This is an exceptional, vintage, versatile, highly interactive, nuanced preamp/distortion circuit. A little gem inspired by the well-thought-out 1981 DRV, resulting in something genuinely worthwhile of attention.
1991 Drive delivers refined open and responsive distortion and overdrive tones, all immersed in its distinct vintage aura.
Created by the pure necessity of a vintage distortion pedal able to achieve unique soundings, this circuit was crafted with a lot of consciousness, caring, and passion; and all that shows in the final result. With only three highly interactive knobs, you can find a wide variety of sounds. It has a broad range of great drive waves, from clean tone shaping or shiny drive to entirely saturated distortion tones. It can also be a fantastic dirty boost, shaping your amp to get a perfect distortion with an airy and open breakup that also tightens into more precise playing.
This board includes an internal charge pump powered by IC3 LT1054*. You can use 7660s or Max1044, among others, but you must connect the two pads under IC3 with a little solder bridge.
For further analysis on this circuit take a look at this thread on Free Stomp Boxes Forum.