The first stage is just a unity gain buffer. This gives buffered bypass. Some "true" bypass believers will insist that there should be a "copper" path between input and output when the effect is bypassed, but in practical terms, this way is usually much better. You're guaranteed a low output impedance regardless of mode, and you can drive long leads without loss.
The second stage is where the magic happens (for me, anyway). It's a lowpass filter. It takes all the high end off the guitar altogether. This is deliberate, and is what gives this effect its unique "voice".
The third stage is the usual sort of "diodes in the feedback loop" clipper (asymmetry can be applied here if you like), just like a zillion other pedals, but the addition of the series resistor with the diodes changes the character of the distortion somewhat. Versions have been built with series potentiometers replacing the 1k (I used 22k to get a good range), but the additional change in tone wasn't worth drilling another hole in the box for! By all means increase (or decrease) that value - I've used 4k7 on some builds.
Following the clipper, there's a passive tone control. Put your favourite version of Big Muff Tone circuit if you want, but the simpler one here works really well. The clipping stage introduces lots of harmonics, and the tone control can round them off nicely. The capacitor values can be open to experimentation, of course. If you change the 33n for 22n or even 10n, you can get some really nasty high-frequency content - I don't like it, but some might!
The final stage is another buffer. The output jack "sees" the same impedance whichever feed is selected, so that subsequent equipment doesn't get any surprises.
Overall, this is a fairly low gain pedal (certainly by modern standards), but it will give you a sound that you've heard on some very famous records!
Here's the schematic:
- ReallySmoothOverdrive.png (7.6 KiB) Viewed 6926 times
The LED switching looks complicated, but isn't. It doesn't "click" when switched, and it's pretty smooth to have a bi-colour LED change colour when you kick the effect in. I also found that the green LED needed a bit more current for equal brightness with the red one, but you may find that yours are different - you might have to change the series resistor values to suit. Generally, you should go for the highest resistor values you can in that position, and still have the LEDs visible. After all, the LEDs are just status indications, and it would be pretty stupid to have a 2mA pedal become a 30mA pedal just because of the LEDs!
