Muroidea (ProCo Rat)


Well-known member
The good ol' ProCo Rat. It's probably not far beyond the Tubescreamer as one of the most long-lived, popular, and talked-about circuits. Highly versatile, from overdrive all the way to borderline fuzz. Simple three-knob operation. Simple circuit.

Here's my build of the PedalPCB Muroidea Distortion. Nothing terribly interesting about the build itself. I found a few LM308 opamps in my collection, though I can't remember where I got them (might be sketchy ebay LM308s). I used a 2n5457 JFET instead of 2n5458. As you can see, I had a couple silly-big 1uF film caps waiting for a PCB with sufficient board space, and the Muroidea delivered! I also used a 33pF compensation cap instead of the 30pF specified by the BOM (because that's all I had on hand). I think everything else is per the BOM. ;)

I used to have an actual modern production Rat, but I sold it a while ago, so I don't have anything to use for comparison. But it sounds like I remember!

The bypass mechanism is a relay-based true bypass circuit that uses a CMOS Schmitt trigger inverter (CD40106 or 74HC14) to maintain state and respond the the footswitch. I can't claim credit for this circuit, RG Keen proposed it about 20 years ago. Here's a brief discussion about it over at DIYStompBoxes: Super simple inverter-based relay bypass. I've been a on a microcontroller based relay bypass kick for quite a while. This circuit is functionally the same, but it's a little simpler (no MCU to program) and a little cheaper, at the cost of a slightly bigger footprint. But it works great, and uses virtually no power.


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