Mad professor’s Stone Grey distortion ™ is their take on putting modern metal tones and heavy distortion in a pedal without simply cloning an existing circuit. A very powerful one-knob tone control, outstanding sting to string definition, tight low end, and a gain control that’s not losing crucial definition when dimmed out will stand right in front of you in any fight you and your axe may could have. As a mod, and to make this beast even better we added a toggle where you can switch between different FETs and MOSFET.
I highly recommend using sockets and experiment with different types. 2n7000 for instance. But take your time with that, you probably would dial it in different. Just cranked all the way the differences can be very subtle.
*You could try replacing 2N3819 with a J201 or 2n547, just bear in mind the different pin out of those (Gate and source are reversed, Drain stills the same) They are acting as clipping diodes of the feedback loop at the opamp.
Because of its low amount of components and its wide range of distortion I can recommend this as a beginner project.
For further analysis on this circuit take a look at this thread on Free Stomp Boxes Forum