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Re: Looking for any good info or intro to OTL

Posted by Duke (D) on December 11, 2006 at 00:26:09

In Reply to: Looking for any good info or intro to OTL posted by Xenithon on December 8, 2006 at 13:16:09:

Briefly, OTL amps are (in my opinion) superb at low-level detail, liveliness, and natural timbre Speaker matching is often an important issue - the speaker's impedance curve will often modulate an OTL amplifier's power output, changing the system frequency response. I have some experience designing speakers specifically for OTL amps.

OTLs typically have disturbingly high THD figures, but the correlation between low THD and listener preference in controlled listen tests is very poor. In fact, some researchers find a negative correlation - which means listeners are likely to prefer the amplifier with higher THD! The reason is, low THD comes at a price - the designer has used large amounts of global negative feedback to get rid of high levels of low-order harmonic distortion in exchage for fairly low levels of odd-order upper harmonic distortion (which the ear finds objectionable even in small amounts). In fact, I'm friends with a physicist whose study of the audibility of different types of distortion found that 30% second order harmonic distortion is statistically undetectable! And among those who do detect it, it's perceived as a pleasant warmth. So one of the benefits of OTL amplifiers is a distortion characteristic that is much more compatible with the way the ear/brain system processes sound. The key thought here is that amplifiers should be designed for ears, not for test instruments.

Now in all fairness many single-ended triode amps have distortion characteristics that are compatible with the ear/brain system, as do many Class A solid state amplifiers. A discussion of the trade-offs between these types might be a bit too salesman-ish for this board, so I invite you to shoot me an e-mail if you'd like for me to go into depth a bit more on the subject. Just for the record, I sell or have sold all three types of amps.

Duke

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