One of the pedals that made me want to start making the mini pedals was the old module fuzz unit from a NY effects company. It's a great little fuzz circuit in both of it's forms as a transistor version and the later Op Amp version from the mid 70's. I had PCB's there waiting but never got around to making and developing them. I figured since these originally came out as "modules" that you plugged into your guitar, they would be great as a little foot switched pedal, like this. The artwork is based on the Big Muds I already make and they worked really well in this form.
The transistor version uses the standard 2N5088 transistors I use in some of my bigger Big Muds. I have a few different batches of gain in these so I did a bit of R&D to work out what sounded best. After settling on the one I liked, I added a volume control as the original only had a gain knob, which is essentially a pot at the end like it's bigger brother. This meant that the circuit was always running "flat out" and the volume just adjusted the amount of signal that came out. So with another pot at the start of the circuit you can now adjust how much signal get's into the circuit and how much comes out. Two VERY different results gain wise.
The Op Amp version was introduced in the mid 70's to take advantage of the new chip that was flooding the market. It's a similar circuit but uses the now classic and legendary JRC4558 Op Amp, the one I use here in mine. I've used the same pre and post volume control on this version too as it allows for much more versatile little circuit.
Both of these versions are very different little fuzzes with the transistor type being smoother with more harmonics (I think) and the Op Amp one having more bottom end and "fizz". The lack of tone control doesn't seem to matter much as the tone is pretty nice and the addition of a tone control would essentially get you ..... a ........ Big Mud.