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I'm repairing an old amplifier Sansui A-40 which I got from a friend. Here's a portion of the original schematic:

Channel 2 - Sansui A40

What does the red marked area do, and how does it work? I'm not familiar with that circuit, and I won't dive in for a repair unless I understand the whole circuit.

From what I understand, the left portion of TR06 as an emitter-follower used as an input buffer for the preamp and amp stages. What does the right side of TR06 do?

(In this case the failing in the equipment isn't important, I want to learn the inner workings)

Just in case you need it, the double PNP transistor is a 2SA798. (The full schematic is everywhere on the web, just look for sansui A40 schematic and there you have it.)

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It's a standard differential pair. The left transistor takes a signal from a previous stage and the right transistor takes a fraction of the output signal and creates negative feedback to control the gain of the amplifier.

Here's an example that uses NPN transistors as the differential pair: -

enter image description here

Your circuit is made slightly more complicated because of the Baxandall tone control circuit sat right on the right transistor's base which, is essentially doing this: -

enter image description here

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  • \$\begingroup\$ thank you, now that you explained it i feel foolish for not figuring it out myself. \$\endgroup\$
    – ndelucca
    Oct 18 '16 at 15:27
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The circuit given in your question is a power amplifier and actually has no significant difference from an opamp.

The marked area is a differential amplifier (the two bases are inputs) and has the same job as Vin- and Vin+ pair in an opamp.

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