[sdiy] Buchla 295 10-band comb filter topology

[Guy McCusker]

Would a workable digital implementation have been possible in 1978? I
always assume the answer is no but one can be surprised.


On Tue, Nov 22, 2022 at 11:25 AM Mike Bryant <mbryant at futurehorizons.com> wrote:
>
> > Buchla’s later filterbank, 296 Spectral Processor does a much better job of leaving the signal alone when all the bands are zeroed.
>
> I read this far and immediately assumed he'd done it digitally
>
>
> > But, those filters are 3 stages deep and the module has almost a million trim pots.
>
> But then I discovered not :-)
>
> ________________________________
> From: mark verbos <mark at verboselectronics.com>
> Sent: 22 November 2022 11:20
> To: synth-diy at synth-diy.org <synth-diy at synth-diy.org>
> Cc: Roman Sowa <modular at go2.pl>; Lanterman, Aaron D <lanterma at ece.gatech.edu>; Mike Bryant <mbryant at futurehorizons.com>
> Subject: Re: [sdiy] Buchla 295 10-band comb filter topology
>
> 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>
> >
> >
> >
> >
> > _______________________________________________
> > Synth-diy mailing list
> > Synth-diy at synth-diy.org
> > http://synth-diy.org/mailman/listinfo/synth-diy
> > Selling or trading? Use marketplace at synth-diy.org
> >
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy
> Selling or trading? Use marketplace at synth-diy.org
>
>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy
> Selling or trading? Use marketplace at synth-diy.org