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M2 Direct Digital Amplifier - Stereophile Review

March 10, 2010

NAD M2 Direct Digital Integrated Amplifier

By John Atkinson • March, 2010

A decade ago, many predicted that amplifiers with switching or class-D1 output stages would come to dominate high-end audio. In a post–Peak Oil world in which the price of energy would always continue to rise, a class-D amplifier's very high efficiency in converting AC from the wall outlet NAD M2 Direct Digital Integrated Amplifierinto speaker-driving power would be a killer benefit. Although a conventional push-pull class-B amplifier has a theoretical efficiency of 78.5%, which would seem usefully high, this efficiency is obtained only at the onset of clipping; the need for the output devices to carry a standing bias current reduces that efficiency considerably, typically to around 50%. Class-A amplifiers are even less efficient, with a maximum of 25%; ie, three times as much power is dissipated by the amplifier as waste heat as is used to drive the loudspeaker (see "Sam's Space" in this issue).

"[M2 is] a paradigm-breaking component."

By contrast, a class-D amplifier can, in theory, be 100% efficient, and practical circuits are at least 90% efficient. Watt for watt, all of the expensive parts of an amplifier design—the power transformer, output-stage heatsinking, and chassis—can be smaller and thus less expensive: a high-power class-D amplifier can be small in size, light in weight, and cheap. Class-D amplifiers appear, therefore, to have had every competitive advantage. So why do audiophiles still mostly buy and use amplifiers with class-AB output stages?

The answer is the ultimate sound quality—so far, class-D amplifiers have found widespread domestic acceptance only as subwoofer amplifiers. There are exceptions: Bruno Putzeys's Hypex modules, as used in the Channel Islands amplifiers, have their followers; PS Audio's GCC-100 is a favorite of Stereophile reviewer Robert Deutsch; and the latest generation of Bel Canto's e.One Reference 1000 monoblocks are to be found in "Music in the Round" columnist Kalman Rubinson's system.

Still, when I encountered NAD's Masters Series M2 class-D integrated amplifier, which sells for a respectable $5999, I was torn between respect for the technology its design demonstrated and skepticism about its ultimate sound quality. How good could a class-D amplifier sound? The only way to find out was to review it.

The Masters Series M2

Although it's convenient to refer to the NAD M2 as an integrated amplifier, it's actually something rather different: the M2 is a multiple-input D/A converter with an output stage that can drive a loudspeaker. Although it has two pairs of analog inputs—one pair single-ended on RCAs, the other pair balanced on XLRs—these are immediately converted to 24-bit digital, with a user-selectable sample rate of 48, 96, or 192kHz. Sources are selected with the buttons below the front panel's blue fluorescent display or with the remote.

Both digital input signals and the converted analog input signals are fed to a digital signal-processing section, this specified as having an internal data path 35 bits wide. The DSP section includes the volume control; as the maximum bit depth handled by the control is 24, the 35-bit data path, in theory, allows there to be up to 11 bits' worth (ie, 66dB) of attenuation with no degradation of signal resolution. Level adjustments are made either with a front-panel rotary encoder or with the usual Up/Down buttons on the remote control, in 0.5dB steps. The DSP section also allows the user to choose from seven impedance-compensation filters, to allow the amplifier's top octave to be tuned to match the chosen speaker impedance in that region.

The rightmost front-panel button is labeled Menu; in conjunction with the rotary encoder, it allows the user to select the sample rate of the analog inputs' A/D converters, the speaker impedance-matching filter, a gain offset for each of the analog inputs (from 0dB to –9dB in 3dB steps), the amplifier's absolute polarity, and the alphanumeric name of each input. The volume control can be bypassed if the owner wishes to use the M2 with a separate preamplifier, while digital data can be routed via an external processing loop if desired.

"the soundstage was wide and deep, the high frequencies silky smooth, the lows warm and rich."

The M2's rear panel offers two pairs of plastic-shrouded binding posts for each channel, to either side of the IEC AC inlet and main power switch. (A front-panel button switches the M2 in and out of Standby mode.) The balanced and unbalanced analog input jacks are on the far left of the rear panel, with the digital inputs and loop sockets vertically arrayed next to them.

