The input stage

Hello guys! Tell me about the power amplifier, what function does the C308 capacitor perform? How exactly will the sound change (addition /decrease of treble / bass) with denominations, for example, 47 pF, 100 pF, 220 pF ?
 

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R302 C308 is an overload protection correction.
It is necessary to select it in such a way that the gain curve crosses 0 dB at the desired, smoother angle and that the phase shift of the amplifier without an input filter is no more than 120-150 degrees at the 0 dB point.

The larger the C 308, the more stable the amplifier is, but at the same time there will be more distortion on the treble and a worse clip.
 

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By the way, which type of capacitors is more preferable there? Film, ceramics, mica? According to the service manual, ceramics are supplied for Europe, and film for Germany. In some other models, mica is placed. Maybe it is possible/ better to combine different types?
 
C317 increase the ULG frequency and methink that C308 is here to compensate this increasement and bring down the ULG frequency such that the circuit phase margin keep being within safe values.

That being said some sims would be necessary to be sure if it s for this purpose, keep in mind that even if C317 value is small it still have a big influence given that R326 has a quite high value.
 
Its rated at 125V I think. Rate it too small, it will fail, and the amp may self destruct. Or did you mean its type or value? Just use the correct value in C0G with high enough voltage rating. C306 is the Miller compensation cap and its not part of the input section, its part of the VAS (voltage amplification section).