[sdiy] Debugging Hammond filter circuit

[Aaron Lanterman]

One of my students (cced here) is working on the circuit from this patent:

http://users.ece.gatech.edu/~lanterma/patents/hammond_fet_vcf.pdf

It's 4,023,113. It's a weird lowpass filter circuit that works by putting a highpass filter in the negative feedback loop of an op amp. (The Buchla 191 works similarly, except it works one-stage-at-a-time, and uses diodes as variable resistors in the highpass cores instead of JFETs).

It seems like the "tone input signal" is appearing in an attenuated form at the + terminal of OA2, and it looks like it's being replicated at the negative terminal, and it seems like this same signal is (more or less) appearing at the output of OA2.

A nice highpass version of the output of OA2 is appearing at the + terminal of OA1, and twiddling the (negative) control voltage on the FET gates changes the cutoff, and you can trace through each of the stages and see the additional filtering effect of each stage, and this highpass signal seems to be appearing nicely buffered at the output of OA1.

But there's no lowpass action happening at the output of OA2. There is a slight difference in the output of OA2 as you change the CV on the JFETs; you see slightly more or less overshoot ripple at the transition of a sawtooth or square wave, but not very much difference.

If we disconnect R148, it doesn't look like the output changes, so this network doesn't seem to be doing anything (or maybe it's not doing anything because something else is wrong). I'm not used to seeing an op amp circuit allegedly operating in linear mode that has positive feedback in addition to negative feedback so I'm not sure how to think about this.

I've read over the patent but my patent-fu is rather week.

By any chance does anyone have any thoughts on what to check next? Any insights?

Should probably mention she's using TL08x or TL07x type op amps.

- Aaron