I haven’t posted much in a while, mostly because I have been very busy building pedals and introducing new products. So here’s a little snippet I’ve been working with for a while. The Electro Harmonix Big Muff Pi uses a very simple tone control to mix between a high pass and a low pass filter. This arrangement of filters lets you get a wide range of EQ curves from flat to scooped depending on the values use, and allows the pot to function as a tilt control. It cuts bass as it adds treble and vice versa.
This style of control has been used on lots of other effects as well, such as the Boss DS-1 for example. While toying with it, I realized that in some circuits, I prefer the bass response of the tone at Noon and would rather have it not cut bass when adding treble. By adding 1 resistor and shorting pins 1 and 2 of the pot you can do just this.
In the alternative configuration, R4 controls the fixed mix of the low pass path. A good starting point would be to make R4 half of the original tone pot value. Reducing the value of the tone pot will increase the amount of treble in the lowest setting, but leave the maximum fixed (when pin 2 is shorted to pin 3). R4 could be made a trimpot internally to tweak the bass mix as well.