[sdiy] Buchla 295 10-band comb filter topology

[Neil Johnson]

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.

> 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

Neil