Reworking my CFA3 for ZF/SS switching.
Inspired by ang728's hat board, I designed a split-board version.
It works fine with ZF but has issues with SS:
R channel: 0.2V offset, fluctuating ยฑ0.1V.
L channel: <30mV offset, fluctuating ยฑ0.1V.
Bias: Stable.
Tried those solutions from previous posts but unsuccessful to solve any issues:
-Removed hat board; use original jumpers for ZF/SS.
-Added 680pF between B-C of output transistors.
-Removed C9, C10 (20pF).
-Replaced all PZTA56/06.
-Swapped THAT340 / K389-J109 / K170-J74.
Changing R171, R211, R272, R322 from 300R(silkscreen) to 600R(schematic) fixed the 0.2V offset, but both channels still fluctuate.
Suspecting trace interference, I directly wired the jumpers to SS mode. The fluctuation is gone.
So I redesign the hat board.
SS resistors(R44, R49) sit on hat boards.
SS jumpers and SS resistors are removed from cfp3smt2splitpre.
My final result to get stable and low DC offet will be:
-Using the hat board
-R171, R211, R272, R322 = 600R.
-Remove C3, C7, C9, C10 (600pF & 20pF)
Thanks ang728, his posts helped me a lot.