Digital Processor Reviews

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.

Figuring that all was well, I ran some music from the Meridian Sooloos Control 15 (footnote 1) via the coax input and noticed right away that instruments jumped out at me a bit more than I was used to at my standard preamp volume setting. Wow, this DAC was sounding different already! I plugged the Benchmark DAC1 back in and the level sounded more normal. I then got out my RadioShack SPL meter and ran pink noise from Test CD 2 (Stereophile STPH004-2) through the system. Sure enough, the NAD, set at "0dB," measured 1–2dB louder than the Benchmark at my listening position. (JA can check this in his tests.) I reduced the Fixed level by 1dB and all sounded normal again.

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Footnote 1: Meridian's line of Sooloos products has been officially renamed Meridian Digital Media Systems, but I just can't bring myself to use such an unwieldy and generic-sounding title. Hopefully they'll change it back.