NAD M51 High Res D/A Converter DAC

For sale is a lightly used NAD M51 direct digital d/a converter.  I am the second owner and have had the unit in my system for the last year.  I believe that the previous owner purchased it about 2.5 years ago.  There are no scratches or obvious signs of use.  Comes with original remote and a standard power cord, but no box or manual.  Updated to the latest firmware. Below are some comments from Stereophile from the following link:  https://www.stereophile.com/content/nad-m51-direct-digital-da-converter

Who wants only a digital-to-analog converter
when you can have a DAC with benefits? How about if those benefits also
come with some high-resolution attitude?

That's what I pondered while setting up the NAD M51 ($2000). Sure, it's a
basic DAC, but it also has extras—like HDMI inputs, remote-controlled
volume, a polarity switch, and one of my favorite features on any DAC: a
display that tells you which sampling rate the thing is locked to.

The honey badger of DACs?


Then there's that attitude: The NAD resamples everything you throw in
its direction and converts it to a pulse-width-modulation (PWM) signal,
the native format for DSD, at a sampling rate of 844kHz, all controlled
by a clock running at 108MHz. It doesn't care if you want this done or
not—it just does it. This is done within a 35-bit architecture using a
similar PCM-to-PWM approach as the company's M2 integrated amplifier,
which John Atkinson favorably reviewed in the March 2010 issue.

So is this in-your-face hi-rez arrogance, or bitstream brilliance for $2000?

NAD claims a key advantage of their approach: that it eliminates jitter
at the conversion stage, and that the subsequent digital filters exhibit
zero ringing. I'll leave it to JA to confirm those claims with his test
gear, while I use my ears and brain to hear what happens when you slice
and dice a lowly PCM input signal into a million little pieces and send
it back out again.

Walkabout

The M51 is an attractive, full-width component with a half-inch-thick
aluminum front panel and a minimalist case of medium gray that remained
cool to the touch at all times. It's surprisingly robust and hefty, with
little going on out front—and not a lot crammed under its thick metal
hood. At the left of the front panel is the Standby off/on button, at
the right an identical button to select inputs. In between is a rather
wide blue-letter vacuum-fluorescent display (with adjustable
brightness!) that you can read from across the room; this indicates
input, volume status, and sampling rate. That last benefit is one that I
wish more DACs had—it comes in handy when you need to verify that, yes,
the USB driver is sending the right resolution through the cable, or
that the file on your server that you forgot to label is in fact 88.2kHz.

At the left of the rear panel are high-quality analog outputs, balanced
and unbalanced; at the right, a socket for the detachable AC power cord
and a power switch. In between are AES/EBU, coaxial, optical, USB, and
two HDMI inputs. To the right of those is an HDMI output, which is
simply a direct pass-through of only the signals from the HDMI
inputs—handy for looping your Blu-ray player into the system to extract
two-channel audio while sending the video along. To the right of that is
a USB jack labeled Upgrade, strictly for updating the M51 via thumb
drive. Then come some automation jacks: RS-232, 12V trigger in, IR in.
All inputs can handle PCM audio data of resolutions up to 24-bit/192kHz.

The sturdy remote control is festooned with buttons and is essential for
setup and volume control, dimming the front panel, and switching
polarity. It also has buttons for NAD transport and CD-player functions,
and makes it easy to directly select inputs via the M51.

Attitude adjustment

I unplugged my reference Benchmark DAC1 USB
D/A converter and set up the NAD M51 with my usual intention: I'd run
it for several weeks as my main D/A converter, getting familiar with its
sound and operation, and then do some serious comparisons. I wasn't
sure how I felt about this new PWM-conversion thing entering my PCM
domain, and wanted time to assimilate into the house this new digital
stranger.

The M51's digital volume control lets you run it as a preamp directly
into a power amp and control your system volume with NAD's remote.
Because of the HDMI inputs and output, you could even use it as a
limited two-channel audio/video preamp, though I didn't try that. The
only drawback I could see using the NAD as a preamp is that there are no
volume buttons on the M51 itself; you'd have to decide where to put the
remote and leave it there. If you lose it, you're screwed.

The M51's volume-control technology is impressive, though. According to
NAD, "The extreme headroom afforded by the 35-bit architecture allows
for a DSP-based volume control that does not reduce resolution. Even
with 24-bit high definition signals, the output can be attenuated by
66dB (very, very quiet) before bit truncation begins."

And, in fact, I found running the M51 via its variable volume control
sounded indistinguishable from its Fixed volume mode, and transparent at
all practical levels. To compare the NAD with my other DACs without
having to worry about the level changing by mistake, I set it up in the
Fixed volume mode and ran the outputs into my Marantz AV7005
preamp. The NAD lets you trim its Fixed output, so I set it to "0dB,"
assuming this would match the output level of my other DACs. The
remote's Mute button still works in this mode, but the volume up/down
buttons are disabled.

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