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RE: OTL amps rule!

"But to most people,some OTLs output PS caps look like there in parallel not in Series"

I'm never entirely sure what people mean when they say that the power supply capacitors are "in parallel with the output." But if they find those words more satisfying than "in series with the output," then I suppose that is fine, although it seems to me to be a misleading way of saying what is happening.

But what is not in doubt is that the audio signal is passing through the capacitors. If we consider a circlotron, for example, then there are two "signal current loops" in the output stage. One loop comprises the upper tube (or bank of tubes), the corresponding power supply capacitor(s), and the loudspeaker, daisy-chained in series. The other loop comprises the lower tube(s), the capacitor(s) of their associated power supply, and the loudspeaker, again daisy-chained in series. Any signal current that passes through a given bank of tubes necessarily also passes through the capacitor(s) of the associated power supply in that daisy-chained loop.

So provided one accepts that the audio signal "passes through the output tubes" (which seems like an uncontroversial enough statement!), then one necessarily must accept that the audio signal also passes through the power supply capacitors.

As far as class A OTLs are concerned, there do exist rather low power single-ended OTLs, which are necessarily class A. (They are also super-inefficient, even by OTL standards.) Such an amplifier would almost certainly have to have an "explicit" output capacitor, to keep DC current from flowing through the loudspeaker.

For push-pull types of OTL, whether totem-pole or circlotron, they are essentially what one would normally call class AB. Of course, for sufficiently low output levels they are "operating in class A," in the sense that if the signal is low enough that the tube (or bank of tubes) that is being driven towards non-conduction in a particular half cycle is not being driven fully into non-conduction, then for such a signal level both banks of tube are conducting all the time. But once the signal becomes large enough to take the current in the conducting tube(s) to about double the quiescent current, or higher, the current in the other bank of tubes will have gone essentially to zero, and so the amplifier is then operating in genuine class AB. In practice, the OTL will only be operating in class A for rather low power levels compared with the power it is capable of delivering in its class AB mode.

Chris



Edits: 12/07/14

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  • RE: OTL amps rule! - cpotl 09:45:59 12/07/14 (1)

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