I’ve already built the Micro V, and it’s a really cool-sounding envelope filter, so I thought I might try the Musitronics Mu-Tron III. It’s generally regarded as THE envelope filter of its day. As much as I love a bit of fuzz, I really do enjoy sitting in a groove with an envelope filter running….
Note read comments below – currently unverified.
MUSITRONICS MU-TRON III – VERO LAYOUT
Same again, but bigger… bit more room on the board.
Recommend that you check it against the schematic below if you plan to build this before I do. I’ll post photos etc once I have it made, or at least a progress report from the bench.
The placement of the trimmer is a bit clumsy, but I think I will test some values and hopefully replace it with a standard resistor. The trimmer is R23 on the schematic – see note re value.
Once (if ever) working, I’ll need a suitable funky enclosure to fully release the seventies vibe.
ON THE BENCH
Seems to be partially working, but it’s not fully wired up with all the switches etc yet. I used the larger layout for this.
The homemade vactrol is working, although white heatshrink still lets light through. Needs to be blacked out more – the case should take care of that, but it’s not helpful trying test it like this.
It’s being very tricky to dial in, which may be the homemade vactrol, and/or the size of the trimmer on the LED. Seems to have a very small spot where it works, but even then, the sweep is not great.
Apparently, these are notoriously difficult to get going properly – maybe a Micro V is a better idea if you consider trying this.
Suggested mods/notes to self to address issues
1) bigger trim pot. 220k
2) change 180k resistor to 500k trimmer
So still a work in progress – UNVERIFIED.
MUSITRONICS MU-TRON III – SCHEMATIC
Note that the schematic above shows where to place an external control input for the envelope filter.
The strange part labeled O805 is a single LED with dual LDRs. It is not available to buy, so your options are make your own with LDRs and an LED, or try a couple of single optoisolators and see how that works.
Google “R.G. Keen Neutron Filter” for more details on options.
MUSITRONICS MU-TRON III vs EHX Q-TRON
As it happens the EHX Q-Tron is a direct progression of the Mutron III, albeit appearing over twenty years later. They share a lot in common in terms of topology – perhaps due to the fact that they were both designed by Mike Beigel of Musitronics fame.