MUSITRONICS: Mu-Tron III

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.