How does OTL amp get its power?

I agree with virtually every point Ralph raises, but as everything in life, it goes both ways . . .

Advantages of OTLs:

1. No output transformer. Really, this is THE advantage and rasion d'etre of OTL amps, hence their name (refers to something they don't have). Whether the performance attributes of audio transformers offset their disadvantages is of course a multi-decade debate.

2. Gain efficiency - actually, really something I hadn't thought of until Ralph mentioned it.

3. Higher potential slew performance.

Disadvantages of OTLs:

1. The transconductance characteristics of vacuum tubes operated in an OTL push-pull fashion is both inherently non-conjugate and non-complimentary - essentially similar to a the "all-NPN" solid-state amplifier designs of the early-1970s. Class-A biasing helps tremendously, but this will always be a fundamental source of large-signal even-order non-linearity, even at higher harmonics. A tranformer-coupled push-pull topology is still non-cojugate, but is inherently complimentary, and provides reliable cancellation of even-order distortion.

2. The plate resistance of virtually all vacuum tubes is WAY too high for effecient power transfer to a typical loudspeaker load. Paralleling a bunch of output tubes is the usual solution, and power-efficiency of OTLs is still very poor, even worse with all of those filaments to run. Now when direct-coupling to electrostatics, it's a whole different story . . .

3. OTLs may be gain-efficient, but they're definately NOT voltage-efficient, and require split high-voltage power supplys (or capacitive coupling, but then what's the point?). The primary inductance of a transformer, in contrast, makes for a VERY efficient use of power-supply voltage, as the maximum AC voltage peaks can be much higher than the B+.

4. Vacuum-tubes have comparatively poor DC-offset performance, and while solveable, this can present significant engineering hurdles. Conventional transformer-coupled topologies (should be) inherently DC stable.

5. The poor power-transfer-efficiency eats up virtually any possible advantage in slew performance, and stability issues for application of global NFB are identical for both types.

In the end, it's obvious that transformers are inherently imperfect . . . but also obvious that a perfect transformer could solve SO many engineering problems. Virtually every aspect of building an acceptable OTL amplifer involves huge sacrifices in efficiency . . . and (but?) efficiency isn't that great with ANY tube amplifier to begin with . . .

Any way you slice it, there are some significant obstacles to making the perfect amplifier.

Kirkus,
What do you mean, "Now when direct-coupling to electrostatics, it's a whole different story . . ."? Are you saying the step up transformers in Quads can be bypassed?

Are you saying the step up transformers in Quads can be bypassed?

Actually, I meant that remark in the sense that there's nothing inherently "wrong" with the fact that tubes have relatively high impedance characteristics . . . it's just very different than the low impedances of dynamic loudspeakers.

For a high-impedance (i.e. electrostatic) loudspeaker, then there are indeed methods for directly coupling a tube amp to its electrodes, and a very efficient transfer of power is possible . . . Acoustat did this in the late-1970s, with very well-received, albiet unreliable results . . . I serviced a few of these many years ago, and can personally attest to both of these characteristics.

A direct-drive electrostat these days is still a pretty intimidating engineering project - the best common tubes suitable for the task (i.e. TV sweep tubes) are long out of production, and custom transformer(s) will probably still be required, for the high-voltage supply. And if you're talking about the ESL-63s, there's the bit about the multi-tapped delay line that would probably be a complicating factor.

If I wanted to build an active electrostatic speaker with a tube amp, I'd probably look more to using a conventional push-pull output stage with conventional, readily-available audio tubes, with a special low-ratio step-UP output transformer to match the electrostatic panel. But it'd still be a LOT of work to get it right.

THANKS TO ALL! I read the white papers on Ralph's website.
Ralph--thanks for the additional input.
Audiofeil--I will be contacting you.
I think that I will try the Atmasphere 60 on the Utopias.

Pepe

Hello,
While Athmaspere OTL amplifiers are superb, and no question about it! I still wish to have fairness to other OTL designers who do not participate in on-line forums. One of them is Jud Barber of Joule-Electra. Not many audio designers have so many truly prestifeound awards like Golden Year by Harry Pearson as Jud has.

His web site is not maintain very well so you have to do a google search. Below is review of of David Robinson, The Editor-In-Chief of Positive Feedback
-http://www.positive-feedback.com/Issue6/joule.htm

In no way I am saying that one designer's equipment is better then another. Its all up to the listener to judge.

Regards