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Stupidly Wonderful Tone Control 2
A Simple Passive Tone Control for Effects Pedals

This is the basic layout of Mark Hammer's "Stupidly Wonderful Tone Control" (SWTC). It is an excellent idea and has numerous uses, especially on the output of fuzz pedals that have a lot of harmonics to be tamed. The idea is that the amount of high frequency rolloff is modified as the wiper is moved from left to right, while keeping the overall output volume at essentially the same level.

This design works quite well but has one limitation in that the high frequencies will always have some amount of attenuation because of the R1/C1 low pass network. In circuits with an abundance of harmonics and overtones, as with distortions, this is perfecty acceptable, but for other uses, a different response may be needed.

By moving the C1 capacitor, we have totally changed the response of the tone control circuit. With the wiper of R1 all the way to the right side, the frequency response is totally flat as if the capacitor is not in the circuit, which it isn't because the two ends of C1 are shorted together in that position. As the wiper is moved to the left end of the pot, proportional amounts of treble boost are added to the circuit.

With the new "Stupidly Wonderful Tone Control 2", the volume always remains the same but the amount of treble is controlled from flat to boosted as the pot is adjusted. There never is a low frequency rolloff with this circuit.

I used a similar tone output in one of my compressor designs. Compressors often have the effect of making the signal a bit muddy or less clear, and this tone control allows the user to dial a small amount of treble boost back into the signal to open the sound up. I would prefer Mark's design for fuzz and distortions and this new design for compressors and chorus pedals, or other circuit designs where a clear high end is desired.

I recommend R1=50k and C1=0.022uF as starting values for this circuit.

The next logical question is "How can we combine the two circuits?". After a bit of thought, I found a way to accomplish this.

By adding a single resistor and moving the position of the tone control potentiometer, I have made a tone control circuit that has a response that can be altered from high cut to high boost as the knob is turned. As with the previous "Stupidly Wonderful" circuits, the output resistance is constant so the volume does not vary as the tone control is adjusted.

When the wiper is at the grounded end of the tone pot, the R1/C1 network makes a low pass filter exactly as with the original SWTC, and there is a treble cut. When the wiper is adjusted so that it is at the top end of the pot, the C1 cap bypasses R2 and it creates a treble boost as in the previous SWTC2 circuit on this page.

The 100k tone pot and the 100k output volume are always in parallel as a constant load. A linear taper pot is suggested for the tone control and a log (audio) taper for the volume control.

Suggested values for beginning experimentation are R1=10k, R2=47k, and C1=0.022uF.

A limitation of this combined tone control is that there is some signal loss, as with any passive network. However, many pedal designs have plenty of output signal level, and this tone control is an excellent option for those circuits with enough drive.

Use this new tone control circuit as you like but credit where you learned about it. A link would be nice too!


©2007 Jack Orman
All Rights Reserved

This page last modified on Tuesday, 18-Sep-2007 08:51:39 PDT

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