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Re: Quad owners

Posted by Leisure7 (A) on October 30, 2002 at 09:58:27

In Reply to: Re: Quad owners posted by Garibaldi on October 30, 2002 at 07:22:18:

At the risk of annoying the SET amp fans and complicating your search logistics, let me recommend that you listen to Atma-Sphere OTL before you buy anything that you can't return.

SET amps have a characteristic distortion caused by the magnetic grain in the cores of the output transformers. Each SET amp design has a different sonic signature from the interaction of the particular transformer, tube type, and circuit details, but they are all veiled because the transformer core distortion is of a fixed minimum magnitude. As the signal gets smaller, this distortion becomes a larger fraction of the signal. The effect on the sound is somewhat like the effect of a scrim used in live theater or opera on the visual detail of a scene. It is easy to adapt to it, but the effect of its removal is startling.

Push-pull transformer-coupled amps like the ones you listed have this distortion as well as something called "cross-over" distortion. The output tubes change over handling the current near the zero point of the signal waveform, and the changeover is not completely seamless. The feedback loop works differently at this point of the waveform because the tubes' gain characteristics are different near cut-off. This cross-over distortion is what most SET amp fans find objectionable about push-pull tube amps, but, as I mentioned above, the SET design is itself limited by the output transformer characteristics.

OTL amps do away with transformer distortion, and the Circlotron-based output stage used in Atma-Sphere OTL amps does away with cross-over distortion. The drawback is the heat production: estimated to be about 10% higher than equivalent class-A transformer-coupled amps; and high output impedance (scales inversely with number of output tubes). Kentaja would need MA-2s to satisfy his listening tastes and probably would not want the heat, but you might be happy with 60 watts per channel from the smaller, cooler, cheaper M-60s.

The point of worrying about all this is your electrostatic speakers, and Maggies with ribbon tweeters, will reveal more detail than any of these amps, or any signal source, can provide. You need to make an informed compromise among convenience, cost, and performance, and all the chatter the Internet can hold can not substitute for actual listening. There is at least one Atma-Sphere owner in Maine, who posts regularly on the Atma-Sphere Owners' Group site (www.otlamp.com), and he might be willing to let you listen to his setup.

This post is made possible by the generous support of people like you and our sponsors:

Atma-Sphere Music Systems, Inc.

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