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RE: As you might imagine, I'm having a problem with this

"First, we have to pop the bubble about feedback. It does not reduce output impedance at all. If it did, a basic law of electricity, Kirchoff's Law, would be violated. The only way you can reduce output impedance is with more tubes, bigger output transformers, etc. Of course, what is meant by the term 'output impedance' must then be under question, but put simply you can't drive a lower impedance simply by adding feedback..."

It seems to me you are conflating two rather different issues here. The output impedance of a device can be defined in various different but related ways. Perhaps the cleanest would be to say that we take the two output terminals, drive an externally-sourced current (Delta I) through them, and measure the resulting voltage change (Delta V) across the terminals. The output impedance is then defined as
Z = (Delta V)/(Delta I).

The output impedance so defined will certainly be lowered if negative feedback is used. And this lowering of the output impedance will have all the usual concomitant features, such as increasing the damping factor for a loudspeaker connected to the output, and so on. This is an absolutely standard piece of basic circuit theory, which applies to OTLs, operational amplifiers, or whatever.

There is a caveat that must be included in the above. Namely, the discussion is applicable only provided that one is never calling on the device to supply, or source, more current than it is capable of handling. This, it seems to me, is the point you are really trying to get at; the amplifier will not be able to drive any more current through the loudspeaker simply because one has added feedback. To get it to handle more current, and thus produce more power into a given load, one would need to add more output tubes.

This same caveat would apply to any other example too. Thus the low output impedance of an op amp with lots of feedback would only be valid provided one did not try to get it to pass more current than it could handle.

But provided one never asks the amplifier to pass more output current than it is capable of, the output impedance, as defined in any standard way, is reduced by the use of feedback.

Chris

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