The M2 is relatively hefty, but offers a superb level of fit'n'finish, as well as a high maximum continuous output power of more than 200Wpc into 8 ohms. In common with NAD's amplifier philosophy, more power—300Wpc into 8 ohms—is available for short-term transients. Also reflecting NAD tradition, the M2 offers optional Soft Clipping, selectable with a rear-panel switch and realized in the digital domain. The M2 uses three power supplies, all switch-mode types: one for each channel's output stage, and a third for the input stage and control section.

Technical details

In its simplest form, the output stage of a class-D amplifier comprises two complementary switches (usually power MOSFETs), one connecting the output terminal to the positive voltage rail, the other to the negative rail. When no input signal is present, the switches alternately open and close at a very high frequency, sending a series of full-scale positive and negative pulses to the output—in the case of the M2, ±50V. The switches are never on at the same time, and as the average voltage at the output is zero, there is no output signal. With an input signal present, the oscillator controlling the switches adjusts its duty cycle so that the full-scale positive pulses last longer when the audio signal is in its positive phase, and the full-scale negative pulses last longer for the negative signal phase. The higher the signal level, the longer the switches stay closed for each pulse, and the higher the average voltage fed to the output for each signal phase. You have an amplifier! For obvious reasons, this operating principle is called Pulse Width Modulation (PWM). And because the switching devices are either fully on or fully off, no power is wasted and the efficiency approaches 100%. However, a hefty low-pass filter needs to be in series with the output in order to prevent the high level of high-frequency switching noise from contaminating the neighborhood, and to reconstruct the analog waveform.

M2 Direct Digital Integrated Amplifier

In practice, of course, there are many engineering problems to be solved in the design of a PWM amplifier, and many proprietary solutions are offered. NAD has collaborated with a British semiconductor company, Diodes Zetex Ltd., which developed a novel feedback topology in which the output pulses are continuously compared with a reference to produce an error signal. This error signal is integrated, digitized (at 108MHz), and fed back, with noiseshaping, to the PWM modulator. The signal is also monitored at the output low-pass filter, to give a low output impedance. The Zetex team refers to their topology as a Direct Digital Feedback Amplifier, and the NAD M2 is the first commercial product to feature DDFA.

In the main, a PWM output stage follows a conventional small-signal analog amplification stage. However, if the PWM stage can be fed PCM data directly, there is no need for there to be any analog amplification at all. This is what NAD has done in the M2, which is why they call it a Direct Digital amplifier. From the block diagram in NAD's white paper on the design of the M2, it looks as if the PCM data are first converted to a noise-shaped bitstream that is then applied to the 108MHz PWM modulator, along with the feedback signal.

There have been similar products before, in which a digital input signal is directly fed to an amplifier's output stage. The original Wadia company bet the farm on what they called a PowerDAC, but couldn't bring it to market successfully. The TacT amplifiers did generate some marketplace traction, and the Toccata PCM-to-PWM interface used by TacT was licensed to Texas Instruments in 2001. The Sharp SM-SX100, from the start of the century, was functionally very similar to the M2 but was compromised in terms of dynamic range, and never sold in significant numbers.

The advantage of keeping everything in the digital domain is that, provided the math is done with sufficient precision, the only source of noise and distortion is the PWM output stage itself. The M2 thus has the potential for sounding better than a conventional analog PWM design.

Hiccups

A couple of operational niggles were apparent when I first set up the M2. The shrouded EuroNanny speaker terminals accept spade lugs from one angle only, which makes dressing speaker cables a hit-or-miss affair. The terminal's opening is also too narrow to take the thick lugs now found on some high-end cables, such as AudioQuest's K2. Fortunately, the terminals do accept 4mm plugs, which I fitted to the cables I used. I was also initially puzzled by the M2's sample-rate display, which obstinately told me that the incoming sample rate was 44.1kHz, even when I was feeding the amplifier data at a different sample rate. According to Greg Stidsen, NAD's director of product development, even though the M2 does adjust itself to operate at the incoming sample rate, the display shows the sample rate set by the appropriate status flag in the datastream. Unfortunately, if this flag is left blank, as can happen with some source components, the display will default to "44.1kHz."

