I have a bag of 5C3 vactrols....so what to do? Build an envelope follower.....
Here's a pix of it during breadboard stage. Notice Forrest Mimm's learning kit used as breadboard--laugh while you can, monkey boy!
The result is what you see here. If you follow the schematic, this is pretty simple stuff. The first few stages buffer and amplify the incoming signal. The bias control can be a trimmer, but since it affects the levels of the output of the device I put it on a pot on the front panel. The rectification stage (IC4A/IC4B) is straight out of Electronotes--EN#26--a brilliant little circuit fragment I think.
For the rest of it, this was another case of "what do I have lying around to use"....I had a whole bunch of 5V TO220 regulators for instance, so there's my 5V CV source. And I was going to add a A/R type stage, and a switch to go between the 5C3 and a slow-as-snot 5C2, but ran out of time and patience so I put a somewhat yucky .47u filter at the output instead.
As far as using it, I usually follow an electric guitar with a preamp (although one Les Paul I own can drive the circuit directly). Then I set the "Bias" setting to where the LED lights, and then back it off a tiny bit until the light goes out. Then I set the range to where I want the "max CV" for the circuit to be.
When working with line level audio, output from a keyboard, drum machine or whatever, I can just hook it in and go. I put a 1/8" and 1/4" jack on the front for quick-connects. Here is a fuzzy pix of it in the rack--maybe I will get a better camera for my next birthday?
In general the EF works well for solos and following sounds with quick to medium decays. I was not as thrilled with it for long, slow-decaying sounds since the rectification sort of petered out as the sound decayed, and the resulting envelope was "jittery" instead of being smooth. I was stuck until I got an email from Harry Bissell suggesting I might try using a lag circuit to smooth things out. I built his design of what he calls morphlag, an elegantly simple and clever circuit which has log/lin control and independent Attack and Decay. I added some buffered LED's to monitor I/O levls to his design, but otherwise built it "box stock". I put the morphlag in series with my envelope follower and long decays ahoy. Works great. .
OK on to the sound samples. I will spare an entire orchestrated thing and just present a few "solo clips" of this one. This circuit (no morphlag), along with some of the VCF's I have built, is all you hear here except as noted.
Guitar + Preamp + 5C3 EF + 5C2 filter is here.
Another few guitar lead things here and here.
No this isn't a talkbox! Multimode filters in series and envelope follower sounds a lot like it. Go here.
Used to enhance a clavinette and a whirley.
Used on a Virus Access here
More guitar samples: with stomp box chorus and reverb and straight.
Note: Many thanks to Brian Sarvis (Zoetrope Studios) for his help during the brainstorming stage of this project.