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OTL, Output Transformerless Amplifier User Group.

RE: From what I understand the Zero's...

"Chris, You and a few others are the ones taking the purist stance."

Well, I am a theoretical physicist in my day job, always questioning, seeking to extract general principles, universalities, and precise definitions. I suppose I tend to apply a similar approach in the rest of life; automatically questioning, looking for inconsistencies or ambiguities, and so on. (Doesn't always go down too well, I have to admit!)

So in this case too I just like to have a clear, precise and robust definition of what OTL means. For me, it means one thing and one thing only; no inductive impedance-matching device between the output tubes and the loudspeaker. As far as I can see, any attempt to broaden this definition so as to allow some impedance-matching transformers but not others is just opening a can of worms, and it will end up with arbitrary criteria being introduced in order to try to justify something that really cannot be logically defended.

In any case I don't see any need for such a broadening of the concept of OTL; better just to stick to the simple yes/no criterion of whether there is any impedance-matching transformer between the output tubes and the speaker or not.

In a case where a Zero autotransformer is being used, it can perfectly well be described as an OTL amplifier supplemented with an impedance-matching transformer that connects to the loudspeaker.

I guess the thing that prompted me to comment in the first place was that for myself, I would feel it to be a bit of an admission of defeat if I had set out to build an OTL system and then ended up resorting to using impedance-matching autotransformers in order to do the job satisfactorily. Now, I'm sure it is true that certain speakers with especially low impedance might be quite challenging loads for some OTL amplifiers. If I had such speakers, and was determined to use them with an OTL amplifier, I think I would direct my attention to beefing up the OTL so that it could handle the load, rather than using transformers.

In the end this comes down to a matter of personal taste, I suppose. I built my OTL amplifiers because I was fascinated by the idea that tubes could drive ordinary loudspeakers directly. I wouldn't want to give up on that motivation, and if I had very low impedance speakers that provided a difficult load for the OTL amplifier, I would view that as challenge to overcome by improving the OTL amplifier itself.


Edits: 05/16/17

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