How To Eliminate Amplifier Squeal When PreAmp Power is Turned Off
Would adding a capacitor into the mix solve this little annoyance?


There's probably already one on the pre-amp board and this is providing power for the squeal during power down. Adding more capacitance will probably make it worse.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1 (a) Exisiting circuit. (b) Modified circuit.

The pop is a "DC thump". The preamp is powered by +9 V battery so when quiet the output of the internal amplifier will sit around 4.5 V. When you switch off and on there will be a kick on the left side of the capacitor, the right side will switch with it and discharge through the amplifier input.

My suggestion is to use a 3-position switch which will short the audio to ground before disconnecting the battery. i.e., Figure 1b: when switching off SW2 closes first muting the output and SW1 opens. On switch on SW1 closes first, powering up the preamp while SW2 mutes the output then SW2 opens, removing the short.

Posn    SW1     SW2
   A    Open    Closed    Power off and muted.
   B    Closed  Closed    Power on and muted.
   C    Closed  Open      Power on and music!


I don't know where you would find a toggle-switch with those characteristics but a multi-position rotary switch should be obtainable and with a suitable knob would match the other controls on the guitar.