What’s a State Variable Filter?
The basic text book definition is this:
State variable filters use three (or more) operational amplifier circuits (the active element) cascaded together to produce the individual filter outputs but if required an additional summing amplifier can also be added to produce a fourth Notch filter output response as well.
What it really means is that is a sort an EQ with a setting that favors the Low frequencies, the mid frequencies or the high frequencies.
The generic schematic would like this:
State Variable Filter Circuit
What is so funny to me is that these have been around in many text books for sometime but not really utilized in popular effect devices.. They were highly used in HiFi stereo equipment. And became a sort of backbone of some more obscure effects, depending where and how you vary resistance to move the filters. Below you can see 4 different, popular effect circuits that all basically look the same. To Actuate the filter, I mean to shift it around and so 2 “poles” are needed to tug it around. A stacked potentiometer has the ability to move 2 poles at once. Opto-Couplers on the other hand allow for 2,3,4,n poles to be actuated at the same time.
The mutron 3 state variable filter is actuated by the players picking dynamics.
The mutron VolWah is actuated by Opto-Couplers which are actuated by a single potentiometer in a foot pedal.
The Alembic Super Filter is controlled by a stacked potentiometer and is generally static.
The Tychobrahe parapedal is actuated by stacked potentiometer controlled by a foot pedal.
They all are similar circuits, but kind of like different flavors of pasta sauce: one chef add basil, another ads oregano.
I found this to be the bane of my existence for the last 20 years as I was not only mentored by Mike Beigel but also Craig Anderton who wrote the book on the use of Opto-Couplers to servo control points. I may even say that my research picked up where theirs left off. I have been able to experiment with 100s of combinations of Opto-Electronic configurations and utilize them in many other applications of State Variable Filters. My desire was to build a State Variable Filter which could be Manual(Static), Foot Pedal(Wah-Wah), or picking dynamics(Envelope Control). I had already designed and developed products which could be Envelope and Foot pedal controlled.