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Review and Measurements of NAD M51 DAC and Digital Preamp

amirm

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#1
This is a review and detailed measurements of NAD M51 DAC and digital preamplifier. It is on kind loan from a local member. Even though it is still listed on NAD website, other retailers indicate it is discontinued. Looking around, it seems the cost is USD $2000.

The unit has somewhat bland industrial design for my taste:

NAD M51 DAC and digital Pre-amplifier Review.jpg

What distinguishes the NAD 51 from many other DACs are two things 1) custom upsampling DAC and 2) HDMI inputs.

On the DAC portion, this is how NAD describes it:

1549819577390.png


In simpler terms, the sample rate of content is increased to 844 kHz and then what resembles a little class-d amplifier converts it to analog. The higher sample rate reduces the needed bit depth/accuracy in the PWM portion of the DAC. Whatever DSP they use, must have a 35-bit word length and hence the reason that is the accuracy of the volume control.

The second feature, HDMI is something that is very rare in DACs. In HDMI audio is slaved to video so you need a video subsystem to extract the audio. In my past testing of older Audio/Video Receivers, I found that their HDMI performance was much worse than their S/PDIF (sans a very high-end Mark Levinson processor). The NAD M51 gives us another chance to make this comparison and see if there continues to be a reason to avoid HDMI. If not, this will make it convenient to extract audio out of your blu-ray music discs and have high fidelity playback of them.

Let's get into measurements and see if this wizardry translates into better performance and functionality.

USB Measurements
Starting with our typical dashboard in balanced output, we are greeted with competent results:
NAD M51 DAC and digital Pre-amplifier USB Measurements.png


This was not meant to be at first. I always test at 0 dB and performance drops like a rock with that setting. Looking around, it seems NAD found the errors of their ways and in an upgraded version of the firmware, reduced max volume by 1 dB. Of course audiophiles being audiophiles, there is talk online about how much better the old firmware was with respect to bass. JA in stereophile confirms the same:

1549820084460.png


Of course higher volume increase perception of bass so that is understandable. But preferring tons more distortion is not it. NAD was correct in fixing this. You can always turn up your amplifier by 1 dB and get the same effect, without all the distortions.

Anyway, back to our dashboard, we have very good SINAD (signal over noise and distortion) of 108 dB. THD+N beats NAD's spec of 0.0005% in both channels which is comforting. This puts M51 solidly in our tier 2 performance:
NAD M51 DAC and digital Pre-amplifierSINAD Measurements.png


RCA output is nearly as good:

NAD M51 DAC and digital Pre-amplifier USB RCA Measurements.png


And now matches the spec.

Dynamic range lands in the same space with balanced being exceptional:
NAD M51 DAC and digital Pre-amplifier USB Dynamic Range Measurements.png


Good news continues in linearity test where the M51 nails the test:
NAD M51 DAC and digital Pre-amplifier USB Linearity Measurements.png


Usually my audio analyzer struggles to get reliable readings between -110 and -120 dB due to existence of noise. Not here. The measured values were dead on with almost no variation from expected results. This shows very good attention to design. Few DACs do this well.

Jitter is not as good but no audible concern:
NAD M51 DAC and digital Pre-amplifier USB Jitter Measurements.png


Will the good news continue into intermodulation distortion? I am afraid not:
NAD M51 DAC and digital Pre-amplifier USB Intermodulation Distortino Measurements.png


In the past we have seen many instances of this in ESS DACs which I have coined to have "ESS hump." It is a hump because it starts at mid-levels but then goes away as levels increase. Here, it is persistent to almost the end. In that regard this is worse than ESS DACs. I will investigate this further but note the jump in green when I run the test at 0 dB volume. We have a classic, amplifier like clipping occuring at max volume which should be avoided.

Since this IMD distortion is quite pronounced and there have been many questions regarding its audibility, I thought I dig in with more tests.

