[Viewpoint] Ralph Karsten of Atma-Sphere and Class D amplifiers

Since our first review of the T-Amp, way back in January 2005 (that's 20 years ago!), we tried to point the way forward for HiFi amplification. In the meantime, a lot has happened, many historic manufacturers have created Class D amplifiers (PS Audio, NAD, Rotel, etc.), but the core of tube electronics designers seemed to resist. In recent days, an interview with Ralph Karsten, CEO of Atma-Sphere, a company that has always been involved in tube electronics and in particular OTL amplifiers, has been circulating on the web and social media about the future of operating classes (A, A/B, D, etc.).

Karsten began working on his own Class D amplifier project back in 2017, using proprietary technology, not the usual modules found in many other designs (IcePower, Hypex, etc.). Having heard about Class D amplifiers at a trade show, he decided he couldn't be left behind. In 2021, the prototype was ready and working, and since then, these amplifiers, which are simply called Atma-Sphere Class D, have received numerous extremely positive reviews, even when compared to much more prestigious and expensive amplifiers.

Karsten has always believed that Class D technology was ripe to rival his OTL tube amps, but recently he's gone a step further, saying that tubes have definitely had their day. Not only that, but he believes that Class D amplifiers are fundamentally better than traditional tube amplifiers, even for the most discerning audiophiles.

Now, without going into the details of his technical considerations - you can find them on-line and on YouTube (just search for "Ralph Karsten and Class D") - let me point out that slowly, very slowly, as befits this industry, things are changing. And, while traditional amplifier designers essentially apply small modifications to decades-old circuit designs and technologies, there's still a lot of room for development within Class D, which certainly isn't within everyone's reach. As Karsten put it, understanding a traditional Class A or Class A/B circuit is one thing, but understanding all the maths required to design a Class D circuit from scratch is another. Coming from someone who's always designed OTL tube amplifiers, that's a pretty thoughtful proposition.

I don't expect all audiophiles to adapt and understand - they're generally among the most conservative and reactionary people on planet Earth - but I expect them to at least try, every now and then, to free themselves from the prejudices, often based on cosmic nothingness, on which they base their choices and beliefs. Who knows, maybe they'll discover that, outside their ivory tower, other possible worlds do exist.

DISCLAIMER. TNT-Audio is neither a shop, nor a HiFi company or a repair laboratory for HiFi components. We don't sell anything. It is a 100% independent magazine that neither accepts advertising from companies nor requires readers to register or pay for subscriptions. If you wish, you can support our independent reviews via a PayPal donation. After publication of reviews, the authors do not retain samples other than on long-term loan for further evaluation or comparison with later-received gear. Hence, all contents are written free of any “editorial” or “advertising” influence, and all reviews in this publication, positive or negative, reflect the independent opinions of their respective authors. TNT-Audio will publish all manufacturer responses, subject to the reviewer's right to reply in turn.

October 2025 Editorial

Ralph Karsten of Atma-Sphere and his love for Class D amplifiers