Actually this is just a slightly modified and hacked together copy of a clone (or “work-a-like” as it’s put) of a distinguished old music accessory. The original was invented by one Mike Beigel and marketed as the “Mu-Tron III“. It’s an envelope-controlled-filter and you can find an in-depth description of the concept here . The plans I started with can be found here .
The slight modifications do open up a world of new possibilities for this effects box.
Some experimentations are documented on the
over on the next page
- 3 extra outputs for simultaneous output of high-pass, band-pass, and low-pass, besides the normal switched output.
- 1 extra input allowing for external control of the filter sweep.
- a 2-way momentary trigger switch to manually sweep the filter up or down.
- an external LED which follows the envelope detector (copies the vactrol’s LED).
- an extra bypass switch which lets the input go completely around the effect circuit to the output.
- a 3-way power switch to select between batteries, dual-voltage DC adaptor, or just off.
Since the state-variable filter in this design can output high-pass, band-pass, and low-pass simultaneously, I added 3 direct outputs besides the normal output which can switch between the 3 modes. Branching the different stages of the filter to different chains of effects was what I had in mind here, plus it’s nice to have the extra levels of tweekage offered by an external mixing device. The extra outputs also allow for some weird feedback patches to be made, perhaps owing in part to the fact that the original mu-tron (and thus this design) inverts the phase of the audio signals which go through it.
I further added an extra input so that an external signal can control the filter sweep. Plugging in a jack bypasses the audio’s normal branch into the envelope-detector. Basically the just the control-voltage idea here. One application for this might be to route one of the extra outputs through a delay pedal and back to the external input. Even routing any of the extra outputs directly into the external input will change the character of the sound very slightly by altering envelope detector.
The 2-way momentary trigger switch is actually quite handy. The filter is controlled by a vactrol (opto-isolator) which is just an LED with some photo-sensitive resistors (LDR’s) pointed at it. Pulling the trigger one way just adds +9 volts to the LED and pushing it the other leaches the power away so the LED goes blank. The currant limiting resistor I eventually chose for the LED was a 2k2 (I think) because it sounded the best with the parts I was using. I just chose from the range of resistance which allowed the filter operate properly in the effect’s “down mode”. because too much brightness would cause lots of extra noise where there is supposed to be silence. In retrospect, I think that an extra pot might have been more fun in the end. In my configuration the LED operates with a lot of headroom as far as brightness goes, so giving the + trigger the option of giving the LED much more currant than normal could also make for some extra neato sounds if desired.
I “rolled my own” vactrol using one of the suggestions in the project file. 2 LDRs I had sitting around and an LED soldered onto a 14-pin IC connector. This was mounted onto some leftover PCB with the right holes already drilled and then housed in the plastic case of a no longer working 9 volt rechargeable battery. Liberal quantities of hot-glue to hold it together, bolted into the box, and a bit of duct-tape to make it light tight.
Having the open vactrol on the breadboard was fun not only because of being able to play it like a “light-therimin” but also because actually seeing the LED do its work is darn useful. For that reason I added an extra LED on the outside of the box. This sort of behaviour is kind of hinted at in the project file I think and “the more blinking lights the better” is always a good rule regardless their of function. I’m certainly interested in how different types of visual feedback can influence the user’s interaction with an audio instrument, plus, the light always gives a reassuring little ‘blip’ when you switch the effect on so that’s a bonus.
I really must have missed something when I tried to add a “true-bypass” to the effect. It works as expected sometimes and gives a ‘hum’ at other times, but that is probably because I didn’t ground the box with the effect properly. I can live with this because the grounding issues have turned into tons of extra fun sound-wise.
The other extra switch I added was a 4-pole 3-position switch for handling the circuit’s power supply. A friend who had an original Mu-tron III for bass-playing told me many years ago that drained batteries were a real problem with that pedal, so I wanted an on/off switch which could totally isolate the batteries from the effect and from each other. Maybe a bit of overkill but much quicker than bothering to figure out how electricity works. The 3rd position on this switch lets the power come from an AC/DC dual power supply adaptor. Why I wanted the external power to be totally isolated from the internal batteries is a mystery to me but that’s the way it came out.
I really wasn’t sure as to whether I should put this switch on the front panel or perhaps some where a bit more out of the way, but I eventually opted to put all the switches together reasoning that silly mistakes are easier to avoid and correct with everything upfront. The bonus here is that the switch turns out to be very handy for creating extra crazy noises when used with certain patches.
I used 3 pin DIN plug for connection to an external dual-voltage power supply. I have one that spits out 10 volts so all is well and good there. For added expense and extra fun the project file mentions that you can add the special IC7660 voltage inverter mod. According to the plans, this would cut your battery-life in half and probably add a high-pitched “whine” to your sound for you. Actually it would be useful for getting plus and negative out of cheap-o wall-warts.
The project file does mention that the 150K pots used in the Mutron III are hard to find and that you can just use 2 × 100K pots in series to get 200K instead. I used a pot that said 22 on it and 2 × 1M resisters in parallel and ended up with 158K for some reason, so i just stuck with that.
The plans also called for a non-polar cap of 15µF at the output stage, but I honestly didn’t seem to have any problems using a (much cheaper) 4.7µF polarized cap instead. Eventually I opted for the non-polar caps on each output, but only because they look cool (and dangerous) and because I found some for a halfway decent price. What they would do for the sound and how they affect the effect’s performance is a mystery to me.
the case against filter circuit:
After the thing had sat on the breadboard long enough to get everything working I set about soldering all the bits onto some perf-board. This was a little bit sketchy as I didn’t really plan anything out. It’s a real mess of solid copper wires all over the place and putting the lid on the box means just kind of stuffing everything like a can of surprise springy-snakes or something like that.
I put all the output capacitors on a separate piece of leftover PCB because they are quite big and I got some plastic holder/connectors for the 9 volt batteries too. All these bits I bolted to the bottom of the box using rubber grommets to keep the important bits isolated from the base. The case itself was an old printer-port switch-box (thank you very much) and the thing even had rubber feet on it already so that’s good.
The batteries have been lasting pretty well so far, a bonus of the “off” switch I should think. The common practice of putting the switch on the input plug would be silly for me because I always forget. And besides, it’s better to leave everything plugged in if you can.
I was lately using it to split a bass-guitar signal between 2 amplifiers and it was a nice result.
The Audio and Video Documentation Files >>>>>
cover some other experiments with the completed device.