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In Reply to: RE: From what I understand the Zero's... posted by cpotl on May 10, 2017 at 05:42:05
(1) The impedance conversion afforded by the Zero autoformer is much lower than that of any conventional tube output coupling transformer, because the impedance matching can be as low as 2:1 and almost never more than 4:1. (The output Z of the OTL is nearly the same as that of the speaker, usually at least a bit less.) Whereas in a conventional transformer coupled (OTC) tube amplifier, you are facilitating an interaction with power tubes that have output Z in the thousands of ohms to a typical speaker with a Z of 4-16 ohms. This makes for wider bandwidth and lower distortion for the Zero vs the conventional output transformer. The job of the output transformer is much harder.
(2) Because it's an auto former and because of the reduced workload, you would not have hysteresis distortion, or at least much less of it.
I don't say the Zero is perfect, but I do say that to my ears the combo of the Zero with an Atma OTL is still superior to every OTC tube amp I have tried on the same set of speakers. Audio is full of trade-offs. If you want to stamp your feet and walk away in protest, that is your privilege. Or find a pair of speakers that don't need the Zeros.
Edits: 05/15/17Follow Ups:
"I don't say the Zero is perfect, but I do say that to my ears the combo of the Zero with an Atma OTL is still superior to every OTC tube amp I have tried on the same set of speakers. Audio is full of trade-offs. If you want to stamp your feet and walk away in protest, that is your privilege. Or find a pair of speakers that don't need the Zeros."
I can absolutely believe that there are circumstances where a particular OTL amplifier may drive a particular set of speakers better if an impedance matching autotransformer is inserted between the amplifier an the speaker. And I can absolutely believe that the resulting system may sound spectacularly good.
My only gripe is if it is claimed that such a configuration is still adhering to the spirit, and the letter, of the concept of an OTL. I think OTL has a very clear and precise connotation: no impedance transformer between the output tubes and the loudspeaker.
As I said before, if you wanted to claim that with a turns ratio of 2-1 it is still an OTL system, but that with a turns ratio of 20-1 it isn't, then you would have to come up with some specific ratio somewhere between those two values, and assert that it is still OTL one side of the line, but not the other. This would be an entirely arbitrary cut-off, with no difference in qualitative behaviour between one side of the border and the other. Hence I simply go back to the (almost orignal) Dickie-Macovski notion, of the absence of any iron-cored components in the amplifier. (Actually they went one stage further and had no iron-cored components in the power supply either, but that is another story, and a side-issue for the present discussion.)
As with many things in audio, I think it is a shame when the image of a probably superb-sounding system risks being tarnished by virtue of claims that it is something that it really isn't. (And don't get me started on the subject of the ZOTL!!!)
Chris, You and a few others are the ones taking the purist stance. I have no desire to argue the point except to say what I said. Yes, the Zero is a kind of crutch to improve the match between an OTL and a typical low impedance speaker. That doesn't bother me. Most people who have employed zeros with their Atma and other OTL amplifiers report that they are happy with the pairing. I was too, until I found a way to modify my speakers to raise both their efficiency and impedance curve such that I no longer needed to use the Zeros. As you know, too, there are also ways to reduce the output Z of many OTLs, by adding NFB, that also might do the job and obviate the need for Zeros. It depends upon which is the lesser of two evils. It's all good, so far as I am concerned.
I would maintain though that a well designed autoformer, like the Zero, used at a 2:1 impedance ratio (I never went above 2:1; the Zeros permit up to 4:1, I think) is in fact likely to give wider bandwidth and lower distortions compared to a true audio output transformer as found on most OTC amplifiers, driving the same speaker, for reasons noted. If you want to say that an autoformer-coupled OTL no longer fits the strict definition of an OTL, I would not argue with that.
One other small point, the Zero is placed at the speaker end of the path from the amplifier outputs, not at the amplifier end, for whatever difference that makes. You might say it acts on the speaker. But I don't want to go there, either.
"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.
I am an MD/molecular biologist-virologist. That mixed identity is confusing enough during my day job. I also have a view of audio that is analogous to something Frank Sinatra is reputed to have said about religion: "Whatever gets you through the night". He went on to say that in his case that was a bottle of Jack Daniel's.
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