Atma-Sphere Class-D GaN Mono Amplifiers Reviewed

It’s no secret that we at Future Audiophile are big fans of Class-D power amplifiers. We’ve reviewed many, and while not every Class-D amplifier is superb, some of the most neutral, smooth, and transparent amplifiers being made today are based on modern Class-D technologies. To get technical for a brief moment, the topology of Class D can provide very high gain, which allows the designer to use enough negative feedback to achieve low distortion over the audible spectrum. In addition, the load invariance of modern Class-D amplifiers means that the frequency response won’t change depending on what loudspeaker they are driving. In a good Class-D implementation, the result can be Class-A smoothness and excellent bass, without the weight, heat, energy consumption and cost of a Class-A amplifier.

In this review, we consider a rather special pair of Class-D mono amplifiers, the Atma-Sphere Class Ds. (“Class D” is the model name, as well as the class of operation). In the Atma-Sphere line, the Class D (100 watts into eight ohms for $6,120) falls between the M-60 (60 watts, $7,950) and the MA-1 (140 watts, $19,600; all prices per pair) in output power. The Class Ds are special on several counts. They are the first solid-state amplifier offered by Atma-Sphere, which since its founding in 1976 by owner-engineer Ralph Karsten, has made tube-based preamplifiers and output-transformerless (OTL) tube amplifiers. The Class Ds represent a strong commitment by Karsten to the potential of this technology. He uses them in his home system, and he’s said online: “I don’t miss the tube amps – there isn’t anything they did that the Class D doesn’t do as well, with the exception of grace during overload. My speakers are 98 dB/1 Watt so overload really isn’t in the cards.” They are designed entirely in-house by Atma-Sphere, not based on pre-designed modules like many other Class-D amplifiers. Finally, Karsten has stated that they were designed and built to last, using premium parts, a sturdy chassis and elegant front panel, and a beefy power supply. So, let’s take a thorough look and listen at these special Class-D power amplifiers …

What Makes the Atma-Sphere Class D Amplifiers So Special? 

• These are solid-state Class-D amplifiers from a notable maker of tube electronics. Atma-Sphere’s Karsten believes Class D is the future of audiophile amplification, and that his Class D amplifiers are every bit as good as his widely respected tube designs. Here, we have Class-D amps designed by someone with an ear for the best of what tube sound can deliver.
• Atma-Sphere has designed its own modules for their Class D amplifier. It’s common for manufacturers to buy Class-D amplification modules from suppliers such as Hypex and PuriFi, or to license such designs and use them with modifications. In contrast, Atma-Sphere has designed its own modules, which allows them complete control over all aspects, including quality of components and the distortion characteristics of the finished product. 
• Like many Class-D amplifiers, the Atma-Sphere Class Ds are relatively small (13 × 11 × 3 inches) and light (14 pounds) and run cool (five watts at idle). It is a pleasure to use amps that I can lift easily with one hand, set down and pick up without back strain, and that don’t heat up the room. If you’ve ever tried an amplifier that emits hundreds of watts at idle, you know the importance of high efficiency to user comfort, especially in a relatively small room. And today, many of us want to reduce power waste, both for cost reasons and as a small contribution to reducing climate change.
• The Atma-Sphere Class D amplifiers use GaNFET output transistors. In my view, the use of GaNFET or MOSFET is simply a designer’s choice, as based on gear I’ve heard, it’s possible to make a great amp with either technology. Still, GaNFET is exciting new technology, and if that’s on your wish list, you get it with the Atma-Sphere Class Ds.
• The Atma-Sphere Class D amplifiers offer both balanced (XLR) and unbalanced (RCA) inputs. I prefer balanced connections when available, to minimize chances of noise. Still, some preamplifiers offer only unbalanced outputs, and the unbalanced inputs of the Class Ds make it easy to connect to such components, with no need for adapters or special cables.
• Signal output to the speakers is via rugged and sturdy Cardas copper binding posts. I appreciate that they are not plastic and definitely won’t break anytime soon. Their use illustrates Atma-Sphere’s attention to quality and detail.
• The Atma-Sphere Class Ds are attractive amplifiers. Many of us want gear that sounds good, and also looks the part. With their champagne front panels and classic black-wrinkle cabinets, the Atma-Sphere Class Ds are handsome in a restrained way. They will be at home in an elegant audio room or living room, not just in an audio space that sets aside appearance to focus on performance.
• Atma-Sphere claims that their amps’ distortion signature will lead to smoother sound, free of irritation. The amps’ measured distortion is quite low, less than 0.1 percent total harmonic distortion at full power. Given that, when higher harmonics are masked by lower harmonics and by the noise floor, we expect tonality to be sweeter. I can’t verify the technical details, but I can verify that the Atma-Sphere Class D mono amplifiers have very smooth, non-irritating treble.

Why Should You Care About the Atma-Sphere Class D Mono Amplifiers?

