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LPG --> bandpass, highpass response? multimode gate
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author LPG --> bandpass, highpass response? multimode gate
Hi all,
does anyone know of a way to obtain a bandpass or highpass response from a Buchla 292 -style LPG?

Specifically I have a quad thomas white LPG that I am wondering if I can mod somehow.

I have heard that inverting the LPG output and mixing with the source signal yields a highpass response, is this true? (My polarizing mixer is in the shop right now so I haven't been able to check since reading this).

And then a bandpass would mean highpass -> lowpass in series?

Does anyone know how the boogie filter obtained BP and HP modes, or is this a different filter topology from a 292-style LPG that just happens to use vactrols?
This might be a good resource to start with:
its a sallen key filter so basically you should be able to get high pass by swapping the place of the two resistors and capacitors in the filter circuit, this could be done with a switch. bandpass by just swapping one resistor with one cap IIRC. Google sallen key filter and check your lpg schematics, should be relatively easy. If I have understood this correctly, never tested it, just thought about doing this at some point ....
Yeah, I came across that page searching google. The Mutant Vactrol was several 292c LPGs wired into a Sallen-Key filter topology (in theory anyway).

I was thinking with a quad LPG you basically have 4 separate "poles" that could be patched into various filter responses using polarizing mixers and a lot of patch cables. How to actually do it, or whether its possible, is eluding me at the moment.

Dead Banana
Just in case anyone else is interested, I did find this page on the Doepfer site, which I had never come across before. It explains their vactrol filter, which is essentially a Steiner filter with the diodes replaced by vactrols as I understand. But it shows the basic low pass sallen key, and then the modified Steiner-type version with LP/HP/BP inputs. So assuming the TW 292c clone is a sallen-key filter topology, there might be a way to add BP and HP inputs at strategic locations...?


I think the 47k/1k voltage dividers at the inputs are not necessary, they just raise noise level, they are necessary in the original Steiner VCF because it uses diodes for vc, the vactrols can take the synth-level.
The HP is only -6dB, but resonant.

A post by Andrew Simper from sdiy group shows how it´s done best for 2nd order outputs:

You could also do pole mixing, summing the input stage as well as 1st and 2nd stage outputs with different coefficients (e.g. +1,-2,+1 for BP).

There´s an excellent Electronotes article about it, but Olivier Gillet´s writeup takes you a lot of the way:Pole mixing pdf

Another possibilty is to try a state variable topology with VTL5C3, maybe LPGs don´t need to be Sallen-Key???
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