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OTL, Output Transformerless Amplifier User Group.

Sorry no, its designed in distortion numbers are quite poor

If you have read the data sheets you ought to have noticed that. Look at the transfer curves?






When used as a cathode or concertina phase splitter they are all but blameless, but that's with 100% local negative feedback.

Do a bit of digging and I'm reasonably sure you will find that the 12AU7 does have RF antecedents.

When I first got into audio - as an idea - it was via getting to know recording engineers for our national broadcaster - down here in Aus. - who would often record the cathedral choir I sang in, with just one single-capsule stereo-mike. I was a tween at the time.

Valves ruled back then, and DIY too, and I don't recall anyone telling me how wonderful a 12AU7 was as a gain valve. All who commented said what I am saying.

The 12AU7 is/was used by mfrs because its current draw is low and thus the PT doesn't have to be so expensive. It's strange fame with rollers may also contribute to its ongoing use.

Nevertheless this is a perfectionist hobby, and keeping overall/total NFB down is a pretty good idea, no?

But, using a 12AU7 for gain duties means extra NFB.

My current (version 10 ???) pre-amp still (since v3) uses a 12AU7 at the output, it's currently a selected Brimar 13D5 but it's in CF line-driver role. Good economical choice.

If I ever do build with a balanced output stage I might just double up but add CCS's under each valve.


Tim Bailey

Skeptical Measurer & Audio Scrounger

Edits: 10/17/15 10/17/15

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