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!!!! UNDER CONSTRUCTION !!!!
CPF is short for 'Complementary Feedback Pair'. It is a combination of an NPN and a PNP transistor which most interesting property is a very good linearity, and thus low distortion.
The two transistors in the differential amplifier at the input of the outputstage are each replaced by a CFP. The original transistors (or FET's, as can be seen in the figure here on the right) T1 and T2 are still connected to the output of the passive filter, but now drive two new transistors, type BC560C. Two extra 6k8 resistors supply the base current for the PNP transistors.
The quiescent current through the differential stage has remained the same and is still 2mA, or 1mA on each side. The FET's operating point is set by the 6k8 resistors at about one-tenth of the original current. The rest (about 0,9mA) runs through the PNP transistor.
From listening tests and simulations this appears to be the best setting. Because the amplification of the CFP is a bit higher compared to a single transistor, the emitter resistors R7 and R8 have been raised a bit.
Here is an interesting link to a page where a number of different input-circuits with FET's and transistors are compared, like the common emitter circuit, de standard differential amplifier and the complementary feedback pair, with and without emitter resistors. It shows that when the current changes, the change in transconductance or gm is smallest for the CFP-circuit (variant 'G'), which means that this circuit will produce the least amount of distortion when it's driven with an audio-signal.
The next article: Compound vs. Darlington by Rod Elliott from Elliott Sound Products shows various applications of the Compound Pair, which is how the CFP is also called. It is compared with the classic darlington circuit.
Printed circuit board
Schematics and partslist
Printed circuit board designed by Steve