What an OTL is, and why you should care


Member Sponsor & WBF Technical Expert
Jun 23, 2010
Monument, CO
Just a note to thank Ralph for this thread, cool stuff!

Disclaimer: I owned and/or worked on a number of tube amps through the years, including Futterman (to me sound was "ok" and reliability, well, good for a college kid working as a tech ;) ), and did build a couple of OTL amplifiers based upon a push-pull approach with feedback to get a reasonable (to me) output impedance and a servo circuit to control DC on the outputs. The latter, along with a matching preamp, was a differential circuit so complicated as all get-out. It measured and drove speakers better than any of the other ARC or CJ amps I had around but I never went further with it. Sold the prototypes, but the main feedback I got was "too SS" -- it was much cleaner than the tube amps of the day but that was not what people wanted to hear. I'd love to hear an Atma-Sphere some day.

IME tubes (non-OTL amps) on ESLs are gorgeous in the midrange but have trouble controlling the bass and get harsh in the highs as the impedance drops out of the "good" region of the output transformer. The latter is also a problem for many SS amps IME but most did better than tubes -- if they could handle the low impedance!


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