[sdiy] Buchla 291 analysis?

Richie Burnett rburnett at richieburnett.co.uk
Fri Aug 28 18:52:38 CEST 2015

A very odd schematic indeed!

There's some strange things going on there, like connecting the outputs of 
op-amps 1 and 3 together with a direct capacitive path via C2 & C3.  I'm not 
surprised that someone has written on that schematic about needing 
additional resistors to tame HF oscillations!!!


-----Original Message----- 
From: Pete Hartman
Sent: Friday, August 28, 2015 4:35 PM
To: Walker Shurlds
Cc: Synth-Diy
Subject: Re: [sdiy] Buchla 291 analysis?

The Stroh schematic is here:  http://home.comcast.net/~r3cogniz3r/291j.pdf

Major difference that jumps out at me is the use of an op amp instead of the 
pair of FETs at the start of the main block and after the main block.  The 
one after is obviously just a follower for buffering, not so sure about the 
role of the FETs/op amp at the start of the section.



On Fri, Aug 28, 2015 at 10:12 AM, Walker Shurlds <walkershurlds at gmail.com> 

I think I changed my mind: the first op-amp doesn't seem to have a pole that 
actually moves with the control voltage, so probably not a SVF.

On Fri, Aug 28, 2015 at 9:47 AM, Walker Shurlds <walkershurlds at gmail.com> 

I'm looking at this one: 
http://www.synthfool.com/docs/Buchla/Buchla_200/291/Buchla_2910.jpg Is it 
different than the one you're looking at?

It looks like SVF to me, but you have to remember than the "states" (I'm 
about 90% sure I'm really abusing the control systems terminology here, but 
bear with me) include all three possible outputs, all three possible inputs, 
every possible branch...

So to me it looks like *both* the input and output are in different nodes 
than in the common tow-thomas and KHN topologies. And there seem to be extra 
branches added for stability. And C6 is making everything unnecessarily 
complicated, analysis-wise. Really tempting for me to analyze it instead of 
listening to my assembly coding professor lecture for the next hour.

Hope that helps,

On Fri, Aug 28, 2015 at 1:52 AM, Pete Hartman <pete.hartman at gmail.com> 

Are there any analyses anyone's aware of about how the 291 works?

Looking at the Dustin Stroh schematic, it looks *sort of* like a state 
variable filter, but not quite, so that throws me off a bit.  Maybe it's 
just that the schematic is oddly drawn, I think the last time I asked a 
question like this it was a matter of re-arranging the bits to see it :)

Trying to determine if what I'm seeing with one is "normal" or not, and with 
Buchla circuits, I can't just fall back on "does it behave like all these 
other filters I have". :)



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