[sdiy] Buchla 295 10-band comb filter topology

[Michael E Caloroso]

Don kept his cards close to his chest and was a member of the 1960s
counterculture where he didn't trust the government, then didn't trust
corporations (after his brief stint with CBS in late 1960s), then only
trusted a handful of people.  I spent a couple of evenings with him talking
shop, where the typical exchange was I would mention "wouldn't it be great"
and his response was "I've done that".

Don majored in Physics from UofCA Berkeley.  His talent in electronics came
naturally as a child and he claimed he had never studied them in academia.
Before analog synths his career was in NASA and nuclear physics research in
the university.  Physics background seems to be a common element among the
synthesizer pioneers - Dave Rossum, Al Pearlman, Hugh Le Caine, Bob Moog
(engineering physics).

MC

On Wed, Nov 23, 2022 at 4:40 AM mark verbos via Synth-diy <
synth-diy at synth-diy.org> wrote:

>
>
> > On Nov 23, 2022, at 12:42 AM, Neil Johnson via Synth-diy <
> synth-diy at synth-diy.org> wrote:
> >
> > Hi Aaron,
> >
> >> Then there’s the ultimate question, I often ask when looking at Buchla
> circuits, which is: HOW DID DON COME UP WITH THIS?
> >
> > Without asking him I don't think you'll get a definite answer (so
> > that's a "no" then).  My guess is through a lot of actually designing
> > and building circuits.  And reading.
>
> Even with asking him, I assure you that you wouldn’t have gotten a
> definite answer!
>
> His circuits came from books in the analog computing world of the time. I
> have found cases of him lifting ideas from patent papers that had come out
> the same year as his circuit. I guess he didn’t worry too much given his
> tiny quantities!
>
>
>
> >
> >> In some cases, it’s clear he had a basic vocabulary of circuits and
> generally plunked them down until he got the effect he wanted (there seems
> to be no effort to go back and simplify things, it seems he was more
> interested in moving on to something else), but there’s a lot of Buchlaland
> that just seems like it’s from another dimension.
> >
> > Yes - imagination.
> >
> >> There’s two main grand mysteries two me:
> >>
> >> 1) How did he come up with the “diodeless deadband circuit” in the 259
> and Music Easel (I scoured three different books on nonlinear
> circuits/analog computing trying to find anything remotely like it), and
> >
> > It's an optimised deadspace circuit.  Instead of using two diodes
> > connected to defined +Vband and -Vband he instead used opamps with
> > almost rail-rail outputs and powered them from +/-6V instead of the
> > system rails.  That is optimised engineering.  It's the same
> > underlying idea of using a single rail low-voltage opamp buffer before
> > an ADC running off the same rails as the ADC: you get signal
> > conditioning AND range limiting for free.
> >
> >> 2) How did he come up with the Buchla 291 topology (I read on a forum
> that someone asked him and he said something along the lines that it was
> based on filters Max Matthews used in his vocoder work, but I was never
> able to find any more information about that to dig in further.
> >
> > It's called a bandpass twin-T.  Buchla wrapped an opamp around it.
> >
> > The earliest source I have to hand is "Filter Theory and Design:
> > Active and Passive", Sedra and Brackett, 1978.  See Figure 8.34 on
> > page 467.
> >
> > Amusingly, Terry Watson's Masters thesis from 1965 found the bandpass
> > twin-T purely by mistake, but didn't find it interesting (see page
> > 35):
> >
> >
> https://scholarsmine.mst.edu/cgi/viewcontent.cgi?referer=&httpsredir=1&article=7686&context=masters_theses
>
>
> On page 44 there is a passive version of the 295 circuit.
>
> Mark
>
>
>
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