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

[HOSHUYAMA]

Hello Aaron,

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

I tried a similar structure using diodes for APF not HPF, and failed ;-)
The reason I failed was signal coupling through CV*s,
which caused RF oscillation.
Voltage followers were not sufficient for decoupling when CV*s were high.

When with transistors, the signal coupling is much reduced: 

http://www5b.biglobe.ne.jp/~houshu/synth/Vchp0302.GIF

For CV cancellation, I used a feedback structure using U2 
instead of a simple inverting amplifier, because perfect 
complementary pair is not possible.

> Anyone have any  
> intuition as to what a useful range of CV*s would be?

I my case, the range was 0.3v to 0.55v.
Another issue is temparature compensation.
Q1 in the above schematic (Vchp0302.GIF) does the job.

Hope this helps.

Sam HOSHUYAMA
Saitama, JAPAN

Aaron Lanterman wrote:
> 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.
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