How does OTL amp get its power?

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

To Atmaspere:
Hello Ralph, do you agree with five disadvantages of OTLs described by Kirkus above?
I always enjoy reading your technical explanations and your educational role on this forum deserve our deepest gratitude.
Thank you in advance.
Rafael

Rafael, no I do not agree with all his comments, although at at an earlier time some were true.

For example, the comment

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.

applies to Futterman amplifiers only. The Circlotron output circuit (we were the first to use this in a practical real-world OTL) eliminates this problem in both tube and transistor circuits, allowing one to use non-complementary pairs (and for the record, complementary transistor pairs, such as NPN and PNP are never exact matches, so the argument is really a red herring).

I also have to clarify something about this statement:

2. The plate resistance of virtually all vacuum tubes is WAY too high for efficient 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 . . .

There *are* tubes that have low plate resistances. The 6AS7G is an example, as is the 6C33. So in an OTL, you will not find anyone using 6550s or EL34s! It is true that you still have to parallel tubes, but of any higher power amp that is a fact of life. Kirkus is correct about the filament issue, although the filaments often get blamed for excess heat, which they are *not* responsible for. That comes from the class of operation.

Comment #3 applies to Futtermans OTLs only. Circlotrons don't use a split-voltage supply, nor is there any need for an output coupling capacitor. Many people think that if its an OTL, it has to be set up like the old Futtermans, which do have these 'design features' but that is not true.

Comment #4... not IME; many customers of ours have commented on the fact that they can set the DC Offset of the amplifier and it will be exactly right 6 months later. The trick (and you would think this is obvious) is to be able to control the power tubes.

Comment #5 was never true- even the old Futterman amplifiers from the early 60s had slew rates far in excess of their transformer-coupled counterparts. We've measured slew rates of 600V/micro-second in the output section of our amps. This translates to extremely wide bandwidth. Our early prototypes exhibited this trait right away- they made very capable RF booster amplifiers at frequencies as high as 50MHz without oscillation. In our production amps we limit the bandwidth in the driver circuit to minimize RF issues, but the output section retains its speed.

Dear Ralph, As usual your comments are extraordinarely appreciated. I truly thankful to you for your (at very least) educational role for me and I am sure many other members of this forum.

To my regret, I never in my life auditioned your amplfifiers and only a few weeks ago became introduced to OTL amps at all (what a wonderful experience - I could not stop listening to music.... and I auditioned amplifier or two in my life). The efffortlessness, the "magic", the...
.(no words) ... You and Jud Barber are magicians !!!!!!
Thank you again,
Rafael

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.