Re: Puzzling audio circuitry
The top valve in an SRPP acts both as an active load and as a (rather modified) cathode follower. The net result is a lowering of output impedance, although not by as much as you might expect. However, this will have little or no effect with a high impedance load so most audio uses of SRPP must be regarded as a popular fad.
The SRPP was invented to drive low impedance loads. There is an optimum load impedance (for lowish distortion) which results in both triodes acting a bit like push-pull, but in this regime the gain is quite low because the optimum load is usually much smaller than the output impedance.
The SRPP is related to two other circuits:
1. the mu-follower
2. the grounded cathode stage with cathode follower and bootstrapped anode load
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