Littlewings was asking in
this thread how to increase the decay on the DOD FX25 envelope filter. When studying
the schematic, I realized it might be helpful for everyone if I sort of gave a basic breakdown of what to look for in a circuit like this, so that you can modify, improve, and copy to your heart's desire. For those not familiar, the FX25 is your standard issue whaow whaow envelope filter.
It's actually a rather simple circuit, and surprisingly, not that different from something that your average DIYer would come up with. Starting from the left, there's a basic Vbias generating circuit so that the pedal can use op amps. Below that is the switching circuit (4007 and associated components), which we can for the large part ignore. We can also ignore the two FETs - just pretend the J113 is a wire, and the J201 is an open circuit (not connection - that's how the circuit works when NOT bypassed, when the effect is engaged). Directly after the input you have two op amp circuits - first a buffer, then the envelope follower. The buffer doesn't do much sonically - it basically just makes the rest of the circuit "work". The second op amp (lower one) is more interesting - it, and the diodes and capacitors after it, are the envelope follower circuitry. The op amp is configured as a comparator - it's function is to basically output a pulse (or gate) whenever the signal is loud enough. The diodes and capacitors generate an attack/release voltage after that - read up on this kind of circuit
on the wiki page for envelope detectors. This control voltage is sent to the two LM13700's. They're configured essentially like my
favorite filter circuit. If you're not familiar with LM13700's, they're Operational Transconductance Amplifiers - basically just chips that make voltage control easy. In this case, they're a VCLPF, with the input coming from the buffer, and the control coming from the envelope generator. LM13700s take their control input at the little visa symbols (if you will), so if you were to cut the trace before it splits into the two 10k resistors R7 and R9, you could replace it with any control voltage source - a sequencer, LFO, manual sweep, etc. Also, by following the other filter circuit from Tim E, you can vary the resonance. After the LM13700's, the circuit is over. Lots of pedals are similar in that they're fundamentally fairly simple, using common (and nice) circuits, and can lend themselves to a lot of modifications to extend their capabilities.
-Colin
