[sdiy] So just what *is* the Buchla 291?

[Tim Stinchcombe]

> Well, that's my attempt at saying random things in hope that 
> one of them lands on an insight.

Works for me!

I too had been labouring under the (mis-)apprehension that the filter was
some sort of bastardised state-variable set-up, but the idea that it is some
sort of multiple-feedback set-up made me look at it in a different light. I
have posted a quick gif of what I think the basics of the filter might be:

http://www.timstinchcombe.co.uk/synth/buchla_291_topology.gif

To me this now looks like a two amplifier multiple-feedback circuit: k1 is
FET buffer + opamp2 + VT2, thus is variable gain from about +9dB to -25dB
(I'm ignoring C4 whose affects are apparently below audio); k2 is FET buffer
+ opamp3 + VT4, a simple inverting set-up, with gain from about +7dB to
-47dB (ignoring C7, which looks to be stability-related); the rest is
hopefully obvious.

As Don has already pointed out, this is similar-looking to the well-known
Delyiannis-Friend bandpass circuit, only here we apparently have two
amplifiers (? - I'm open to conjecture here). Unfortunately there are
_dozens_ of these sorts of things in the literature, especially when you
start taking standard lowpass circuits and employ tranformations to make
them into bandpass ones etc.

Judging from the dates of several old books I have, this was a very active
area in the early/mid seventies, so my guess is Buchla came across an
interesting circuit and 'vactrolized' it - whether he had enough insight to
actually tinker with the topology, by adding feedback loops etc., who knows
- maybe we should try asking him directly? Other than that, I guess it is
just a matter of getting lucky, scanning as many old books and papers as
possible until maybe the same structure leaps out at one!? (I spent several
hours last night, but to no avail.)

This also highlights something that frustrates me immensely - we are often
interested in *analysis* of these old filters, i.e. are trying to find out
what they are/do, but most books are written with *synthesis* in mind, i.e.
'this is how you build a filter to your required spec'. I also find it
interesting that there doesn't seem to be a universally-accepted
'classification system' for filters, to enable one to establish if any given
new topology is equivalent to any already extant - I have a couple of books
which attempt to do this, basing the classifications around different
aspects of the polynomials that arise in the transfer functions. However,
they seem incredibly involved, at least one requires a good deal of
familiarity with signal flow graphs, and clearly requires making the time
for some serious study, which so far has eluded me. (And Googling 'filter
classification' will just turn up the usual low/high/band pass distinction -
next to worthless...).

That's my current take on it all!

Tim
__________________________________________________________
Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at tstinchcombe.freeserve.co.uk
www.timstinchcombe.co.uk