Here are my 2 cents about the NAD M51...
Last week I received my new (used) M51 and connected it to both my setups with several different inputs and file qualities.
The first setup is my test setup, it has good gear except for the speakers. The used gear is as follows:
PC with files from MP3 to 24/192 FLAC connected with USB
Decware CSP2+ tube pre amp
Bel Canto S300 power amp
Mission 'something something' cd player connected with digital RCA
Pioneer SP-BS22 speakers
PS Audio UPC200 power conditioner
The cables are generic or basic cables, nothing fancy.
The inputs I used were USB and Digital RCA.
Ouputs used were RCA to the Decware pre amp and XLR directly feeding the S300 power amp.
Eagles, Hotel California 24/192 FLAC
Norah Jones, Come Away with me 24/192 FLAC
Metallica, Nothing Else Matters 24/96 FLAC
Lenny Kravitz, Are you gonna go my way, crappy MP3
Red Hot Chili Peppers, Under the Bridge CD
Smashing Pumpkins, Siamese Dreams entire CD
I compared the M51 with a Musical Fidelity M1DAC (MF). The MF M1DAC is one of the most neutral DACs I have ever heard and will be a good base line.
After playing my test songs on the MF M1DAC and getting used to that sound I connected the M51 and played the same songs and what I heard was both surprising and I was a little disappointed... (the disappointment has more to do with my taste as you'll read later)
The pre-review info will be longer than the actual review...
Biggest surprise was how it handled the crappy MP3, there was a HUGE improvement over the detail heard compared to the MF and it sounded less 'digital'.
The low end sounded good, it was there, it sounded nice and tight, but it was missing some 'oomph'.
Mid range was great, voices sounded clean and very separated from the instruments, which is a weak point of many DACs that I have heard.
High range was crisp, but way too clean for my taste, crazy detailed. I heard more little things than I remember hearing with any other DAC.
From an audiophile point of view this DAC is the best I have heard, beating the MF, PS Audio PWD, Bryston BDA-1 and Rega DAC in detail, instrument separation and again.. DETAIL! You will hear little nuances hiding in the music that stayed hidden with most other DACs.
From a music lover point of view this DAC lacks some emotion, some character, warmth. You miss that little raspy sound in a voice, the sound of the fingers touching (plucking) the snares of the guitar. Qualities that the Bryston and Rega definitely have.
This DAC will divide the camp in 2 groups:
1. the lovers of detail and pure sound quality, that will rave about the M51.
2. the lovers of a more 'live music' sound that has more warmth and might not sound as perfect.
At that point I was still divided, until.....
I added the M51 to my main setup:
Oppo 103D as cd transport Optical
Sonos ZP90 with W4S 96kHz mod Digital RCA
Primaluna Dialogue 2 with KT120 tubes
Goldenear Aon 3 speakers
All audio cables are Anti-Cables
All Power cables are Wireworld Silver Electra
API power conditioner
I played the same songs, but all at 16/44.1 (cd) quality.
To compare I used my faithful Bryston BDA-1.
First I played all the songs with the BDA-1 in place and it sounded like perfection (in my ears). Not the greatest detail, but when you close your eyes you are in that 'smokey bar' and the band is there playing in front of you.
Plugging in the M51, letting things warm up and playing the same songs I could not believe what I heard....
Compared with my other setup and test this sounded so lean, thin, uninteresting and bland... WTF!
I disconnected everything, checked again if everything was connected right and working as it should. Played the same songs again and crap.... well.. crap? not crap, but not what I heard in the other setup.
The bass was almost gone, mid range was still very good and the high end was real sharp. Don't get me wrong, the incredible amount of detail was still there. I was still hearing little details that I did not hear with the BDA-1.
I think that in this setup classical music will shine a whole lot more than what I prefer to listen to. Just from a detail and separation of the instruments point of view.
It may sound like that I am biased and will pick the BDA-1 over about anything, but I am looking for a serious replacement for the BDA-1 and was critically listening to both DACs.
I will keep on playing with the M51 over the next few days and see if things change after more time settling in and if anything changes I will let it know...
I hope it all made sense...
After a week I found a helpful tip on a forum. Play with the volume control of the M51 until you hit your sweet spot. With the volume turned down on the M51 and the volume on the amp turned up the issue I had with the M51 disappeared. It became more fun to listen to, bass was more present and mid range more realistic.
The M51 is still in my system as we speak...