I had a problem with the first sample of the M2 (serial no. H99M200085) I received for review. After a couple of weeks of operation, during which time I left it on continuously, I got home one evening to be greeted by the sound of clicking relays and the front-panel message "Overheat." Unfortunately, I had no idea how long the M2 had been in this state. Following the instructions in the manual, I turned the amplifier off at the rear panel, unplugged the AC cord to make sure there would be a hard reboot, and left the M2 powered down for an hour. When I turned it back on, it passed signal for about 5 seconds, then displayed the "Overheat" message again—the amplifier was now stone cold—and shut down. NAD shipped me a new sample (serial no. H99M200094), and I continued the review, including the measurements, with that sample, which performed perfectly.

Sound

I initially set the M2 up in its Fixed Gain mode, for use as a power amplifier, but after a New York minute's reflection, I realized that that was not going to take advantage of what the NAD could do. So I rethought the system's architecture, feeding to three of the M2's digital inputs the digital outputs of the dCS Puccini (for CDs) and Ayre Acoustics C-5xeMP (for DVD-Audio discs) players, as well as my Mac mini server via a Bel Canto USB-to-S/PDIF converter. For SACD playback, I fed the Puccini's analog output to the M2's balanced inputs, making sure its maximum output level was set to 2V so as not to overload the M2's own A/D converter (see "Measurements" sidebar). I left off the NAD's Soft Clipping feature. I also set the speaker compensation to ">8 ohms," which seemed appropriate for the Aerial Acoustics 20T V2 speakers with which I did most of my auditioning. The Aerial has a low impedance in the midrange, but much higher in the top two octaves; setting the M2 to "4 ohms," as I initially did, resulted in too much high-treble energy. For the PSB Synchrony Ones I used the "6 ohm" setting, to match that speaker's top-octave impedance.

I wasn't sure what to expect from the M2. My experience of class-D amplifiers has been somewhat limited, but from that experience I anticipated taut, dry lows, somewhat threadbare highs, and a flattened soundstage. I got none of those things. Instead, when I fed the M2 the 24-bit/88.2kHz master files of Cantus's While You Are Alive (CD, Cantus CTS-1208), the soundstage was wide and deep, the high frequencies silky smooth, the lows warm and rich.

With iTunes playing back CD data, well-balanced rock recordings such as Mary Chapin Carpenter's version of Jagger and Richards' "Party Doll," from Party Doll and Other Favorites (CD, Columbia CK 68751), had a delightful palpability in the midrange, and where reverberation had been used in the mix, the source moved back appropriately in the soundstage.

M2 Direct Digital Integrated Amplifier

And even when the recording was not good, its poor qualities were handled by the M2 without exaggeration or tonal emphasis, allowing them to be mentally categorized and put to one side. One of my favorite Miles Davis albums, for example, We Want Miles, recorded in 1982 (CD, Columbia 469402 2), was not well served by the engineers, sounding bright, hard, and in-your-face, even on the original LP. Yet with the M2, the brashness seemed to be reproduced in a plane different from that of the music. "Kix" begins with a jaunty figure from Marcus Miller's bass, with first congas, then full drum kit accompanying, before Miles enters with a typically sparse melodic line. Miller uses a funky, hammered-on percussive style for this passage, followed by a mellower, thumbed walking-bass line in the solo trumpet, sax, and guitar sections. The M2 fully distinguished between these different tonal qualities and did a good job of retrieving the ambience around the bass and congas. And the M2's treble didn't exaggerate this recording's splashy-sounding cymbals.

