Optical sallen/key wah project

[VectorHorrific]

Hullo folks, resident lurker back with more filter nonsense! This time I'm trying to demystify the S&K filter topography. I generally dislike regular wah pedals but have always adored resonant filters, so I wanted to find some way to build an inductorless wah/bandpass filter with some more radical and less "organic" sounds than your regular wah pedal would providse. Here's what I have so far as a starting point for this project.





Based from what I've learned about the famed Steiner-Parker filter (which I only recently have fallen in love with after playing with a mate's Microbrute last week) I think I've been able to reduce it down to as few parts as possible in the above circuit: showing the points at which the input would produce differing filter effects, and operating on my assumption that the diode string is acting as a variable resistor. R1 and R2 substitute these diodes, and my plan is for these to be a pair of vactrols which will then control the frequency sweep. I'm not a huge fan of high-pass filters so I may include a switch to just bounce between the Low and Band inputs on the end design.

Currently I believe that even though this is an active filter there will be some gain loss at play here. As this is designed around an op-amp I think this can be easily rectified by using a dual op-amp IC and using the second half as a buffer at the end of the circuit to make up for any lost volume.

I'm a little bit worried about what to do with the negative voltage in this instance, too. Most sallen/key filters I've seen have been designed for euroracks and so usually have this sent to a negative rail which isn't something most pedal powers supply. I'm sure I can probably just tie it to ground but I worry about what limitations this will impose on the effect. If there are any limitations then I would like to try and build around them if possible.

I'm also interested in finding a way to tame the oscillation somewhat. I love these oscillations on synths but guitar signals don't seem to play very nicely with them. I will be experimenting with ways to make this more musical or at least easier to recover from when backing off on the frequency.

I'm also conscious not to have too many external controls. As I plan on building this in to a Wah shell there's going to be very little space for pots and switches on the outside.

I'll be updating with my progress as it comes, but if any of you guys have any tips or ideas on the above then I would love to hear them!

[Scruffie]

http://www.doepfer.de/a1011_tec.htm