First, let's look at the spectrum of our IMD distortion at two levels, one where IMD is not dominant and one where it is:

NAD M51 DAC and digital Pre-amplifier USB IMD Spectrum Measurements.png


IMD test is dual tone: a 60 Hz and 7 kHz tone. These are the two tall spikes. An ideal DAC would reproduce those two tones and nothing else. When the DAC is not linear, it will then produce sideband distortion products that show up symmetrically around our main tones.

The blue spectrum is the distortion with digital samples level at -45 dB before our IMD distortion start to rise up. We see our noise floor in blue and maybe a few spikes hugging our main tones closely. When I increase the digital value up to -25 dB, the sidebands (in red) multiply and substantially rise in level. We also get a mirrored cluster of them around 14 kHz. the IMD graph above simply sums these and hence the reason distortion rises substantially. There is simply a lot more junk there.

Taking our 7 kHz tone, its level is about -32 dB. The tallest distortions are -108 dB. The difference is 76 dB. Put another way, our distortion is 76 dB below our music level. That makes it very difficult to hear.

Let's look at the same problem, this time with a 1 kHz tone:
NAD M51 DAC and digital Pre-amplifier USB 1 kHz spectrum Compared with -25 dB Measurements.png


This time the graph in red is at full digital amplitude. We see a spray of red spikes indicating harmonic distortion. We also see a rise in noise floor which is typical of noise-shaping which would be mandatory as the bit depth is decreased. What is not expected is continued distortion spikes all the way up to 90 kHz. These should be well below noise floor as it increases.

The related problem to IMD is seen when I *decrease* the signal level to -25 dBFS. Ordinarily decreasing the source level should result in less distortion. Not here. We actually have higher harmonic distortions than at full scale (in blue)! The differential is similar to what we computed in IMD test. The main difference being that perceptual masking would not hide them as harmonics continue at full level until nearly 8 kHz or so.

Again, this lands in difficult to hear category but something is seriously broken. WIth music that is recorded well below max 0 dBFS, you are liable to increase the volume to compensate and then, you will have a better chance of hearing those artifacts.

Back to our suite of measurements, the noise shaping and harmonic distortions in ultrasonic range hurt THD+N versus frequency with 90 kHz bandwidth:

NAD M51 DAC and digital Pre-amplifier USB THD vs Frequency Measurements.png


The measurements actually became so noisy that the analyzer gave up a few times (as indicated by "T" symbol and sudden jump). Even without that, we have much elevated THD+N compared to "ordinary" budget DACs that cost less than the shipping for the NAD M51.

Multitone test was not very revealing other than not being able to get a stable reading every time:
NAD M51 DAC and digital Pre-amplifier USB Multitone Measurements.png



HDMI Measurements
I had a hell of a time finding a way to access HDMI audio. My normal monitor is 4K and the NAD being an older unit of course doesn't support that. I reduced resolution to 1080p, 1080i and even 720p and no cigar. I would get no signal from my monitor when I routed the output through the M51. By itself connected to the computer was no problem so it was not a capability issue of my monitor. I dug out another cheap PC monitor I had sitting around and that worked at 720p.

Next problem was how to play audio to the HDMI port. My Nvidia card shows up as a sound device but ASIO4ALL does not see it. WIth no native ASIO interface, I was out of business using the Audio Precision analyzer software to drive the port. Fortunately I could access it using WASAPI exclusive mode using Roon media player. That limits me to a couple of specific tests so let's get into that.

FIrst is the dashboard:

NAD M51 DAC and digital Pre-amplifier HDMI Measurements.png


Ah, most excellent! Performance is nearly identical to USB!

Jitter test is where I had seen issues with HDMI but not here:

NAD M51 DAC and digital Pre-amplifier HDMI jitter Measurements.png


The graph is a bit more busy than USB but nothing audible.

I did run into a problem here though. Using fixed output I was just getting microvolts output. I switched to variable and all was well. Not sure what is going on there.

Conclusions
I am always concerned about custom DAC implementations as reimplementing the wheel is not easy. The NAD M51 at first showed me they had gotten the basics right with very low noise and good distortion metrics with full scale signal. Alas, that fell apart when we look at distortion and noise at levels below max. There is a sharp increase in distortion, costing us some 30 dB or so in performance. What a shame.

