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.

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.

I actually tried to speak very generically about some of some of the challenges associated with designing a successful OTL amplifier . . . which seemed to be somewhat relevant to the original poster's question. I was NOT referring to disadvantages with Atma-Sphere in particular. NONE of these are necessarily of any particular disadvantage to the end user of a competently-designed amplifier, but they ARE obstacles for the circuit designer.

I personally have a little experience with the Futtermans, and am reasonably familiar with the topology-usually-refered-to-as-"Circlotron" (which isn't necessarily/originally an OTL) . . . and these circuits REALLY aren't all that different from each other -- they're all variations on "push-pull". An excellent analogy would be the QSC solid-state amp vs. the conventional topology - it's disorienting to look at the schematic, but all of the same elements are still there, doing the same things. I share Ralph's view of the "Circlotron" as being the more elegant arrangement, mainly because of the equal-amplitude drive voltages. And I'd speculate that we might agree that the capacitor-coupled "totem-pole" arrangement as being the most problematic.

Engineering is very much an "in-spite-of"/"because-of" kind of discipline . . . and IMO it's the ability to keep this in balance that defines whether or not a design is ultimately successful. Atma-Sphere's longevity as a company is a strong testment to their product being hugely, vastly improved over the NYAL (Futterman) amplifier, and they get good reviews for sound quality in-spite-of/because-of (choose one) the fact that it's OTL, tube, low-feedback, class-a, high-output-impedance, etc. etc. etc.