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OTL, Output Transformerless Amplifier User Group.


Futterman amplifiers are capacitor-coupled. The Berning ZOtl amplifiers are not; they work by raising the audio signal so it "rides" on a frequency around 250 KHz, then the carrier frequency is filtered out at the output by use of a small inductor, rather than an output transformer. Because the carrier frequency is so high, only a small inductor is required which makes it possible to achieve very wide bandwidth and low distortion. I'm sure this is a gross over-simplification, but for sure David Berning is a brilliant guy. Pretty much nothing he designs is ordinary. Absent that small inductor, the Berning ZOtl amps are direct-coupled.

EDIT. I'm sure there's an electronic network involved too, in filtering out the carrier frequency, not just an inductor.

Edits: 12/25/15

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