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Review and Measurements of NAD M51 DAC and Digital Preamp

Techsmart

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#22
Very happy to come across your test results! Bought brand new M51 a year ago and used very little as Im not sure it benefits me as my only possible sources would be bluray, cable box and an Integra Pre/Pro. Have tried experimenting with it from cable box to M51 to pre/pro via hdmi and not sure Im hearing any benefits. Really want to like this unit and make it worth the grand I paid but im on the fence about selling it.

My intention was to use it at max volume as I control volume on pre/pro. Read in one of the reviews( maybe Stereophile) that it was performed better stepped back a little from max. Anyhow, based on what Im reading here with your results wont I get least amount of distortion keeping it on max and then I have a unit that could shine and be recommended as a keeper?

One last thing if I may ramble a little more.... the comments from Garbulky and Manbeard seem to be important about the firmware update. JA said the unit at max performed better with the newer firmware with even less distortion if Im reading things right. And I think you or someone mentioned that
 

Techsmart

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#23
You can turn up the volume on the amp to compensate. Would love to see you do the tests with the new firmware to see if this makes this a recommended unit or at least a unit I can be happy with at max volume.
 
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#26

m8o

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#27
Very happy to come across your test results! Bought brand new M51 a year ago and used very little as Im not sure it benefits me as my only possible sources would be bluray, cable box and an Integra Pre/Pro. Have tried experimenting with it from cable box to M51 to pre/pro via hdmi and not sure Im hearing any benefits. Really want to like this unit and make it worth the grand I paid but im on the fence about selling it.

My intention was to use it at max volume as I control volume on pre/pro. Read in one of the reviews( maybe Stereophile) that it was performed better stepped back a little from max. Anyhow, based on what Im reading here with your results wont I get least amount of distortion keeping it on max and then I have a unit that could shine and be recommended as a keeper?

One last thing if I may ramble a little more.... the comments from Garbulky and Manbeard seem to be important about the firmware update. JA said the unit at max performed better with the newer firmware with even less distortion if Im reading things right. And I think you or someone mentioned that
Edit: After refreshing my memory of the operation of the 124a from the manual, I've corrected the dBV references below. My use of -10dBV was incorrect for the onset of distortion (seemed low, and it was, by 20dBV).

You bring up some important use cases for me. I didn't go into it before because I didnt want to sound like I'm defending something found to have failings simply because of pride in ownership. But I am one that probably uses this the most with a feed from a cable box. I use the volume control, in 'amplification mode' as HDMI level is significantly under line level. And I have an Aphex buffer in the chain, which visibly 'monitors' for overload. So I'll just share my experience with the community.

So, my bedroom/office headphone listening/work seating area is at the far corner on the other side of the room from the corner where my cable box is. When I had the room redone, I ran two 50 foot runs of Mogami 4-wire mic cable, and one 50 foot run of a high speed flat hdmi cable, all in the wall under the baseboard molding. The progression of 'feed' to my headphone amp was as follows:

1. First used RCA out (unbalanced) from the cable box to my headphone amp via one Mogami 4-wire cable. This yielded hum city. Surprisingly, if I used a FiiO K7 dac-amp docking station as a kind of in-out buffer (no amplification), it squelched the hum. No doubt thanks to the ground lift the external walwort PS provided. But regardless, the subjective sound quality of line out from the cable box was noticeably "soft and mushy".

2. TOSLINK out from the cable box to a DacMagic Plus sitting near the cable box. Then either balanced or unbalanced out thru the Mogami cables, to my headphone amp. Nice, powerful, authoritative sound. When I wanted tone control, I'd run unbalanced from to the aforementioned 'buffer', to a tone control, to the headphone amp. Yielding sweetened quality authoritative sound.

But still, I'd never be happy. I found if I played with the selected Dac's filter, or phase, it would have an effect on the presentation. There was always a (to use a highly technical term) "schmearing" of the detail present. It was musical and enjoyable. Just lacked the detail I felt should be present.

