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OTL, Output Transformerless Amplifier User Group.

RE: I never said it was....

Well, my own OTL amplifiers are home-built. Two of them use 6C33C tubes in the output stage, totem-pole style. (One pair of tubes in the output stage.) These give about 25W maximum into 8 ohms. One of the OTLs is based on a design by Hans Beijner, the other a design by Tim Mellow. I built another OTL that is a circlotron, based on a design by Alan Kimmel. This one allegedly gives up to 100W into 8 ohms, but I've never tried pushing it to measure that. Finally, I have one I really built just for fun, loosely based on the very early Dickie-Macovski design, with no iron-cored components whatsoever.

The speakers that I use are all high or fairly high efficiency. Lowther DX3 drivers in bass reflex cabinets; Klipsch Forte II; and Paradigm Monitor 7. With these, I've had no problems at all driving them from any of the OTLs. In fact the Lowther DX3 drivers have paralleled Audio Nirvana drivers too, so I suppose they are then nominally 4 ohm. But the sensitivity is so high that running out of headroom is never a problem.

I fully agree with you that if you have already an amplifier and speakers, and if the amplifier is not able to drive the speakers comfortably because of impedance and current-delivery issues, then Zero transformers may well be a reasonable solution to the problem.

Personally, I would be left feeling slightly unsatisfied if I were having to do that with an OTL amplifier, since it does, to some extent that different people can hold different views about, partially negate the whole OTL philosophy. For me, if I ran into such a situation it would be fairly straightforward for me to rebuild one of my OTL amplifiers in a beefed-up form, for example by doubling up on the output tubes. (In fact it would give me an excuse to build another one, which I have been wanting to do for a while, but currently find it hard to justify the need for a 5th one!) I can see, though, that if one is buying ready made amplifiers the financial considerations would be somewhat different.

Regarding whether Zero transformers should be called output transformers or not, I think that all that can be said on the subject has been said already. My main point in my observations on the topic is that one is going to be somewhat hard pressed to come up with a universal definition of what is, and what isn't, an output transformer if the definition is not simply going to be crafted for the express purpose of "allowing" Zero autotransformers while disallowing everything else.

Chris



Edits: 05/21/17

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  • RE: I never said it was.... - cpotl 02:29:16 05/21/17 (1)

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