Envelope filter.

[Devius]

Anyone build the dod 440 (ggg) and have a problem with the decay? It engages beautifully but the decay on mine is sort of... choppy.
It trails out nicely for a short time then "steps down" if you pick up what I'm layin down.

[Liquitone]

I haven't built that one, but i think to alter the attack/decay you have to change the value of the capacitors between the LED and op-amp that triggers the effect, in this case the 4u7 and 1u caps.
If you raise the value of the 1uf it should take longer for it to discharge after an attack, so the LED will stay on for a little while longer, thus increasing the decay. maybe adding a resistor in series may help to even out the decay, but not sure about this.
I think the 4u7 cap does the same for the attack, but not 100% sure about that one either.
R.G has an article on geofex.com that goes into great detail considering this subject that can explain things much better than I can. (The Technology of Auto-Wahs / Envelope-Controlled Filters)