As I mention below, I did run into the power limit of these amplifiers in one extreme listening session. How relevant is that to you? Many audiophiles use high-efficiency speakers, and the Atma-Sphere Class D could be an ideal match. (That combination is what Karsten uses himself.) For those whose preferred speakers have lower efficiency, the Atma-Sphere Class Ds could well be a good match if the speakers aren’t too inefficient, or if playback volume is kept reasonable, or if bass is diverted to subwoofers by a crossover. Those who contemplate using these amplifiers with low-efficiency speakers may be best off listening in the near field or choosing amplifiers with a higher power rating.

As I audition more modern low-distortion amplifiers, many of which operate in Class D, I’ve especially appreciated a sonic characteristic that the best amplifiers give you: natural clarity. The exceptional clarity can be startling at times, such as when mumbled lyrics are understood effortlessly, or when it’s suddenly easier to follow instrumental lines in classical, bluegrass, acoustic folk, and jazz recordings. The clarity I’m describing is not the etched presentation that comes from exaggerated or peaky treble; instead, it’s from low distortion and low noise floor. The Atma-Sphere Class D gives you plenty of that clarity, and I think any audiophile will enjoy that eight days a week. Along with clarity, the Atma-Sphere Class D can provide treble that’s exceptionally smooth and, in the bass octaves, plenty of warmth.

Some Things You Might Not Like About the Atma-Sphere Class D Amplifiers

• The Atma-Sphere Class D Amplifiers are not Class A. Let’s clear up one thing … amplifier classes were assigned sequentially, thus don’t indicate relative merit based on an associated letter in the category name. Still, in some traditionalist circles, Class D is looked on warily. Whether that reflects the mediocre Class D of 20 years ago, the marginal performance of some very cheap Class-D amps, or simply outmoded prejudice, I don’t know, but I’ve often thought that if Class D had been labeled Class AA instead, it would have an easier time in the audiophile market. Please don’t let the outmoded notion that Class D is somehow less enjoyable than Class A influence your thinking.
• Will 100 watts per channel be enough? If you have high-efficiency speakers, the answer is very much yes. If you have lower-efficiency speakers and listen at high levels, the answer may be no. I found the Atma-Sphere glorious on most occasions, but when I played a Brahms piano concerto at concert level volumes on my reference speakers, which have a rated efficiency about 12 dB lower than Karsten’s speakers, I did hear the sound begin to get murky, as the Class Ds started to reach their limits. If the subwoofers in my system had been engaged, the Class Ds would not have been handling the deep bass, and likely their limits would not have been approached. So, a little thought about system matching and listening habits seems to be in order here.

Listening to the Atma-Sphere Class D Amplifiers… 

I put the Atma-Sphere Class D amplifiers into my reference system, setting the Anthem STR Preamp (read the review) to drive only my main speakers (with no subwoofers), so that the power amplifiers were handling the full audio range. With this setup, I listened to a wide variety of music: vocals, instrumental jazz, and classical music, with an emphasis on natural-sounding recordings. The lasting impression made by the Class D amplifiers was of naturally smooth midrange and treble (particularly welcome on vocals, trumpet, and violins); strong, warm lower midrange and bass; and excellent soundstaging and imaging.

Even this jaded audiophile/reviewer sometimes is surprised at what he hears. Here, the surprise was that the Atma-Sphere Class D amps sounded pleasantly warmer and with deeper bass than other excellent amplifiers I’ve used lately. I didn’t expect a frequency-response plot to show anything along those lines, and when, after some head-scratching, I made measurements, it did not. So, simultaneously, my expectations were met, and I still was puzzled. Though I believe there is an objective explanation, I won’t speculate about distortion spectra, damping factors, or other possibilities. I’ll just invite you to listen for yourself.

Moving on to specific tracks, I started with a landmark of mid-1970s music, Stevie Wonder’s Songs in the Key of Life. The album debuted at number one on the Billboard Pop Albums chart and won Album of the Year at the 19th Grammy Awards. More to the point, it had great beats, catchy, memorable songs, and political messages without stridency or anger. Altogether, this is music that only Stevie Wonder, that beautiful soul, could have produced. Listening to “Love’s in Need of Love Today” (a ballad ringing as true now as then), I noted that the 24-bit, 192-kHz Qobuz remaster sounded FAR better than my old rip from a Compact Disc. With the Atma-Sphere Class Ds, I was struck by the naturalness of the vocals, the wide soundstage, and the powerful, well-defined bass line. It was a moving experience to hear this old favorite played back in such engaging sound. 

If Etta Cameron’s name isn’t familiar to you, and you enjoy gospel, blues, or jazz, look her up as soon as you get a chance. The Bahamian-Dutch singer had a rich and expressive voice, used to full effect in the inspired version of “You Are My Sunshine” from her album Etta. Again, I was struck by the natural character of the sound, with piano, bells and other percussion, muted trumpet, and bass in the extended introduction casting a gorgeous sonic spell. As Cameron comes in, first humming, then singing the lyrics, the percussion continues, and the voice is free of grain or grit. This is a beautiful and mesmerizing arrangement that, heard through the Atma-Sphere Class D Mono Amplifiers, is truly a demonstration-quality track.

