This is a really simple design for an overdrive pedal. My goal was very low noise, low power consumption and an overdriven amp type distortion sound. The circuit uses a very low noise OPA1652 to boost the input and control overdrive level. The signal then goes into a single voltage starved Jfet operated at 5 volts to create the distortion. The biasing of the Jfet is fairly critical, in order to make the distortion symmetrical. If other Jfets are used, one might have to tweak the bias resistor values. The pedal is very low noise and provides signal boost – all the way to heavy distortion. What I like about it most is that the pedal is very responsive to playing dynamics, allowing for subtle all the way to crunch type distortion depending on how aggressive you play lines or chords.
In the schematic, Q1 and Q3 are generic PMOS and NMOS fets- nothing special here – many will work. The PMOS is used as reverse polarity protection. The NMOS is for the LED activation. Also the 5v regulator can be anything – even a zener if you like.