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But while betatesting the new Bugbrand Synth Voice modular frame, which doesn't have an LPG (but lots of other goodies), I came up with another, even simpler trick, which also works with old Bugbrand filters, and probably with a lot of other brands too - as long as the filter has a DC coupled output. (I'm sure others have done it before, since it is really quite trivial.)
The recipe is simple:
* Use Low Pass mode
* On the OldBlue SV Filter, set Cutoff Freq to lowest
* On the new filter, either use lowest Cutoff setting at Audio Rate mode or 2 o'clock at Sub mode (to start with)
* Set Resonance at zero
* Patch audio to both Audio In and CV in
* Set CV amount to positive maximum (to start with)
You get the envelop at Audio Out. It will track the positive peaks of the signal, but will disregard the negative part. In most applications this doesn't matter.
The idea is simple. When the wave goes high, the filter goes high and lets it through. When it descends again, the filter slows down and introduces a lag so that it doesn't follow the wave down so quickly. A lowpass filter is a kind of slew limiter, or a leaky integrator. In this way, the positive parts of the wave are given proportionally much more influence over the output signal, resulting in an output following the positive contour of the signal.
Here's a picture of a typical setting with the OldBlue filter, with oscilloscope output (spoken word input):
And here's the filter of the new blue Synth Voice (mine is green, since it's a prototype):
Variations:
* You can use the V/Oct in instead of CV in.
* Adjust the Cutoff Freq and CV amount to adjust slew rate and general behaviour. If Cutoff is set too high, you get some audio ripples on top of your envelope signal. If it is set too low, it will not catch fast changes in the sound.
* You may have to amplify the envelope signal to reach full swing - the easiest way is to patch it to both inputs on a DC Mixer.
* On the SynthVoice frame filter you can set the CV to negative, and you will get a negative envelope countour out. This can be used to patch up a kind of pumping compressor (with sidechain) on a VCA, and other things.
* Patch the generated envelope signal to the Gate In of an envelope module, and connect that envelope out to a VCA - and you have a simple noise gate.
I hope this was helpful. It should work with other filters, too. Just for fun, I tried it on a CGS Wasp filter, and it sort of worked!
awesome patch tip, otoscope!
and it can also function as a vc-slew limiter, as i've been testing out this past weekend.... hmm, is this bugbrand or serge? 
If the filter doesn't go low enough, apply negative offsets.
