How about if the NFB is disabled, loop broken, eg 27k leg lifted?jarrodthebobo wrote: ↑Tue Jan 09, 2024 5:16 am … if the presence knob is not cranked to maximum (or slightly below maxium), I have some pretty obnoxious 120hz buzz. Additionally, switching in the additional gainstage in front of the plexi topology leads to some strange oscillations unless the presence knob is at MAX.
Is the 4k7 NFB shunt resistor grounded to the same point as the presence control?… rewired the presence pot to the modern 25k pot with 4.7k parallel resistor). Disconnecting the presence pot ground from the circuit and attempting to ground it at different spots has lead to changes in the buzzing tone wise, but never gets rid of it.
Disconnect the ground where exactly, eg is the LTP still passing DC, do its anode voltages change?Disconnecting the ground COMPLETLY however still allows the buzzing to stop if the presence knob is at maximum again,
Sketches of the schematic and layout, and build photos, would be very helpful, may save pages of to and fro.