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Anderton's envelope follower questions...

8/3/1999 12:58 AM
nic Anderton's envelope follower questions...
I am wondering how I could reverse the output voltage on Anderton's EF. I want to be able to run both selectable with a switch. Would I have to have another op amp?  
8/3/1999 3:47 AM

Another opamp is the simplest way. The Mutron 3 (and Neutron at GEO) do this. Uses one opamp to rectifiy, another to add/subtract from a selectable level - low for the normal, high for the reverse.
8/3/1999 2:40 PM
Thanks R.G.!
8/4/1999 1:09 PM
Another aproach that's sometimes used is to include a control voltage mixer stage that allows one to pan between inverted and non-inverted control voltages, with the centre position resulting in a cancellation (i.e., no CV out). The EH-Y-Triggered Filter uses this approach. I've seen schematics of theYTF posted, but the schematic was not condusive to reverse engineering. I have a schematic for such a CV "pan" circuit in a back issue of Electronics Today. I'll see if I can scan it when I get back from holidays.  
Incidentally, this approach allows the CV pan/balance control to double as an "intensity" control. Now that I think of it, parallels to the Big Muff Pi tone control are appropos:the amount of each type of output depends on how far clockwise or counter-clockwise the control is turned.
8/4/1999 1:11 PM
Mark Hammer

Uh, that was me, using my brother-in-law's machine from far away from home.  
Now I know how all those other anonymii (anonymae?) happen.
8/16/1999 2:02 PM
paul perry

With a simple half wave rectifier used in some followers, you can reverse the envelope voltage by reversing the diode (or switching between 2 diodes). Not in the Anderton one, because of how the led/ldr is.  
Anyone looking for an amp with a gain control from -1 to +1,  
see "The Art of Electronics" by Horowitz and Hill. I cant see how anyone can design anything without it.

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