Class-d-elightful

WHETHER YOU

fall into the “all well-designed amps sound the same when used within their limits” camp or the “amp selection is critical” army of true believers, it’s arguable that prior to the turn of the millennium amps designed for high-performance audio had fallen into a rut. They were so good that the advertising for them had to become increasingly creative. But a parade of skilled designers remained convinced that the new concepts they had come up with were superior, and audiophiles still lined up to buy them. The turf was always familiar: tubes remained tubes with their lovable quirkiness, and solid state was dominated by class-a/b designs as it had been since the transistor was invented.

Meanwhile the digital era gathered steam, although digital implementations were slow to crash the well-entrenched amp party. An early design from Bang & Olufsen called Icepower was dubbed class-d for obvious reasons.

There were undeniable benefits to class-d, particularly its high efficiency: While class-a/b designs are lucky to achieve around 60 percent efficiency with the remaining energy simply turned into heat, a class-d amp can reach above 90 percent.

A few years after birth of class-d, a young engineer at Philips in Belgium, Bruno Putzeys, designed his own version of a digital amp. He called it UCD for “Universal class-d.” A range of variations followed as Putzeys moved on from Philips to Hypex, where the latter marketed (and still does) products based on his designs.

Putzeys is now with Purifi, a company he helped found along with Lars Risbo, Peter Lyngdorf, and others. Purifi’s first class-d digital module is Putzeys’ latest brainchild: the 1ET400A, or as it’s better known, the Eigentakt (for self-clocking). But a class-d amp requires more than just a module; it also needs a power supply, an input

The M28 is one of the first products to feature Purifi's new "Eigentakt" class-d amp module. driver stage, and high frequency filtration to strip away ultrasonic noise arising from the pulsewidth modulation used in class-d processing.

NAD is one of the first licensees for the Purifi module and is currently using it in both its Masters Series M33 integrated streaming amplifier (reviewed in the December 2020/January 2021 issue and on soundandvision.com) and the NAD Masters M28, the sevenchannel power amplifier reviewed here. A two-channel NAD power amp using this module, the C 298, is also available. (The company’s current A/ V receiver line doesn’t use the Eigentakt design.)

"Midrange was open and transparent, with the system sounding as good or better than I've ever heard it."

DESCRIPTION

The NAD M28 is rated at 200 watts per channel into 8 ohms and 340Wpc into 4 ohms, in both cases with all channels driven at a specified THD of 0.003%. Both balanced and unbalanced inputs

are provided for all seven channels. While not a featherweight at just under 47 pounds, the

M28 is easy to maneuver into position.

A power indicator on the front panel glows a dim red when in standby with the unit plugged in and the rear main power switch on. When you touch a contact sensor located at the top of the front panel, the indicator turns bright red and then white after a few seconds, indicating full-on. A tiny, well-hidden switch on the back panel lets you select three different brightness levels, or you can turn it fully off. After 30 minutes with no input signal the amp automatically switches to standby (there’s no option to defeat this timed shutdown). There’s also a single 12-volt trigger input to turn the NAD on or off remotely.

The M28 is not just solidly built but also one of the bestlooking amplifiers I’ve ever reviewed. You won’t want to hide it away—a good reason for that dimmable front-panel light switch.

SETUP

While the M28 is certainly compatible with quality surround sound preamps such as NAD’S own M17 V2i

(not surprisingly, this is NAD’S recommended pairing), it’s also compatible with the preamp outputs of a good A/V receiver for users who might want to maintain their existing frontend—at least for now—while first upgrading their amplifiers. For this test, I used a Denon AVR-X6700H A/V receiver (also reviewed in the

December 2020/January 2021 issue), supporting a 5.2.4 speaker configuration.

I began my listening with twochannel music and no subs, with the Denon’s left and right preamp outputs driving just two of the NAD’S five channels. (It’s far easier to get a grasp on the sound of a product or system without introducing the aural complexity of multichannel.)

Source components used for this review included a Marantz UD7007 disc player (for music) connected to the Denon via its coaxial digital output and an Oppo UDP-203 Ultra HD Blu-ray player (for movies) linked via HDMI. Speakers included a pair of Monitor Audio Silver 10 towers, a Silver C350 center, and Revel Concerta dipole/ bipole surrounds.

The response of the front speakers in my current setup gets little in the way of low-end support from the room, so subwoofers are required to produce adequate bass. There’s also a bit too much energy in the 100-200Hz region— enough to make the sound excessively warm. To overcome these limitations without relying on Audyssey room EQ or the system’s two SVS PB-3000 subwoofers (more on those later), I instead used the Denon receiver’s graphic EQ controls

at

63, 125, and 250 Hz, finalizing the result by measurements at the listening seat (using the Parts Express Omnimic system). The L/R speakers’ bass dropped off significantly below 50Hz, but I still got superb results. And the 4- 6 db bass boost required at 63 Hz didn’t overtax either the speakers or the NAD amp— even in my very large listening space.

