ATMA-SPHERE CLASS D AMPLIFIERS - REVIEW

ATMA-SPHERE CLASS D AMPLIFIERS

In a perfect world, an amplifier would be a “wire with gain,” a box that magically increases voltage or current or both, without adding or subtracting anything. Trouble is, nothing is perfect. That said, modern amplifiers of canny design, whether they be solid or hollow state, approach that ideal quite nicely. Typically, the more money you throw at the problem, the closer you come to that golden ideal. For this review, we have that rare beast; an audiophile separate that exhibits excellent execution without a stratospheric price tag…

Ralph Karsten and his merry men at Atma–Sphere have been building fabulous hollow state amplifiers since the bicentennial. His products are known for their rugged and reliable designs, their sonic transparency and very reasonable prices. Who could ask for more? I could! In 2017, he began work on a project that would change the face of his product lines.

All of us have taken missteps on the path to better quality. Lord knows, I have had my share of bogus beliefs and misguided choices. For a long time, I was one of those who thought DSP was an inherent degradation in a hi–fi system. Only later did I realize that it was the poor implementation that was at fault, not the foundational technology itself. Back at the turn of the millennium, I was working for loudspeaker wizard Ken Kantor on various projects, including his groundbreaking IA-643 active subwoofer. At the time, Ken was fascinated by both DSP and Class D amplifiers, but the off–the–shelf versions then available were decidedly low fidelity. In the ensuing 20 years, Class D moved from smirk-worthy to seriously sublime. Hypex, Purifi, AGD, Orchard Audio, and GaN Systems have all moved the technology forward, but it’s the latter three that have advanced to incorporating GaN or Gallium Nitride semiconductors.

FETs or Field Effect Transistors have been used by many talented designers to create some memorable amplifier products. “Pappa” Nelson Pass has been a marvellous proponent of FET technology for amps, having used rare silicon nitride FETs to good effect, but it’s GaN or gallium nitride devices that have changed the Class D landscape. Originally commercialized for those blue LEDs we all now take for granted, GaN FETs found a place in radar circuits and motor controllers—think electric cars, where their extreme switching speed and tolerance of high current made them an exemplary match. As it turned out, their response to varying voltage and current also looks surprisingly like a vacuum tube. This made them perfect for the high speed “chopping” or proportional on/off cycling† that makes Class D work.

If you are a clever designer, you can create a low distortion, low power, wide bandwidth amplifier using vacuum tubes. However, if you want more than flea strength or clean robust low frequencies, it’ll cost you. Running a tube amp will also cost you in expendables and power bills, in that vacuum tubes have a decidedly short service life compared to transistors, and tubes are far from efficient in converting power from the wall outlet into music from your speakers. Class A transistor designs address the reliability issue but still eat sadly large amounts of energy transmuting raw alternating current into musically pleasing AC.
Enter Class D…not really “digital” but sharing some similarities, Class D amplifiers work like really clever light dimmers, except that instead of dimming a bulb, they vary the raw power being delivered to your speakers in a binary fashion such that music comes out. In the past 10 years, Class D tech has reached some semblance of a plateau, with deep engineering transforming what was once a rather grating and rough approximation into something quite wonderful. Then came GaN FETs.

In a popular online forum, Ralph noted that, “The result is a solid state amp that has all the smoothness in the mids and highs that you expect from an excellent tube amp but with considerably lower distortion, (lower) output impedance and obviously higher efficiency. Class D…isn't going to be as romantic as a zero feedback tube amp, even if it has the same distortion profile, simply because it has so much less distortion overall!” As with other Class D amplifiers, efficiency is also, for me, a strong selling point. At idle, only 10 Watts of power are drawn from the wall for a stereo pair of his Class D monoblocks, about the same as a typical LED lightbulb.

Karsten noted that his new amps offer a lot of power for difficult speakers. “…even though our Class D easily doubles power as the load impedance is halved (right up to its current limit protection) I’m of the opinion that the easier the speaker is to drive the better, especially since (our) Class D has a really nice first Watt.” Let’s explore that first Watt…I’ll begin my listening observations with a hoary old prog rock classic, Yes’ 1971 The Yes Album [Qobuz 2023]. Newly re–released as a 192k “super deluxe edition,” the rambling fourth track has been a perennial fave for me. With its very artificial mix and slightly dirty analog sound, plus real double-tracked Anderson vocals, extreme swings of loudness and extra long fade at the end, I've Seen All Good People is a bit of a toughie to reproduce. The Class Ds present as good a rendition as I’ve heard, especially when paired with Silversmith Audio’s wonderful FIDELIUM speaker cables feeding Scansonic HD’s MB2.5 B towers. More on that in a moment, but the midrange was, dare I say, lovely. The low end was solidly quick and tuneful, and the top, well, the top was somehow tamed, velvety and extended. To digress a moment: if there’s one down side to Scansonic’s MB line, it’s their very thin veneer of distortion, which to my ears announces itself as a high frequency resonance. With lesser electronics and cabling, they can be slightly raspy, unduly emphasizing sibilance. Ralph’s little amps are a synergistic fit for the minitowers, somehow suppressing that bite, which makes for a magical, marvellous combination.

