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OTL, Output Transformerless Amplifier User Group.

RE: stupid OTL question

"OTL" is an abbreviation for "Output Transformer-Less", meaning there is no transformer between the output tube stage and the speaker. Some OTLs are direct-coupled, like Atma-sphere products, and some are capacitor-coupled to the speaker, like Transcendent amplifiers. Burning ZOTL amplifiers are sort of in a class by themselves. An OTL amplifier can be push-pull (PP) or single-ended (SE), but by far the vast majority are PP types.

The advantage of getting rid of the output transformer is to get rid of nonlinearities, bandwidth limitations, and distortions introduced by coupling transformers. The disadvantage of not using a coupling transformer is that the output impedance of an OTL tends to be fairly high, which may introduce issues with driving low impedance speakers. (The output transformer converts voltage to current and allows the output tube therefore to transfer energy into the speaker. Most speakers have much lower input impedance compared to the output impedance of the average power tube used in transformer-coupled amplifiers.) To ameliorate that issue for OTL amps, one uses a plethora of output tubes in parallel. Very high current, low plate resistance, high transconductance output tubes are also to be preferred. That's not all there is to it, but that's the basic scoop.

Edits: 06/21/17 06/21/17

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  • RE: stupid OTL question - Lew 12:57:43 06/21/17 (0)


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