Class-D audio amplifiers: What’s the argument for them? Class-A audio amplifiers: What’s the argument against them? Class-AB amplifiers: Why does everybody make them? Each of these amplifier output architectures has strengths and weaknesses. Each will interface more or less successfully with a given loudspeaker in a given room. Each has a distinctive sonic signature. Consequently, as knowledgeable audiophiles with a laundry list of system-building goals, we are required to choose the amplifier type that best suits our speakers, our room, and our individual musical-aesthetic predilections.
So: To set the stage for my descriptions of Schiit Audio’s new Aegir power amplifier ($799), I must begin by summarizing my empirical observations about the sonic qualities of the three most common output-section architectures: class-D, in which the conducting done by the output devices is related not to the angle (0° to 360°) of their input signal but to a fixed-rate pulse; class-A, in which the output devices conduct for the whole 360° of the input signal (and because the current going through the output device is related to its input voltage, we call this transconductance, abbreviated as gm); and class-AB amplification, in which transconductance occurs for more than 180° of the input signal but less than 360°.
Class-D creates a simulacrum of the original signal, which feels slightly spartan and texturally different than class-A or -AB. Class-D generates power easily into a wide range of speaker loads. It can be superdynamic. It appears cleaner and more transparent than class-A. Higher in contrast. Quiet passages and empty spaces can feel a bit electronic. Images are well-outlined but devoid of warm flesh. Voices and instruments display a saturated but distinctly limited palette of tones. The feedback and low-pass filtering required for class-D add hardness to the sound. The best-executed class-D amplifiers deliver music with a refined, grainless quality that many audiophiles find appealing.
Class-A is naturally low distortion. Maximum information-density. Less distinctly drawn and impactful than class-D. More overtly textured, painterly, colorful, and full-bodied. Lower in contrast than class-D but with a wider, more complex palette of tones. Empty spaces seem charged and appear connected to performers. The lower feedback required to achieve linearity allows for a more supple and relaxed presentation. Properly executed class-A sounds trippy and lifelike.
Class-AB is a compromise that allows amplifiers to produce class-A power up to some predetermined bias point (typically no more than a few watts), at which point it crosses over to almost-class-B, thereby allowing more watts (of a reduced quality) before apocalyptic clipping sets in. Driving benign loads of reasonable sensitivity, class-AB amplifiers, especially some tube models, can and sometimes will sound superbly class-A-like. Hence their popularity.
But: What if I told you that the Schiit Audio dudes, Jason Stoddard and Mike Moffat, have devised an operating strategy, called Continuity, that gets you possibly all the benefits of class-A while avoiding some of its heat and efficiency issues?
As Jason Stoddard told me in an email, “Depending on your point of view, Aegir Continuity is either (a) a lower-power, higher-cost version of our Vidar amplifier, displaying Schiit’s unquestioned engineering decrepitude, or (b) a really interesting way to get around the problems of transconductance droop and the mismatch between NPN and PNP output devices.”
Schiit claims their Continuity output stage enables “class-A-like” performance up to the Aegir’s full rated power. “Class-A-like” is an interesting description, but what does it really mean? According to Jason, “Continuity extends the benefits of class-A (linear transconductance) far past the class-A bias region. In theory, the Aegir should sound pretty much the same whether it is operating within its 10W of class-A bias or putting 100W into a 4-ohm speaker in mono.
“Continuity allows us to run more efficiently than class-A. It is not a panacea, it doesn’t run cold, but half the standing current of a class-A design is a pretty decent reduction.”
According to Jason, Schiit’s Continuity circuit was derived from the work of super-smart amplifier-engineering mavericks Bob Cordell and John Broskie. Here’s how my amp-designer friend J.C. Morrison (Silbatone) describes Bob Cordell: “There isn’t a human alive that can research, measure, and explain what is or isn’t happening in a circuit more thoroughly than Cordell. John Broskie is an extraordinary circuit-analyst—like Cordell—but also a historian. His blog is an international treasure for audio hardware engineering.” (footnote 1)
Seems like those Schiit dudes hang with some sharp people!
The Aegir
The original Aegir was a Norse sea giant, but this one is extremely compact: a 19.6 lb, 9″ by 13″ by 3.9″ stereo amplifier which, conveniently, is fitted to the same chassis as Schiit’s lower-priced/higher-powered amp, the Vidar, which is named after a silent and reliable Norse god. According to the Schiit website, the Aegir is “a hot-running amp, with over 10W of Class-A standing bias.” Beyond those all-natural class-A watts, the Aegir generates a total of 20Wpc into 8 ohms, doubling to 40Wpc into 4 ohms. Users may effortlessly convert their stereo Aegir to an 80W (into 8 ohms) monoblock by simply connecting the XLR output of a true balanced preamp to the single XLR input on the Aegir’s rear panel.
The Aegir is made in California and, according to Stoddard, “employs a 100% discrete, fully complementary, current-feedback, linear power supply. And features intelligent microprocessor management of all critical operating conditions, including DC offset and standby mode, as well as relay-activated shutdown for overcurrent, thermal, and other faults.”
With Wharfedale Lintons
While working on last month’s reviews, I connected the Schiit Aegir to Wharfedale’s $1198/pair 85th Anniversary Linton Heritage loudspeakers—and brother, oh brother! That sweet-sounding combo helped me remember why I hold the Stanley Brothers in such high lonesome reverence. The Stanleys’ “Hard Times,” from The Complete Mercury Recordings (44.1/16 FLAC Mercury/Tidal), reminded me what standing close to a full-tilt mountain band actually feels like. Think pure acoustic electricity! The Aegir lit up the Lintons and gave them delicacy and lucidity, with an undercurrent of strength.
The Aegir’s ability to balance generous bass power with high levels of filigreed class-A detail made finger picks plucking banjo strings and wood bows rubbing fiddle strings into titillating physical pleasures. I could live, satisfied, forever with this reasonably priced pairing.
With Harbeth P3ESRs
If I could keep only one pair of all the stand-mount speakers I have stowed in my bunker, it would be the Harbeth P3ESRs. These small, finely crafted loudspeakers sound more right-of-tone and correct-of-balance than any other speaker I know. Therefore, I am always searching for the perfect amp to drive them—and damn, if the Schiit Aegir didn’t wave an excited hand saying, “Pick me! Pick me!”
The Aegir made an excellent case for itself by how it reached in and exposed every tiny texture and spatial nuance of a very complex, difficult-to-reproduce recording: Cabaret Modern: Night at the Magic Mirror Tent (44.1/16 FLAC Harmonia Mundi/Tidal). Cabaret Modern is a sound-collage night at an imaginary nightclub, performed by French avant-garde guitarist/composer Noël Akchoté and a cohort of players. Inside the Magic Mirror Tent, I was treated to a full-on 1930s-style German cabaret, with drinking, spontaneous applause, sing-alongs, and stomping on a wood stage. Best of all, there is a band with an accordion, a harmonium, and a honky-tonk piano—all this mixed together with car sounds, doors slamming, and people talking. On the happy “Via Con Me,” the performers were directly in front of me, in the front row by the stage. The Aegir made these divertissements feel solidly and effectively presented.
Footnote 1: Designing Audio Power Amplifiers, Bob Cordell’s encyclopedic book on audio amplifier design, is now available in a second edition from Focal Press: .—John Atkinson
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COMPANY INFO
Schiit Audio
24900 Anza Drive, Unit A
Valencia, CA 91355
(323) 230-0079
schiit.com
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