[sdiy] Useful CV ranges for Buchla 191-style HPF

[Aaron Lanterman]

While we're on the subject of damn fool crusades I've sent my students  
into, I've got another student looking at the idea of making a 4-pole  
HPF by taking an RC passive highpass filter, replacing the C with two  
diodes - one to a positive CV, and to the negative of that (changing  
the CV to change the dynamic resistance of the diodes), and then  
buffer that.

The Buchla 191 LPF uses this kind of HPF in a feedback loop of an op- 
amp-like-thing to form a lowpass shelf:

http://userdisk.webry.biglobe.ne.jp/000/024/65/N000/000/000/Buchla191vcf0707.GIF

Here, we're basically thinking about what's shown as Q6 and Q7 and C1,  
with the input going to C1, and an op amp replacing the Q3/Q4 buffer,  
and then having non of the goo in the upper left.

Here's a rough sketch I drew for my student, Thomas, to get him started:

http://users.ece.gatech.edu/~lanterma/buchla191_inspired_hpf.jpg

My thinking was:

1) A +/- 5V signal should be divided down to a "small AC" signal  
relative CVs used. I set up the op amp to get around +/- 0.2 V going  
into the filter. Does that sound like too much attenuation? Too  
little? I know the guidelines for an OTA but not sure what's best here.

2) On the diagram, the CV would go in the unlabeled pin at the 100K  
resistor in the bottom circuit. I thought that one could try CVs from  
a keyboard, so they might go up to 10 V. I somewhat arbitrarily (OK,  
totally arbitrarily) divided that by 10ish, so the CV at the diodes  
(which I'm calling CV*) would be between 0 and 1. Anyone have any  
intuition as to what a useful range of CV*s would be?

Thomas isn't getting any signal output at the moment, but I think  
there are wiring issues somewhere. While he's working that out, I  
figured I'd ask some questions along these lines.

Also, I should, as usual, ask if anyone sees anything completely nutty  
about my idea and/or schematic sketch.

- Aaron

P.S. Nobody will believe me, but I had the idea to build an LPF like  
this some time ago... I thought of it when thinking about the  
transistor-ladder to diode-ladder thinking, what would a diode version  
of the Moog HPF (which I never really understood) look like? Then a  
few years ago I had a student trying to build a LPF like this, using  
varactors replacing the Cs in an RC filter. That didn't work at all  
since it turns out varactors kinda suck at audio frequencies.