Moving from the slightly lo–fi Brit rock to a more modern and much higher fidelity neo–folk track, I dug into the layered and engaging A Perfect Hand from David Byrne’s masterful 2010 Nonesuch release Here Lies Love [Qobuz 96k]. In particular, the Class Ds tamed Steve Earle’s rough and road–weary vocal without dullness, while each instrument in the mix sat comfortably in its spectral and panning pocket. As I listened, I became fascinated with mixer Pat Dillett’s clever handling of the snare and bongos as they move horizontally and shift in volume from verse to chorus. Another old skool fave, Stan Getz’s classic Desafinado [Qobuz 192k Verve Reissues] beckoned with crack players in a beautifully recorded, stripped-down production. More nuanced than The Girl From Ipanema, Jazz Samba through the Class Ds speaks to me with a lush, relaxed and beguiling beauty in keeping with the swaying palms and caipirinha vibe that the samba exudes.

Time for a comparison…My high bias Parasound JC 5 afforded a more forward upper bottom, with a slightly more spare and immediate midrange. The Class Ds seemed “bottom up” while the JC 5 was more “top down”in voicing. Though I feel the Parasound presents a more honest representation, perfect for reviewing, I have to grimace slightly (’cause I love my JC 5 and desire to throw no shade) and state that the Atma–Spheres, being more euphonious, were more fun! The more I thought on that conclusion, the less surprised I was. During our conversations, Karsten said he thought that tube power amps are “…on borrowed time.” He rhetorically asked online, “…why bother with (tube amps) when you can get sound just as good and not worry about tube replacement?” I have to agree. I love his valve–based wares, but now I have to say, why bother? Why heat the planet and screw the pooch for your kids, why pay for short-lived parts, why piss off your significant other with large homey though usually homely tubes? I know, because they can sound good. Ralph Karsten has been building exceptional hollow state amps for quite some time now. He’s distilled all his knowledge, as well as his house sound, into a solid state product that competes with costly high-end vacuum tube amplifiers and offers the same rich, polished and full spectrum sound quality without the drawbacks.

Since I had them in the house for an upcoming review, I wanted to try the stylish little Boenicke W5 stand–mounters. Surprisingly, the W5s seemed leaner, more spare in the upper bass when driven by the monoblocks. Also, the soundstage wasn’t as widely expansive, which was equally enigmatic, though I dare say the complex interaction of loudspeaker, cables, and amplifier can result in some unexpected responses. I’m not saying that they weren’t quite enjoyable. It’s just that, unlike the MB2.5 B pair, which made for a celestial match, I much preferred the JC 5 driving those cables and speakers. With that realization, I retreated back to the Scansonic HDs.

Beethoven’s hoary old Sixth Symphony may be overly familiar to some, but I for one luxuriate in that first “Pastoral” movement. A recent release, Beethoven: Symphonies Nos 6 & 8 from the National Symphony Orchestra with Gianandrea Noseda on their own label [Qobuz 192k 2023], affords an up-close very high-resolution view into the performance. What the Class Ds lack in sizzle and sparkle, they more than make up for in clarity and fullness. Their beguiling propensity to involve the listener is simply remarkable, and this Beethoven piece demonstrates that clearly. Were textures a touch dialled down on strings and brass? Yes, ma’am. Did I care? Not one iota.

By the way, as I was listening, I not only switched amplifiers and speakers to have a contrasting alternate, I also swapped player apps. With lesser electronics and cabling, player apps do not seem to make any difference at all. During these listening sessions, the darker, slightly veiled voice of Audirvana Studio was easily found wanting in the fine detail department when compared to Amarra Luxe. They both deliver the exact same bit–perfect stream to my reference exaSound e22 Mk. II, but what is the cause? I conjecture it’s their signature eye pattern, how they modulate the DAC’s power supply, or both. Frankly, I do not yet know the mechanism, but I clearly heard the difference through the Class Ds.

From old favourites Wolf Alice and Waxahatchee to new crushes Ludwig Göransson and Duo Deloro, the Atma–Sphere Class D is a class act. When I was away cooking in the kitchen, they would draw me in and invite me to sit a spell and listen. They made for a difficult review because all I wanted to do was let go of my analytical side and enjoy my catalogue of best–loved tracks and albums. We audiophiles come to the table for music, not audio. Atma–Sphere’s hand-built Class D amps represent over five years’ worth of work. Karsten is focused on how the human ear/brain system perceives distortion, having tuned his products to be fast and transparent without being bleached or analytical. “I hope we can agree that what we really want is something that sounds like music rather than a particular technology.” Amen, brother. Go give a listen.

Atma–Sphere

Saint Paul MN
www.atma-sphere.com
Class D power amplifier — $2920

† — Light dimmers and Class D amplifiers both employ PWM or Pulse Width Modulation, which works by varying a square wave’s pulse width or “duty cycle.” This is simply the amount of time the square wave is “high” and the output is “on.” Mostly off results in small power; while on a lot yields lots of power delivered to the load. As with digital, both DSD or PCM, you do have to “smooth” or integrate that square wave so it’s transformed into music.