3. Once I got the M51 in the rack, I was finally able to make use of the 50 foot HDMI cable I had in the wall. When I listened to the recordings I had sitting on the DVR for years (Adele in NYC, some SNL performances, some late night and last call TV show musical performances) _THERE_ was the detail I'd always been longing for from the DacMagic Plus. Attack and decay of each note was so much faster, without any loss, rather had an increase, of musicality and enjoyment. Now I know this is cliche to say, but neverthless in some of the "pop" recordings I had many instances of "oh, I never heard that instrument's melody", or "oh, that is both a guitar and a synth, not just a synth" when the M51 was the DAC in the chain. (Fed by HDMI, but this test shows that that is thankfully not a factor). I am still a real fan of this DAC.

Now, on to "volume"...

What I also found was that the level out from my M51 was lower when input was hdmi vs. any other digital input. I found the cable box was the source of the attenuation. But on the plus side, this also served as a test bed for me to experiment with the volume control of the M51. I knew from before owning the M51 that people said it could distort easily if you put volume above 0db. I was able to confirm this with USB or spdif in, and my ears.

I split the unbalanced output of my M51 out. One of the lines drive an Aphex 124a buffer (used to get a unbalanced signal from a balanced one, or a balanced signal from an unbalanced one, as necessary). When there is any audible distortion on drum hits or heavy bassline beats, it coincided with the 124a's overload light lighting up. (Overload Light lighting up does not denote actual overload of the 124a's input. It clips at +15dBV, and the light activates at +10dBV)

So I can confirm if I am using the M51 (using v1.50 of the firmware) fed by a digital signal that reaches 0bBFS, and set volume to +1dB or greater, [audible] distortion can set in on those loud drum hits or bass beats, coinciding to my Aphex buffer's overload lights lighting up.

However if the digital input is attenuated, as I find it is from my cable box, I can a) rely on my Apex 124a buffer to tell me when unbalanced output exceeds reference, and b) reliably use the volume control in the M51 to amplify up to whatever is necessary let's say +8dB, without any audible distortion. But if the Aphex 124a's overload light is lighting, I'm hearing distortion too. I'll back the M51's volume down a dB or two.

This is all to say, the M51 can provide clean [to my ears at least] amplification of the converted digital signal in the analog domain well, but only as long as its output does not exceed its specified nominal output.

So I am saying feel free to use that volume control. But be very aware of the [digital] level of the input. Only go over 0dB (with firmware over v1.39, or -1dB at v1.39) if you know the digital level is attenuated. (Like if you're using ROON and using its DSP based volume control, or like my and maybe your cable box that outputs an attenuated audio signal over HDMI)

To give context, this is my main headphone rig where all these pieces are in play...

20171227_135247.jpg
(click to see larger)
 
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amirm

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#29
Upgraded to latest firmware and ineed, the 0dB level problem is resolved. Here is the output at full scale:

1549929217907.png


Notice that voltage is the same as the old firmware with volume = -1 dB.

There is no performance difference in IMD:

1549929366814.png
 

m8o

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#30
But it really shows that Topping DAC that's a fraction of its cost who is boss between -2.5 - 0.0 bBFS. :p :facepalm:

Thank you for the add'l effort. It definitely puts any curiosity to rest. I did really dislike seeing that spike before the 2nd harmonic in the previous dashboard. Not nutz about seeing all those higher order spikes still there either tho.
 
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#33
Hey @amirm, would you say that what we're seeing in the ultrasonic range on the NAD M51 is similar in characteristic to what you saw on the DAC DAC? Just wondering if I'm reading these results right.

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.
It makes me wonder whether it's just a coincidence that two custom DAC designs both show flaws in the ultrasonics, or whether that might point to some deeper problem that some custom designs tend to overlook...
 
