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AMZ Dual Tone Control

Expanding 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 values shown here are from the vintage Muff pedal, but even on that classic box, the values sometimes varied slightly from those shown.

I had long thought about a simple tone control with independent Bass and Treble knobs to replace the basic tone network shown above and suddenly it occured to me that it would be easy to implement one with just a few component changes.

We replace the 22k fixed resistor in the bottom section of the tone control with a potentiometer of about the same value. It becomes the new treble control. The existing 100k pot is reconnected so it is across the 0.01uF capacitor of the upper section and it becomes the bass control.

To reduce interaction between the sections, a pair of 47k isolation resistors (R1, R2) have been added on the output to mix together the signals from the two sections.

Voilą! The single tone knob of the Big Muff type control has been easily modified into separate bass and treble controls!

The many BMP tone control mods that are posted on this web site or available on the AMZ CD will work with this circuit as well.

Here is a variation that reduces the mid-range scoop that is characteristic of the original tone control. Note that I have changed the value of the treble pot as part of the mods to reduce the notch.

The values of the isolation resistors can be increased to 100k for less interaction of the controls but will cause more overall loss. The 47k is a good value to start.

It would be easy to mod an old Muff to have independent controls over tone. It will open up a new palette of tones for your distortion sounds. Try it on your next pedal build!

The circuit will function best if the input is driven by a low impedance source such as an opamp, and the tone control output is connected to a high impedance input such as a jfet; however, it works just fine with the bipolar transistor stages of a Muff. I did a test using a jfet buffer on the input, then the Dual Tone control and a mini-booster on the output. This configuration works well and gives a wide-range of controls over the sound of the booster.

Click here for another graphic that outlines the development of this mod. Did you know that this general tone control network was patented in 1940? It expired long before E-H used it and you can find a copy to read in my blog.

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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