Class-D-elightful
Sound & Vision|February - March 2021
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.
By Thomas J. Norton

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 driver stage, and high-frequency filtration to strip away ultrasonic noise arising from the pulse-width 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 seven-channel 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.)

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 best-looking amplifiers I’ve ever reviewed. You won’t want to hide it away—a good reason for that dimmable front-panel light switch.

SETUP

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