Driving both the Aerial and PSB speakers, the M2 got right both the clarity and the weight of the piano's left-hand register. However, I did wonder if the bass region was a touch too ripe: while Marcus Miller's Fender on the thin-balanced We Want Miles sounded tonally right, Phil Lesh's bass on the Grateful Dead's Live/Dead (CD, Warner Bros. 1830-2) sounded fuller than I'm used to, though not so much as to interfere with the music. In fact, this fullness helped add a degree of majesty, not only to rock recordings and large-scale classical works, but even to smaller ensembles, such as the collection of Mendelssohn's complete String Symphonies, with Lev Markiz conducting the Amsterdam Sinfonietta (BIS 1738, more than four hours of music on one SACD), which I'm slowly working my way through.

In fact, I kept returning to the M2's retrieval of subtle sonic cues in the mix, particularly of ambience. There was more there there, in the immortal phrase coined by Sam Tellig channeling Gertrude Stein, compared with the Simaudio Moon Evolution W-7, itself no slouch in this area. "Blizzard Limbs," from Attention Screen's Live at Merkin Hall (CD, Stereophile STPH018-2), for example, starts with Mark Flynn playing a repeated figure on kick drum, hi-hat cymbal, and a snare rimshot. In mixing this album, I used what I then thought was just enough of the distant omni mikes to give a sense of the hall's acoustic while preserving the immediacy of the instrumental sounds. With the M2 driving the Aerial speakers, and playing back the hi-rez master files, there was more of that hall's character evident than I remembered. Not that it didn't sound musically satisfying, but the soundstage was now both deeper and a little more reverberant than I had originally intended.

On Attention Screen's purist-miked Live at Otto's Shrunken Head (CD, Stereophile STPH020-2), the image of the four musicians was more solidly resolved. And on Robert Silverman's set of the complete Beethoven sonatas, which I recorded in 2000 (CD, OrpheumMasters KSP-830, now sadly out of print), the acoustic of the relatively intimate performing space was sufficiently well resolved that the fact that the room was a little small for the Bösendorfer 9' grand piano could be more readily accommodated to, as it would be heard live.

All of the above comments describe the sound of the M2 as driven from its digital inputs. The analog inputs are certainly of high quality, but feeding the dCS Puccini's balanced analog out to the M2's analog input gave a sound that, with CDs, wasn't quite as well resolved as when I used the Puccini's digital output to drive the M2. (Setting the balanced Input Offset to –9dB on the M2 resulted in a 0.4dB difference in level that could be compensated for by adjusting the Puccini's own volume control.) During the review period, I was auditioning the gold CD reissue of Arturo Delmoni's recital of works for solo violin by Ysa?e, Kreisler, and Bach (John Marks JMR14; the gold edition is available exclusively from this website. Yes, this recording is an ambiencefest, and that came though via the M2's analog inputs—but the violin was a little more forward in the soundstage than via the NAD's digital input. And a brief reference back to the Simaudio pre/power combination I used for last December's review of the Puccini revealed that for SACD playback—in which, of course, the player's digital output is disabled—the dCS system provided the ultimate sound quality with the Aerial speakers, edging ahead of the sound of the M2 fed by the dCS's analog outputs.

Overall, however, my time with the M2 was among the more enjoyable periods I have spent reviewing an audio component.

Summing up

This review proved a more difficult undertaking than I had expected. My system has been locked into the paradigm of Source Component(s) to Preamplifier to Power Amplifier(s) for the past three decades. When I decided to review the NAD M2, I had not appreciated just how radically it would shift that paradigm. The integration of a D/A processor and power amplifier into a single chassis eliminates the need for an actual preamplifier, instead substituting a digital-domain volume control and switching. Of course, the M2 does have two analog inputs, but as these are digitized ahead of the volume control, it doesn't affect the picture I have painted of the M2 as a paradigm-breaking component.

Once I had changed my system approach, the NAD M2 provided many nights of extended listening, with one album leading to just one more. And one more. While the M2 is relatively large and heavy for a class-D amplifier, runs warmer than you might expect, and is not inexpensive, when fed high-quality PCM data it offers sound quality that competes with that of the best conventional amplifiers. Given my long-term skepticism about the sonic benefits of PWM amplifiers, that was not what I was expecting. NAD's Masters Series M2 is a winner all the way.

Click here to read full review and measurements done by Stereophile

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