Tube Power Amp Reviews

Thanks for the trip down memory lane.

I would be willing to chip in to get Art or Herb a fresh example of this amp and see how it holds up vs. the current crop.

What a classic piece of kit!

I would not hold my breath :-) ...........

Headphone people like our Tyll & Steve G will boast about the OTL Bottlehead Crack. I claim that the Schiit OTL Valhalla is superb.

Loudspeakers don't get built with high enough impedence for OTL Amps so we don't have anyone offering the darn things,

do we ?

Tony in Michigan

OTL amps can work wonderfully. I have a 2 X 30W Atma-sphere and the sound it produces in combination with a pair of 16 ohm Sterling LA3/5As is remarkable and addictive.

But with an electrostat? OTLs are a compromise of sorts...the design is forcing a low voltage and — relatively — low impedance output from high impedance / high voltage devices by consuming a ton of power: it really wants to see a transformer. An ESL is a reciprocal compromise — it has very high impedance and requires high voltage and it uses a transformer, its and all non-OTL tube amps’ Achilles heel. So the far preferable way to drive an electrostat is with a “normal” tube amp driving it without a tranformer. Like Acoustat did in the 70s and which I do with a Berning tube amp driving Martin-Logans.

I met with Julius Futterman while he was still building OTL amps himself. I saw his power transformer winder and potting setup. He liked to make his own because the transformer had to be very low impedance to provide the current needed for 6-8 output tubes.

The key design feature of the Futterman and its clones is very low output impedance which is achieved with gobs of feedback which is properly applied and sounds great. I recommend Futtermans early papers which can be found on the internet.

He gave me a copy of an impedance chart he had run on the KLH-9 ESL. He noted it was high and never below 12 ohms or so. Not at all like modern ESLs that dip to an ohm. Even the ESL 57 gets down there at 18Khz.

As one who has designed Futterman type OTLs (the Counterpoint SA-4 and my own) these are not the mate for modern ESLs where the impedance falls to an ohm or so. The famous marraige of the OTL and ESL is particular to the KLH-9 and Futterman OTL.. ONLY.

Beveridge and Acoustat had the right idea in making a Direct Drive amplifier at high voltage and reasonable current. The Beveridge Model 2 and 2SW amps put out 1,500 VA (like watts in this application). The Acoustat amp was not quite as good on current, construction or reliability. It used the wrong output tube.

Since few amplifiers can put out 1,500 VA nor can many step up transformers easily pass it, the DD amp is the way to go. I have made several that put out 2,000 VA for use with the Acoustats, ESL 57 and ESL 63.

One other comment on OTL. Sadly the Atmasphere amps do not drive ESLs or any speaker with varying impedance well as they have very high output impedance (low damping). So lets be careful when considering how OTLs behave with ESLs and in general.

We published a characteristic letter from Harvey Rosenberg in 1999: www.stereophile.com/content/scientists-ivsi-audiophiles-1999-more-letters-part-4.

John Atkinson
Editor, Stereophile