[sdiy] Buchla 295 10-band comb filter topology

[Mike Bryant]

yes that was what I thought.  But surely for the traditional comb effect you need to sweep the frequency up and down with the keyboard.  This would just kill certain frequencies pretty dead forever.  Of course you can adjust the Q and freq of the resonance to move these dead-spots, but that's on a per filter basis so couldn't be ganged pots (I assume unless they built a special megagang pot)
________________________________
From: Roman Sowa <modular at go2.pl>
Sent: 22 November 2022 10:58
To: Mike Bryant <mbryant at futurehorizons.com>; synth-diy mailing list <synth-diy at synth-diy.org>; Lanterman, Aaron D <lanterma at ece.gatech.edu>
Subject: Re: [sdiy] Buchla 295 10-band comb filter topology

I believe they are inverted like that to cause more dramatic changes to
the sound as some frequencies will be canceled out between bands.
Otherwise it would be just plain boring graphic equalizer.

Roman

W dniu 2022-11-22 o 11:41, Mike Bryant pisze:
> They are just bandpass filters - one pot in the feedback changes the
> freq and one changes the Q.  And they both interact with each other of
> course.
>
> But why he inverts every other band into the summing amplifier is a
> mystery to me.
>
> ------------------------------------------------------------------------
> *From:* Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of
> Lanterman, Aaron D via Synth-diy <synth-diy at synth-diy.org>
> *Sent:* 22 November 2022 10:01
> *To:* synth-diy mailing list <synth-diy at synth-diy.org>
> *Subject:* [sdiy] Buchla 295 10-band comb filter topology
> Does anyone recognize the topology Don is using here in the bandpass
> sections?
>
> http://fluxmonkey.com/historicBuchla/buchlaFiles/Buchla_2950_200.jpg
> <http://fluxmonkey.com/historicBuchla/buchlaFiles/Buchla_2950_200.jpg>
>
> The low and high bands are 3-pole Sallen-Keys, but I’m not sure what’s
> going on in the middle filters. It looks kind of like a multiple
> feedback filter, but instead of having a resistor to ground like in
> Figure 1 here:
>
> https://sound-au.com/project63.htm <https://sound-au.com/project63.htm>
>
> There’s some weirdness with a 3rd capacitor connecting to the resistive
> feedback back.
>
> Aaron Lanterman, Prof. of ECE, Georgia Tech
> My blog on Education and Innovation:
> https://edupocalypsenow.wordpress.com
> <https://edupocalypsenow.wordpress.com>
> My blog on Electronics and Programming:
> https://lantertronics.blogspot.com <https://lantertronics.blogspot.com>
> My YouTube channel on Electronics and Programming:
> https://www.youtube.com/c/lantertronics
> <https://www.youtube.com/c/lantertronics>
>
>
>
>
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