This is an classic, old-school, style distortion pedal that I built. Low in parts, based around a vintage metal can LM748. It is similar to the 741 opamps (used in MXR Distortion+ for example) but it has a frequency compensation capacitor like the LM308 (used in the Proco RAT). The pedal is closer in design to a RAT than a Distortion+.
The design is simple. A non-inverting opamp with variable voltage gain up to about 200. The opamp stage is followed by the clipping stage, two back to back silicon diodes (1N4531) connected to ground. Then a simple tone control that cuts treble and an output level control. Audio-wise the LM748 is a really bad opamp with low slew rate and low bandwidth (but that’s not necessarily a bad thing in a distortion pedal where the clipping of the signal in the opamp is part of the sound). It is fitted with a compensation capacitor (I choose 47 pf) that limits high frequencies, which also affects the clipping characteristics of the pedal.
For this one I designed my own PCB in KiCad and ordered it from JLCPCB. I have only ordered from them once, but it went smooth. I think the most important part is to really pay attention when designing the PCB and to verify the design and all generated files that you will send to the fab house. I might write something about that later.
Controls on the pedal are, left to right, Distortion, Tone and Volume.