Tone Control Bypass

Bypassing the BMP Tone Control

This is the classic tone stack from the Big Muff distortion pedal. It is basically a pair of filters whose output is combined through the 100k mix pot. The 39k/.01 filter passes only low frequencies and attenuates the highs. The 3900pF/22k filter does the opposite; it passes highs and filters out the lows. The 100k pot blends together the signal from each filter and therefore acts as a tone control.

The original tone control, with the component values shown here, has a noticeable mid-range frequency dip around 1k Hz.

We can add a simple method to bypass the tone control. The filtering elements R1 and C1 are connected together but instead of being grounded, they are connected to one end of an SPDT switch. The center pole of the switch connects to ground.

When the switch is toggled so that the R1 and C1 components are grounded, the tone control network will function normally. Flipping the switch to the other side lifts the ground connection on the two components and the tone control is effectively bypassed.

The response of the network is essentially flat when bypassed no matter what the position of the tone knob. The signal output will increase about +6db in the bypass mode but that can prove to be useful.

If a stomp switch is used for the bypass, the tone control can be taken in or out of the circuit by merely stepping on the pedal. A low cost DPDT may be used for this mod with 3 of the switch poles unused and only one section implementing the bypass.

The +6db signal boost in bypass mode can add extra output to the circuit... it's almost like getting a boost pedal for free! Also, if you set the tone control so that it provides a bit of a treble rolloff in normal use, the bypass will sound not only louder but brighter when engaged.

Use this idea as you wish but give credit to where you learned about the idea! A link would be nice too...

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