magenta), –40dBFS (left blue, right red), –90dBFS into 100k ohms (left green, right gray) (24-bit data, linear frequency scale).

Fig.9 NAD M51, spectrum of 1kHz sinewave, DC–1kHz, at: 0dBFS (left channel blue, right red), –3dBFS (left cyan, right magenta), –10dBFS into 100k ohms (left green, right gray) (24-bit data, linear frequency scale).

The same phenomenon appeared when I examined the M51's harmonic and intermodulation distortion, for which testing I usually use tones at 0dBFS. Fig.10 shows the spectrum of the NAD's output while it decoded data representing a 1kHz tone at 0dBFS. Some odd-order harmonics can be seen—these are low in level, at or below –84dB (0.006%), and were commendably unaffected when I reduced the load impedance to 600 ohms—but not only that, the noise floor rises with increasing frequency. This behavior is reminiscent of DSD-encoded data. Reducing the level of the 1kHz tone to –10dBFS gave a more conventional spectrum (fig.11), with the noise-floor components at or below –150dBFS and each distortion harmonic at or below –120dBFS (0.0001%). However, an odd tone can be seen at 1.5kHz at –124dBFS; perhaps an idle tone of some kind? A similar picture can be seen with the high-frequency intermodulation test, with the 19+20kHz signal at 0dBFS (fig.12) and –10dBFS (fig.13).

Fig.10 NAD M51, spectrum of 1kHz sinewave, DC–10kHz, at 0dBFS into 100k ohms (left channel blue, right red; linear frequency scale).

Fig.11 NAD M51, spectrum of 1kHz sinewave, DC–10kHz, at –10dBFS into 100k ohms (left channel blue, right red; linear frequency scale).

Fig.12 NAD M51, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 0dBFS into 100k ohms (left channel blue, right red; linear frequency scale).

Fig.13 NAD M51, HF intermodulation spectrum, DC–30kHz, 19+20kHz at –10dBFS into 100k ohms (left channel blue, right red; linear frequency scale).

Tested for rejection of word-clock jitter with 16-bit (fig.14, blue and magenta traces) and 24-bit (fig.14, cyan and red traces) versions of the J-Test data, the M51's output spectra were free from any jitter-related or power-supply–related sidebands. With 16-bit data, the odd-order harmonics of the low-frequency, LSB-level squarewaves are at the residual level. Fig.14 was taken with TosLink data; repeating the test with 24-bit USB data gave an identical result, indicating the effectiveness of the M51's asynchronous input mode.

Fig.14 NAD M51, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit data via TosLink from AP SYS2722 (left channel blue, right magenta), 24-bit data (left cyan, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

NAD's M51 Direct Digital DAC offers measured performance that is almost beyond reproach. Color me impressed.—John Atkinson

NAD Electronics International
633 Granite Court
Pickering, Ontario L1W 3K1
(800) 263-4641

mrhyfy's picture

Great write up seems like a fantastic dac. 

I love the Marantz AV7005 and believe it offers outstanding performance HOWEVER I think you might be doing the Nad a deservice by feeding a $2k dac into a $1700 home theater preamp.

The particular problem for me is that the signal from the Nad has to run through the ADC first,then the dsp then the DAC in the Av7005.Ultimately you're reviewing the dac in the Marantz AV7005!!

 I realize that you used the pure direct mode but it is not analogue pass through in this receiver. 

Would it be possible to evaluate the Nad dac with the signal DIRECTLY connnected  to your amp?  That's the whole point of having the volume control feature in the dac.

Jon Iverson's picture

Whenever I receive a DAC that can also run as a preamp (with volume, input switching, etc), I always connect the DAC directly to my power amps. I then compare this sound to  running via the Marantz in Pure Direct Mode. Rarely have I (or those who are listening with me) ever been able to detect a difference.  To compare to other DACs (which do not have a preamp function) I usually run everything via the Marantz - maybe not perfect, but as fair as I can make it.

VandyMan's picture



I love that they included HDMI, but who only needs two HDMI inputs? I need five and I don't have that complex a system. Before someone suggests adding an external switch, keep in mind that they sometimes causes problems. I have two devices that refuse to work when run thru a switch (and I tried several brands). Besides, one of the points of a "home theater" pre-amp, two channels or not, is to act as a master control switch to all sources.

Surge's picture


Thank you for the review Jon.  I would be really interested in knowing how it compares to the AMR DP-777 that was reviewed in March.

I know the AMR is $5,000 vs. $2,000 for the NAD; but I am curious to know if in your opinion it is worth the additional cost.

Many thanks

Jon Iverson's picture

Surge - I have not yet heard the AMR in my system.

raykkho's picture

Thank you Jon for the very exciting review.  One thing though is how can someone merge his M51 with an existing MC home theatre setup, when the M51 does not have any analogue input or HT bypass? I hate to add an AB switch before the poweramp but can't see how it could be done any other way. sad

Amir the Great's picture

last Xmass I did a mistake. Paying 15000SDK for the M51.
it sounds vry good as JA's review, if it souns! The USB Windows driver is a hell. Win8.1 find the dac some times!!!
wrot to NAD. answer: M51 was before Win 8.1!!! and?

How... mush time putin on a product? ok, gott back to win7, same hell. the PC find the dac sametimes!

then they came with "NAD USB Audio Driver for Windows 7 and above" wow the king waked up finally!

but problem was the same. M51 sings when Trump write the order!

Even the JA link above is empty. even if the song finish, efter some minuts the dac dyes! turn itself off! and again Windowes can not find it!

I should turn off the dac Not mr. Stidsen! "is there any body at home?"
shall I teche him a warm bird sings better than a frozen one?

NAD does NOT send the first v1.39 firmware to anybody!!! why? Ayatollah's fatwa maybe.

my 15000 NAD does NOT work properly!

a good product is good in evry manner. NAD isn't an eBay $15 dac who NOT works when you want!

ergonomically NAD is NOT wright either. what happen if U can't find the remote? or battery ...
2 small bottoms at front panel (volume up/down) costs $1. it is the Master NAD isn't? after so many years of iconic amps? oh sorry it is a dac not an amp!

the hole gole is playing music when you want. not waiting for the president to push the bottom.

too tired to play on the NAD.