John Atkinson reviewed the M51 in May 2014 (Vol.37 No.5):
I've been using NAD's Masters Series M51 Direct Digital D/A processor ($1999) as one of my references since summer 2013. Jon Iverson had enthused about the M51 in his July 2012 review, concluding that he preferred DACs "that reveal as much as possible about what was captured on the tape or in the digits, and couldn't care less about adding a rose-colored tint to dodgy digital sound. In this regard, the NAD M51 succeeds with a wonderfully detailed and revealing sound best described as honest, with a friendly smile." My own experience of the M51 (my sample was serial no. H33M5103771; his was H1XM5101162), driven by the NAD M50 Digital Music Player and Digital Music Vault M52, echoes Jon's. It is transparent to recorded detail without the sound becoming, in that classic phrase, "ruthlessly revealing."
Compared to the half-again-as-expensive Electrocompaniet Classic ECD 2 ($3100), which I reviewed in December 2013, the M51's sound was lighter balanced, with less authoritative low frequencies but with a deeper soundstage, longer reverb tails, and a filigree retrieval of fine detail. Playing my 2000 recording of Robert Silverman performing Beethoven's Piano Sonata 18 (24/88.2 master AIFF file from CD, OrpheumMasters KSP-830), the NAD was slightly better at decoding the ambient cues and the leading edges of the piano's percussive character. In direct comparison, the Electrocompaniet's overall presentation of the Bösendorfer was more true to the sound of the piano than the recorded acoustic, I felt.
When, in February 2014, I reviewed the Auralic Vega ($3499)—which is almost identical to the NAD in its feature set, including a digital-domain volume control—the Vega gave more weight to Jaco Pastorius's subterranean bass-guitar notes on a 24/96 needle drop of "Overture—Cotton Avenue" from Joni Mitchell's Don Juan's Reckless Daughter (LP, Reprise K63003), while the M51 focused more on the sound of Mitchell's open-strung acoustic guitar. Ultimately, the overall sound was a little more airy through the M51, with a cleaner if leaner balance. I ended up preferring the Auralic processor, but the NAD gets close to it at 57% of its price.
A poster to our website reported that the M51's earlier v1.39 firmware is the best sounding. Turning on the M51 with its Input button pressed revealed that my sample was running the v1.41 firmware, which is supposed to have less bass and to sound not quite as dynamic as v1.39. I downloaded the v1.39 firmware here and rolled the M51 back to that version by loading the code onto a USB stick, plugging it into the Upgrade USB port on the M51's rear panel, and following the instructions in the appropriate NAD software bulletin. All appeared to go correctly, and turning on the M51 with the Input button pressed now indicated that the v1.39 firmware had been installed.
This is hardly something for which you can perform an A/B test, but listening to the Thelonious Monk–influenced "Rainbow's Cadillac," from a downloaded Bruce Hornsby concert from Oregon Zoo, in Portland (16/44.1 FLACs), I discovered that the reader had been correct. The bass guitar and kick drum did indeed sound a little fatter than with the latest firmware. This was a benefit with the Joseph Perspective and Wilson Alexia speakers I used in my listening, but not with the Vivid Giya G3s. Who'd a thunk it? (footnote 1)—John Atkinson
Footnote 1: In his "Manufacturer's Comment," NAD's Greg Stidsen wrote that "Software v1.41 dropped the gain by 1dB, to give that little bit of extra headroom. While 1dB may be at the threshold of human perception, a keen ear will hear the slightly louder playback of v1.39 as "better." Other changes in v1.41 include a revision of the Polarity setting logic, and the addition of an auto standby to comply with EC energy policy. No changes were made that would affect frequency response or dynamics." I reloaded the v.1.41 firmware into my sample of the M51 and rechecked the maximum output level at 1kHz. It was indeed exactly 1dB lower than it had been with this sample loaded with the v.1.39 firmware and with Jon Iverson's original review sample.—John Atkinson
Footnote 1: In his "Manufacturer's Comment," NAD's Greg Stidsen wrote that "Software v1.41 dropped the gain by 1dB, to give that little bit of extra headroom. While 1dB may be at the threshold of human perception, a keen ear will hear the slightly louder playback of v1.39 as "better." Other changes in v1.41 include a revision of the Polarity setting logic, and the addition of an auto standby to comply with EC energy policy. No changes were made that would affect frequency response or dynamics." I reloaded the v.1.41 firmware into my sample of the M51 and rechecked the maximum output level at 1kHz. It was indeed exactly 1dB lower than it had been with this sample loaded with the v.1.39 firmware and with Jon Iverson's original review sample.—John Atkinson
