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Old 5th July 2015
  #7
Lives for gear
 

I cleaned my monitor. ok it is a dual rail power supply.
that means the transistors have to be a matched quad. this circuit should have been set up with a matched transistor array ic instead of discrete transistors.
I can also see where you think the bc transistors have a lot of 3rd harmonic noise, but it only is like that when someone uses them in circuits with high impedance collector loads (like above 100k). this one is like 7k on the collector so it shouldn't be too bad and 4560's don't like below 2k on the input so there wouldn't be much more than some harmonic differences if any between op amps. the drift on the emitter and collector resistors would be a concern as well as the transistors drifting from them being matched too.
other things that I think is slightly unusual. why the switch intersection between c12 and c13 in the first place? and the loading section there seems to be kinda redundant to me. I would think there would be some drawbacks having CRC network in the line stage.