[sdiy] Buchla 291 analysis?

Pete Hartman pete.hartman at gmail.com
Fri Aug 28 17:35:16 CEST 2015

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