MUSIC PERFORMANCE

With this setup, bass was consistently tight and deep, and often dramatic, particularly with the hard whacks of big Taiko drums or the growling, aggressive bottom end on the organ transcription of Pictures at an Exhibition. Midrange was open and transparent, with the system sounding as good or better in my room than I’ve ever heard it. The top end sparkled as well, with no trace of harshness when listening to everything from well-recorded female vocals to crisp percussion.

How did the NAD fare in comparison with the Denon AVR’S internal amp channels? With both level-matched as closely as possible—within 0.2db or less—the class-a/b Denon amps were highly listenable on their own (this will be surprising only to those with a preconceived notion about AVR sound quality), sounding impressively clean and clear from top to bottom. Also, they laughed off the bass boost I

likely come down to personal preference. There were also no frequency response differences worth noting with either amp driving the Monitor Audio Silver 10 speakers (measured at the listening position with the Omnimic system, their response curves virtually overlapped after accounting for a small level difference). To minimize variables, I therefore continued using the Denon as a pre-pro from its preamp outputs to drive the NAD amp for the remainder of my review.

But how would the NAD compare with a well-regarded separate class-a/b amp in the same two-channel, nosubwoofer setup? For that I turned to the Parasound Halo A 52+, a five-channel amp I’ve used in many of my recent audio reviews. At matched levels, my back-and-forth listening comparisons favored the NAD by a hair. The Parasound was slightly darker and warmer (though not by much), and the NAD dryer, with more incisive leading edges on high frequency transients. Any given listener’s preference here might easily shift depending on the system. I ultimately leaned in the direction of the NAD but could live happily live with the Parasound (and have for some time now). The five-channel Parasound does have a $2,000 price advantage ($3,000, or $600/channel), while the NAD offers a premium cosmetic design and seven channels ($4,999, or $714/channel).

Adding in center and surround speakers to complete my listening tests, I limited the setup to 5.0 (5.2 with the subwoofers engaged). The NAD’S seven channels alone aren’t enough to fully power my current 5.2.4 speaker array without adding two (non-nad) amps for the additional two speakers. And using a 5.2.2 setup (which the NAD plus powered subs can support) would have required making significant changes to my physical speaker setup—adding either an extra pair of surrounds or only a single pair of overhead Atmos speakers (the latter requiring repositioning from their current location where they’re optimized for a 5.2.4 Atmos setup).

For multichannel music I went with the 5.0 setup, still without subs. Animusic is a fun Blu-ray that combines computer-generated images with electronic music mixed in DTS-HD MA

5.1 surround. The range of its selections is wide, from an elaborate percussion piece in a highly reverberant space to an odd (but thrilling) transcription of Mussorgsky’s Pictures at an Exhibition. Listening to this disc on my system with the NAD M28 amplifier, to say the results were all I could hope for would be an understatement.

Hans Zimmer Live in Prague’s

ensemble combines a wide range of musicians, soloists, a large chorus, and, of course,

film music composer Zimmer himself (who as a young man was a member of the rock band The Buggles, known primarily for the music video Video

Killed the Radio Star!). It’s a marvelously entertaining disc, particularly if, like me, you’re a fan of film scores. The results were spectacularly huge and room-filling, without a trace of distortion even at high levels. While the NAD certainly wasn’t solely responsible for what I was hearing, it more than held its own.

For a grand finale, I cranked up the two powered subwoofers, crossing them over to the main speakers at 90 Hz. (Using subs took some of the heavy lifting off the NAD M28, but I figured that’s how most potential buyers will use it.) With this setup, the system was cooking. The sound mix for the climactic battle scene in Ender’s Game came across as compelling, and How to Train Your Dragon excelled on both its music score and action scenes. But the most jaw-dropping sound with the NAD M28-powered system

came from Blade Runner 2049. Watching it, I heard deep, hardhitting deep bass and remarkable ambience that enhanced every scene. While most of the deep bass in the 5.2 setup was coming from the subwoofers, it was the overtones above the subwoofer crossover frequency that gave the bottom-feeding growlers texture, dynamics, crispness, and dimensionality, and for that we can credit the NAD amp.

CONCLUSION

The NAD Masters M28 is an exceptional amplifier. Though pricey compared to other multichannel amps, it’s a no-brainer if it fits your needs and budget. I’d love to see an enterprising manufacturer (NAD perhaps?) offer a three-channel amp featuring Eigentakt class-d modules to drive just the front speakers in a system to supplement an otherwise competent A/V receiver. And who will be the first to design a receiver packed with a full onboard Eigentakt amplifier complement? This development is so sonically impressive that I suspect we’ll see much more of it in new products over the next few years.