Thursday, May 28, 2020

Vactrol Envelope Follower--From the Lab of Otto Wahh

I'm going back through my old designs from 2004-2005, laying out new PCBs, and rebuilding stuff.

Why = Whynot?

So let's build a Vactrol envelope follower I initially drew up when dinosaurs still roamed the earth:

Another crappy Lazertran decal job--don't know what I was thinking with the bun-n-weiner logo thing on the left, it's really stoopid, so, the front panel will have to redone.  Oh well.

What the VAC? In 2004 I had even less idea what I was doing then I do now, but I think the unit came out OK, you can hear a sound clip of the module here.  Pretty everything you hear is processed in some way through this module, to create auto-wah, side chain amplitude effects, amplitude = modulation on a VST via Expert Sleepers interfaces, etc.

USING THE VAC-Envelope Follower:

The VAC-EF uses amplitude seen at input to generate a corresponding DC voltage at its output. If you've ever messed with an auto-wah device such as a Mutron III or any sort of optically based compressor/limiter you've probably messed with amplitude to voltage sub circuits, a small bit like this one, before.

Fun begins fast; an easy application: change a bandpass filter's center frequency in response to the amplitude of an audio signal. Sounds like Jerry, right? Also fun: side chain effects: so a drum track gets louder when another track is soft, for instance. Or: change phase when there is a loud passage for phase shifty sounds, crank up a distortion effect when the instrument you want to mangle is really loud, but not so much when it's soft, or for any of this, do the visa versa.

Hours of fun.  


Here's the schematic:

Board is designed to fit behind a 1u Frac panel:

Temp Front Panel!!! But does it surpass the weiner?

Overall the vactrol gives a slow, drunken, sloppy feel to the module, which to my ears is sometimes a good thing.

But Why, Scotty, Why: I am not sure if the current source for the Vactrol is a smart design choice, but for me it works, I used a voltage regulator to feed current to the opto because I have about a million (a bit fewer maybe?) of those in my junk box. How about putting the 2222 transistor in the feedback loop of the op amp? Is this a good idea? Not sure--I am not even sure it's necessary, but regardless, I've used this module  for the past 15 years and have been happy with its performance.

The TL084 does get a bit hot sometimes depending on the module's settings which means the design could be improved, but I'm too lazy to bother fixing it.

If you do build it, mess with R5 value to set the overall performance of the circuit. You might even consider putting a trimmer in there in series with R5. This may help ameliorate issues you have with Vac A behaving completely differently than VAC B.  

Here are some bench photos:

My initial test PCB used normal 9mm PCB pots and small knobs but as you can see the ergonomics sucked.  Hence I replaced them with tall trimmers which you can buy from Thonk:

FIXES: I had to add 2 kludge wires to make this work since I screwed up the wiring for V+ and V- for the quad op amp. Nice, but omitting is better than wiring it up backwards and blowing up the op amp right? it's like plumbing--if the water squirts out, hey, get a mop. The Eagle files, PDFs including the fixes, etc. etc. are on my website, here.
OK still need to test it more and redo the damn front panel. May do a dual VAC-EF in a 2u Frac?  Who knows. Until then, have fun VAC-uuming, always suck when you can, and above all (especially now right?) don't breathe the fumes!

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