[sdiy] Buchla 295 10-band comb filter topology

[brianw]

Perhaps I shouldn't believe everything I read during my studies, but my understanding is that the only way to get a "comb filter" is with delay ... and analog delay is way more expensive than digital.

The "phase shifter" is the analog equivalent of "flanging" (a.k.a. comb filtering), and the spacing of the peaks and valleys is quite different comparing comb to phase. Of course, with enough feedback, phase shifting is its own beautiful sound - just not quite like comb filtering.

Comb filter peaks are linearly spaced in frequency, and there are an infinite number of them (they stop only at the limit of the bandwidth of the signal).

Phase shift peaks are logarithmically spaced in frequency.

I suppose the Buchla 295 is not exactly either spacing, and you're limited to 10 peaks. Yes, people label the B295 as a "comb filter," but I consider that artistic wishful thinking. Note that the frequencies on the panel are basically pseudo-log.

Brian


On Nov 22, 2022, at 3:08 AM, Mike Bryant 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
>  
> 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
> > 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.
> >