Here's my attempt at creating a smooth, low-gain overdrive sound, tailored to my own needs.
The drive circuit is pretty standard - an opamp with diodes in the feedback loop (AKA Tube Screamer Clone), set up for symmetrical clipping. The tone control is a bit more sophisticated. A 2 pole Sallen-Key lowpass with a variable cutoff frequency between 484 Hz and 9.1 KHz. I find this solution allows for better control of the sound than the usual 1 pole passive designs. I chose not to use a dc blocking cap between the 2 stages as I felt it would interfere with the filter stage. This also meant that biasing IC1b wasn't necessary. The gain pot is 100K but it could probably be lowered to a 50K since I do not use this pedal for high gain sounds and I have the gain control set at about 9 o'clock most of the time. Using a larger value would increase the amount of gain available.
My main guitar is also diy, has a pretty hot output and is set up with heavy strings tuned down 2 whole steps. I play this through an Ampeg V-4 and that is what the overdrive was tuned and tested on. Your mileage may vary.
Here then is the schematic
Oh, before I forget, diystompboxes.com rules!
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