How does OTL amp get its power?

Go to Atma-Sphere's home page and read Ralph Karsten's white papers on the subject. You can also contact him directly by mail. He is a nice and very helpful person.

You could also wait until I find this thread :)

I'm not sure what you mean by 'OTL amp get its power'... of course the power comes from the wall :) seriously though an OTL amplifier is able to make its power in a way that is
not unlike a transistor amplifier. A lot depends on the type of tube used, some tubes are vastly more suited than others, mostly due to a low plate resistance and high transconductance.

The primary advantage of an OTL is that without an output transformer, distortion is reduced and bandwidth is increased. A less obvious but still important advantage is that an OTL can be a simpler circuit, as with any output transformer the output voltage of the tubes has to be stepped down to loudspeaker voltage, whereas in an OTL this does not happen, so you don't need as many gain stages in the amp. In our case, that means there is only one stage of gain, making for a fairly simple signal path. The less stages of gain, the more bandwidth and lower distortion.

Anytime you reduce distortion, the result is a more detailed sound that is simultaneously smoother. Increasing bandwidth can have the effect of increased impact on the bottom with greater low frequency extension (although right here I will interject that so-called 'tight bass' does not exist in the real world and is a phenomena of excessive negative feedback in an amplifier design), and an obvious increase in speed on top.

The heat is a function of the the class of operation, just like with any other amplifier. A class A amplifier will run hotter, regardless of the technology.

Clipping is a function of the power of the amp. Some OTLs can be unstable at clipping, but that can be true of many conventional amplifiers too. We have built OTL guitar amps that are intended to be overdriven and they work quite well. I believe that any proper amplifier design, regardless of technology will have instantaneous overload recovery and will be unconditionally stable- that is to say it will be stable regardless of the input signal or output load.

'Reserve power' is a term that refers to the class of operation- by definition a class A amplifier will have 0 db of reserve power. IOW, the better the amplifier (regardless of whether it is an OTL or not) the lower the reserve power figure will be (FWIW this term is counter intuitive on purpose to make less expensive AB amplifiers look better).

OTLs can drive JM Labs speakers quite well. This has more to do with the power of the amplifier rather than its technology. JM Labs speakers, IMO, have traditionally been tube-friendly, but they do need some 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.