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#34
But it really shows that Topping DAC that's a fraction of its cost who is boss between -2.5 - 0.0 bBFS. :p :facepalm:

Thank you for the add'l effort. It definitely puts any curiosity to rest. I did really dislike seeing that spike before the 2nd harmonic in the previous dashboard. Not nutz about seeing all those higher order spikes still there either tho.

The M51 was released in 2011/2012.
It was the 1st dac to feature HDMI, USB 2.0 and 24/192 via spdif including optical and have RCA plus XLR support.
The digital volume control is one of the best available, for 2012 if you had to get a Preamp with some fancy volume control, you’d be up for more dollars. In 2012 it possibly raised the bar in what it supported, not many stand alone dac had all of these features and not many support 24/192 to spdif inputs. I still have it and use FW1.39. I don’t listen to it at 0db, -1db or -2db for some strange at lower levels with 1.39 the LF is note detailed and has more weight and feel, with 1.41 it’s a lifeless dac, never tried 1.50.
The digital volume control can go to -65db with no concern loss of detail, now try that with a Topping D50 using the volume implemented in the Sabre chip. Not that I’m going to even if I have both..... becuase I did the comparison with 105, however becuase this site is all about measurements, takenthat with a grain of salt!
 
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#35
The M51 was released in 2011/2012.
It was the 1st dac to feature HDMI, USB 2.0 and 24/192 via spdif including optical and have RCA plus XLR support.
The digital volume control is one of the best available, for 2012 if you had to get a Preamp with some fancy volume control, you’d be up for more dollars. In 2012 it possibly raised the bar in what it supported, not many stand alone dac had all of these features and not many support 24/192 to spdif inputs. I still have it and use FW1.39. I don’t listen to it at 0db, -1db or -2db for some strange at lower levels with 1.39 the LF is note detailed and has more weight and feel, with 1.41 it’s a lifeless dac, never tried 1.50.
The digital volume control can go to -65db with no concern loss of detail, now try that with a Topping D50 using the volume implemented in the Sabre chip. Not that I’m going to even if I have both..... becuase I did the comparison with 105, however becuase this site is all about measurements, takenthat with a grain of salt!
Yes, but ...
If you read a previous posts of mine -- heck you only need to scroll up to #27 -- you'll gather I too am a great big fat fan of the M51. I'm using it right this second listening to the performances of this year's Grammy's.

But I'm not going to ignore reality either. So many of these circulate on the secondary market for a reason. What that reason was, I never experienced. I certainly didnt agree; never selling mine; actually I wanted to pickup a second one. But I'm not going to be an apologist now either when I see emperical evidence suggesting there are tones present in the playback that are artifacts of the processing that far cheaper dacs do not produce that some can hear.
 
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#36
Yes, but ...

But I'm not going to ignore reality either. So many of these circulate on the secondary market for a reason. What that reason was, I never experienced. I certainly didnt agree; never selling mine; actually I wanted to pickup a second one. But I'm not going to be an apologist now either when I see emperical evidence suggesting there are tones present in the playback that are artifacts of the processing that far cheaper dacs do not produce that some can hear.

There’s plenty of them circulating in the 2nd market becuase they flooded the market, on one site the listing for M51 in that classified was around 20 last year alone. The RRP here in Aust was $1500. That was cheaper than in EU or US. At the end of product cycle the were down to $900-1100AU as dealers tried to moved then on, then the prices shot up for brand new! It’s been 7years and you are going to see heaps of these flooding the market as new products shush as Chord etc enter this segment.

Just because there artifact present they may and may not be audiable to some, I do know if you play it at -2db and Definately at 0db you’re going to hear distortion, like I say to everyone; even the products I work on is to work within its limits! I know someone who changes dac more than he changes his underwear, he bought a Topping D50, when he got it he SMS to say that it was a brilliant dac, lovely to listen to, 7hrs later he had it in the classified to move it on, he’s sticking with the Theta dac he bought 2nd hand!
 
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