What the builder had to say:
Here is a chance to own an amazing envelope filter. The patent for the Musitronics Mutron III has expired and is public domain. * Someone designed a clone of the circuit and you can buy the PCB. I doubled up on the parts since I had to order stuff from a few different places. I came out with two working pedals so this one needs to be sold. I want to be clear to anyone interested that I’m not producing these for profit but am only selling this one extra pedal that I made. If you are looking at this you probably know that the Mu-tron III is THE envelope filter made famous by people like Jerry Garcia, Stevie Wonder, Bootsy Collins, and so on. I have tried others on the market without satisfaction. Had the Emm*Dis***bobulator on my pedal board for awhile and I can say this pedal blows it away. I couldn’t afford a vintage Mu-Tron III that may or may not work, so I found this design and spent a lot of time building them. I am quite pleased with how they turned out.
I built a second with some added modifications. The most useful one was the sweep control, so I added it to this one. The circuit is the same as the original complete with NOS original optoisolator. The only exception is the sweep control which just allows you to have a variable resistor value where the PCB calls for you to decide which value resistor to use, (every pedal would have a different value). This pedal is built with a true bypass wired foot switch. Also there is an extra power section on the PCB which allows you to use a standard 9V power supply with the same effect as the original which used a special power supply or 2, 9V batteries.
I decided to leave the pedal unfinished so the buyer could finish it as they choose. You could leave as is, simply label the knobs, or sand it down and finish with spray paint. You can take out the guts without any soldering. Then sand, prime, paint, label, clear coat, put the guts back in, attach the knobs and put it on your pedal board. Use this one first in the chain.
* From Duck Duck Go AI: Invented by Mike Beigel and Aaron Newman in the early 1970s, the circuitry for the envelope-controlled filter was protected under U.S. Patent No. 3,911,776 (granted in 1975). Because U.S. utility patents from that era only lasted for 17 years from their grant date, this patent expired in 1992. As a result, the electronic schematic and underlying technology are entirely free for anyone to copy, modify, or clone.
