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

[Roy J. Tellason]

On Wednesday 30 April 2008 01:43, 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.

For some reason this reminds me of the diode keying that was used in some 
organs.  The problem there is that you need really low signal amplitudes,  by 
comparison with your keying voltage.

> 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/Buchla191vcf070
>7.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.

You don't want to get anywhere near turning those diodes on with the 
signal.  :-)

> 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.

It's not the frequency so much as the amplitude,  because with a varactor the 
signal is going to affect the capacitance as well as the control voltage,  so 
it would seem to me that they'd do pretty well when one is dealing with 
_very_ small signals,  as are typically found in RF circtuitry,  and not so 
much with the sort of signal levels we're going to find in synth gear,  which 
are normally higher than I see anywhere else.

Instead of buying parts specifcied for use as varactors,  you can also use 
ordinary rectifiers in a lot of cases,  too.  Or so I've read,  I haven't 
tried it.

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