[sdiy] Buchla 295 10-band comb filter topology

[S Ridley]

Just for the record, the Serge resonant filter used a different circuit -
used 2 pole MFB filters (spaced maj. 7th).
The EMS filter band (stand-alone and Synth 100) used 2 pole Sallen Key LP
and HP stages.

Steve

On Tue, 22 Nov 2022 at 11:23, mark verbos <mark at verboselectronics.com>
wrote:

> I have one of these. Definitely has “dramatic changes in sound”. In fact,
> everything that goes through it sounds like it went down the tube of a
> vacuum cleaner. The pots on the filters are trims, not on the panel and
> just regular single pots. The panel controls are just level sliders. It is
> pretty similar to the Serge Resonant EQ, which I’m sure is no accident. It
> does a good job of taking normal synth sounds and making them more weird,
> acoustic and less placeable as normal synth sounds. Buchla’s later
> filterbank, 296 Spectral Processor does a much better job of leaving the
> signal alone when all the bands are zeroed. But, those filters are 3 stages
> deep and the module has almost a million trim pots.
>
> Mark
>
>
>
>
>
>
>
> On Nov 22, 2022, at 12:08 PM, Mike Bryant <mbryant at futurehorizons.com>
> wrote:
>
> 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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