Integrated Amp Reviews

When I asked NAD for a sample of their Masters Series M50.2 digital music player, which I reviewed in the December 2017 issue, they also sent me a Masters Series M32 DirectDigital integrated amplifier, which had also been introduced at the 2017 Consumer Electronics Show. Costing $3999, the M32 offers a continuous power output of >150W into 8 or 4 ohms. The M32 is the same size as the M50.2, and its smart-looking combination of matte black and gray-anodized aluminum panels make it look identical to the player, except for the black volume-control knob to the right of the front panel's four-color touchscreen, and the ¼" headphone jack at bottom left. It even has the same eight ventilation grilles inset in the black top panel.

Design
The M32 features NAD's Modular Design Construction, in which a back-plane topology allows up to four extra modules to be installed for additional functionality. While the M32 comes with a USB port and digital outputs, the usual AES/EBU and optical and coaxial digital inputs are hosted on the MDC S/PDIF module. The optional MDC DD-BluOS module ($399) provides Bluetooth aptX and Roon Ready Ethernet connectivity. This module was installed in my review sample of the M32; another MDC module, the DD-HDMI-1 ($299), is available but was not supplied to this home-theater–averse reviewer. As well as the headphone output, there are line-level output jacks that can be set to full-range preamp mode, or to output a low-pass–filtered signal to a subwoofer amplifier.

The heart of an amplifier is its output stage. While NAD's Masters Series M22 power amplifier, which Kal Rubinson reviewed in March 2016, uses a customized version of the Hypex NCore class-D amplifier module, the M32 uses what NAD describes as a "DirectDigital(tm) Feedback Amplifier built under license from Cambridge Silicon Radio (CSR), which uses a form of digital error correction . . . converting to analogue at the speaker terminals. This gives it the shortest signal path in NAD's history."

This closed-loop digital amplifier operates at a very high sample rate: 844kHz. But while the digital signal path may indeed be short, what happens with the analog inputs? There are two line-level inputs and one MM-compatible phono input. All are converted to 24-bit digital, with the sample rate, up to 192kHz, chosen by the user. The Bass, Treble, Balance, and 0.5dB-step Volume controls are all implemented with DSP, so the signal path is identical for the digital and analog inputs.

GUI
While a smart-looking metal remote control is supplied—it awakes and lights up in soft blue when picked up—the M32's ergonomic glory is its front-panel touchscreen. An intuitive menu system permits: source selection; setup options for each source, including polarity and whether to use fixed or variable output level; setting the sample rate for the analog inputs and digital output; the matching of the output stage for loads ranging from 2 to >8 ohms; reversing the channels; and setting the amplifier to conventional stereo operation or to mono (Art Dudley would like that!).

Setup
Though I did try its analog line inputs (though not its phono input), I used the Masters M32 with digital sources for my long-term auditioning. Because I prefer to use short speaker cables, the M32 was placed between the various speakers I used with it, and I at first used a 15' plastic optical link from NAD's M50.2 media player, which sits in my rack. However, I was suspicious that the plastic TosLink connection wasn't getting the best from the M32. So, as I didn't have a sufficiently long coaxial S/PDIF cable, I replaced the TosLink with an equally long AES/EBU cable, using a Meridian 518 in bypass mode to convert the M50.2's S/PDIF output to AES/EBU. (The Meridian works only at 44.1 and 48kHz, however.)

For my second round of listening, I connected the M32 to my home network using the Ethernet port on its MDC DD-BluOS module and fed it audio data using the BluOS app on my iPad to play files stored on the M50.2. As the M32 is Roon Ready, I then played files from a Mac mini running Roon 1.3—and then, when the Mac mini died, from a Roon Nucleus+ server (in for review) running Roon 1.4. (Both the Roon and BluOS apps allow the M32's volume control to be operated from the iPad.)

My auditioning comments are an amalgam of all four operating modes.

The M32's plastic-shrouded speaker terminals wouldn't accept the bulky spades fitted to AudioQuest's K2 cables, so I used brass spade–4mm plug adapters from Michell Engineering.

Listening
Reviewing modern amplifiers is difficult. Unless they are perverse—single-ended triode designs come to mind—all the first-order characteristics, such as frequency-response variations and output-impedance interactions, tend to be indistinguishable from each other. So it is now second-order behaviors—eg, distortion signatures and noise modulation—that characterize sound quality, and these things take a lot of listening to identify. I dropped the M32 in and out of my system during listening sessions for other components I've been reviewing recently, and what emerged as a consistent signature was a clean, clear quality of sound that stepped out of the music's way with every pair of speakers I used.

The M32's treble balance reminded me of NAD's Masters Series M51 D/A processor, which Jon Iverson reviewed in July 2012. JI had summed up the NAD M51 as offering "a wonderfully detailed and revealing sound best described as honest, with a friendly smile," and when I wrote a Follow-Up to his review in May 2014, I described the M51 as being "transparent to recorded detail without the sound becoming, in that classic phrase, 'ruthlessly revealing.'" That is how I would describe the M32. I was listening to this issue's "Recording of the Month," John Butt and the Dunedin Consort's small-forces performance of Monteverdi's Vespers 1610 (16/44.1 WAV files from CD, Linn CKD 569), and realized that the amplifier was doing a great job of differentiating the sound of the baroque trumpet from that of the cornett. The cornett is a woodwind instrument with a conical bore and finger holes like those of a recorder, but is blown with a trumpet-like mouthpiece. I used to play the cornett back in the day, but my asthmatic lungs ultimately couldn't deliver enough breath. I still love its sweet tonal color, less aggressive than a brass trumpet's.