I want to try changing the feedback capacitor, C2, to a polypropylene. Does the value need to remain the same?
This capacitor operates in shunt mode as the AC ground path on the base of a hefty voltage divider. Its influence on the sonics is thus much less – arguably by a factor roughly equal to the gain – than the series signal application on the input. However, a minuscule improvement is available here, but you need not use such a high value. I would suggest a 63V Ashcroft or Arcotronics film of 4.7É F. It is bulky, but could be mounted under the PCB. These are good sounding capacitors.
