fairly simple, good sounding envelope filter circuit... yay

[Brian Marshall]

the drawing is a bit ugly

http://www.subdecay.com/envelope-filter.gif

A few notes-

This could be easilly improved, as i made some minor compromises to fit it in to a 1590B case.  In a perfect world the input and output buffers would be opamps.

the filter is essentially taken straight from the lm13700 datasheet, with positive feedback added for resonance, and values tweaked a bit for stompbox use.
We sold a few of these as prototypes as the gurgle and moan.  it sounds good, but some tweaking could be done in the envelop follower to smooth things out a bit.

Sorry i don't have everything marked on here.  I'll get to that some other time.

[Mark Hammer]

Nice.  I like the FWR and initial frequency tuning pot (or is that a trimpot on the board?).  Looks like it's set for a fairly fast decay, which makes it nice for bass.  Though a long decay is not all that necessary, and a decay pot is not critical, a toggle with fast/slow, or slow/medium/fast, is helpful for matching the device to the song.  My experience with these devices is that time constants and the resulting feel is everything.  Of course YMMV, and I understand the desire to have distinctive sound in a small package.  I also understand that fast decay can make imperfect rectifiers sound better, because the decay is fast enough to obscure whatever ripple remains.  Lengthening the decay time can sometimes make ripple stick out like a sore thumb.

[Brian Marshall]

Hey Mark

It's actually only a half wave rectifier.  It does have some ripple issues, but i kind of liked how gurgle-y it was   Of course all that stuff can be tweaked to taste. 

I wanted to make something simple, that didnt use opto isolators, and wasn't an anderton style multiple feedback filter, which I've always found lacking in tone and range.  It is slightly more complex than a Dr Q, but worth the extra effort I think.

One mistake in noticed, the .0047 cap on the 2nd OTA should actually be larger.  I think i was using a .0082.  Otherwise the range filter Q is a little bit too tight IMHO.

[soggybag]

Hmmm... Looks like you could connect the S&H to this...

I like that it's built from a 13600. These filters usually sound pretty good. The chips are available also.