A vocalist now making waves is the multi-talented José James, whose album For All We Know won the 2010 Académie du Jazz Grand Prix for best Vocal Jazz Album. Born in Minneapolis, James has performed soul, funk, jazz, hip-hop, and probably more. I put on “Good Morning Heartache” from his album Yesterday I Had the Blues: Music of Billie Holiday, then sat back and sank into James’s smooth baritone. The first thing I noticed was that, even on this non-audiophile recording, the vocals had a natural roundness and lack of grain that I appreciated greatly. The spare accompaniment of bass, drums, and piano sounded excellent, with great imaging and delicacy. The whole presentation just drew me in.

Will the Atma-Sphere Class D Hold Its Value?

The field of Class-D amplifiers is growing quickly, and when that happens, older models can lose value. However, I would put the Atma-Sphere in a different category from the average Class-D amplifier. As an in-house design by a firm whose amplifiers are highly sought after, the Atma-Sphere Class D should hold more value than a run-of-the-mill Class-D amplifier. Quality of construction and classic are plusses. Another positive factor is that Atma-Sphere, though a relatively small business, has dealers in many U.S. states and 14 international regions. 

Used sales will carry less hassle because of the ease with which one can pack and ship these relatively light, compact amplifiers, in comparison, say, to packing and shipping a 100-watt tube amplifier. Finally, this the lowest-priced Atma-Sphere amplifier of 100 watts per channel or more. That is sure to increase the appeal of the Class Ds to those intrigued by the brand.

What is the Competition for the Atma-Sphere Class D Amplifiers?

I surveyed available Class-D amplifiers to find some with similar features to or similar pricing to the Atma-Sphere Class D. Here are some standout units to consider auditioning when you’re looking for new amplification.

The first unit that comes to mind is the AGD Production Tempo di GaN amplifier ($5,500), which is also obviously a Gallium Nitride amp. The AGD has more power, which will appeal to people who need more than 100 watts per channel. The AGD isn’t sold by many dealers (mostly sold direct) and doesn’t have the company track record that Atma-Sphere does. This was the first GaN amp that we’ve reviewed at FurtureAudiophile.com and, since then, our publisher was able to compare the AGD with his reference Pass Labs XA-25 amp ($5,150), and the sound was quite comparable, but again, the GaN delivers in terms of power consumption, weight, size, and heat, as well as that resolute Class-A sound. 

One of the most powerful Class-D stereo amplifiers in the market today is the Amped America AMP 2400 ($5,000), recently reviewed here at FutureAudiophile.com. The amp is rated at 400 watts into eight ohms and 800 watts into four ohms, and has both balanced and unbalanced inputs. This amp compares favorably to much more costly amps in terms of that accurate yet dynamic Class-D sound, while delivering much more power than most in the Class-D space today. The component width is standard rack size at 17 inches (important to some), with a rather plain appearance. The manufacturer states the AMP 2400 is designed, engineered and assembled in the United States, and carries a five-year warranty.

The Orchard Audio Starkrimson Mono Premium Amps ($2,700 per pair – read the review) are Class-D mono amplifiers in small, somewhat nondescript black boxes. These use GaN technology, output 150 watts into eight ohms and have balanced inputs only. Jerry loved them. They carry a one-year warranty, which is a little light, compared to the likes of, say, Bryston, which offers many years more warranty. The Orchard Audio amps had an accurate yet dynamic sound at a very compelling price. 

I recently reviewed the Buckeye Amps Hypex NCx500 Stereo Amplifier ($995 – read the review), which I bought for my desktop system. There is no eye candy here and, again, an amp that offers balanced inputs only. Power ratings are 380 watts into eight ohms, and 700 watts into two or four ohms. Using Hypex’s latest modules, this amp produces excellent, neutral sound, and carries a two-year warranty with additional reduced coverage after that.

Final Thoughts on the Atma-Sphere Class D Mono Amplifiers…

It’s not often that a noted maker of tube amplification comes up with an original Class-D design. When you hear the Atma-Sphere Class D amplifiers playing a great recording, you may wonder why so many fine tube and solid-state amplifiers cost much more.

The Atma-Sphere Class D mono amplifiers’ practical virtues include domestic design and manufacture, audiophile construction, nice appearance, cool and efficient running, and a three-year warranty. In the sonic realm – the one we care about most – virtues include clarity, warmth, smoothness, and dimensionality. These are amplifiers that, in the right system, will delight many music lovers. Some will be stunned at just how great Class-D amplification can sound.

I enjoyed the time with the Atma-Sphere Class D mono amplifiers in my system. For anyone looking for new amps in their price and power range, the Atma-Sphere Class D amplifiers deserve a place high on the audition list.