The NAD M51 gives me hope that there are more good HDMI implementations out there. Crossing fingers as I test more units already here for review on that.

Given the mid-level distortion I can't quite recommend the NAD M51. If you need HDMI connectivity though, and can get this unit on the cheap in secondary market, it may work out well.

------------
As always, questions, comments, recommendations, etc. are welcome.

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daftcombo

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#2
Thank you for that review! Another unit where you pay more and get less, that's a shame.

I would also be interested in the measurements of the Nad D1050, much cheaper, if you have the opportunity to get one.
 

amirm

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#3

amirm

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#6

PierreV

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#7
Hmmmm, that behavior seems to be exactly what they claim to avoid by extending the data path to 35-bit to implement a perfect digital volume control. From what's available around, it seems they are based on the CSRA6601 (which itself seems to be a rebadged Zetex)

https://static.qobuz.com/info/IMG/pdf/SCCLZBR1.pdf
 

graz_lag

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#8
Sold mine just before Xmas @ €900, hoping my buyer does not see this review, he cannot read English though so I feel quite safe in any case ... :facepalm:
And happily replaced with the Topping DX7S ...
 
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rajapruk

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#9
If I remember correctly, Stereophile magazine measured this dac with spectacular good results. But maybe IMD distortion on lower levels was missed there?
 
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graz_lag

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#10
If I remeber correctly, Stereophile measured this dac with spectacular good results.
Correct, and that was the only very reason that convinced me to buy it, as second-hand unit though ...
 

graz_lag

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#13
Why did you sell it?
I bought the Topping DX7S after reading the thread here, with the goal to try a well reviewed DAC as my 1st experience with Chinese DACs ...
Tested in parallel to the M51 for some time, I could have lived with either one, decided to sell the M51 because of its value on the 2nd hand market.
 

amirm

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#14
If I remember correctly, Stereophile magazine measured this dac with spectacular good results. But maybe IMD distortion on lower levels was missed there?
The results actually don't match what JA concluded with that you note. Still, he only tested the extremes and did not address the mid-levels we do with IMD tests.
 

garbulky

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#15
I assume the volume control is digital. So if NAD reduced the digital control by 1 db essentially haven't they changed the digital data by doing so? Would this be a cause for the distortion you saw? I remember why they did reduce it by 1 db it's because at 0 db it all went to hell. The designer thought the uh, added dynamics was worth the trade off or something.
 

Blumlein 88

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#16
The results actually don't match what JA concluded with that you note. Still, he only tested the extremes and did not address the mid-levels we do with IMD tests.
In his review he got poor IMD at zero level. He posted results for IMD at minus 10 db which still wasn't any great result. The 32 tone test here was not terrible probably from the lower level of each tone. Seems like the analog section is poor at higher signal level.
 
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amirm

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#17
I assume the volume control is digital. So if NAD reduced the digital control by 1 db essentially haven't they changed the digital data by doing so?
Well, they always change the data by resampling and volume control is somewhere in that pipeline.
 

m8o

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#18
Much thanx for digging in to this unit!

So to fully understand the context, as this unit showed distortion setting in at max input signal when its volume was set to 0db, this unit is still using v1.39 of the firmware?
 

amirm

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#19
Much thanx for digging in to this unit!

So to fully understand the context, as this unit showed distortion setting in at max input signal when its volume was set to 0db, this unit is still using v1.39 of the firmware?
Yes.
 

m8o

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#20
That is a really old firmware. I can't help but wonder about what in the conversion is done in software (can be made better in firmware updates) and what in hardware (unaffected by firmware updates). I would be surprised if the poor 'mid-level signal' IM distortion you measured would be made better with newer firmware, but a guy can hope. Lol. I do keep my firmware up to date. I got mine on the resale market so the 'sting' of less than perfection isn't quite as bad as it may be to others. ;)
 
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