# How To Eliminate Amplifier Squeal When PreAmp Power is Turned Off

I have a bass guitar that has a built-in pre amp, which is switched ON whenever the cable is plugged into the bass. I typically use the bass all day, and leave the amp on and the bass plugged in - for convenience.

The problem is that as long as it is plugged in (whether or not the instrument is in use) there is a drain on the 9-volt battery.

It is not practical to constantly unplug the instrument, so I decided to add a single pole switch that simply cuts the power from the battery to the pre amp circuit.

This preserves the battery life, BUT, when the switch is thrown it produces a pop and short squeal from the bass amplifier (which stays ON while the instrument is not in use).

Would adding a capacitor into the mix solve this little annoyance? If so, can someone recommend the capacitor value needed? If not, can anyone offer a solution? ...or is this simply a bad idea that needs to be abandoned?

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 – 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.

• Might want a bleed resistor in series between position B and ground - to protect the amp's output from large current spike (the one that is currently being dissipated in the speaker coil). – user98663 Sep 21 '16 at 9:03
• This is on the in-guitar bass guitar pre-amp. They take all kinds of abuse when plugging in the jack leads. e.g., When plugging the lead into the amplifier the tip and ring can be shorted during insertion. I think it should be OK but if you have other information ... – Transistor Sep 21 '16 at 9:13
• "Belt and braces", that's all :) Resistors don't cost much and I'm a member of the Royal Society for the Prevention of Cruelty to Amplifiers. – user98663 Sep 21 '16 at 12:11