[sdiy] Buchla 291 analysis?

Walker Shurlds walkershurlds at gmail.com
Fri Aug 28 19:21:52 CEST 2015

Here is my work so far: https://www.flickr.com/gp/pancreas/Dk1sF8

I was treating the FETs as ideal op-amps for my analysis anyway, so no change unless there's a resistor value I missed.

The middle block's large capacitor creates a pole that actually moves woth the CV in a bizarre way, when Rx is high,  there is a low-sheld filter effect ... but for less than a dB and for frequencies below 1Hz. Gonna have to assume that the cap is there for other reasons only.

Other than that, it looks like if we ignore the 470p cap, all the branches are just positive feedback around the two main blocks, the second of which has one moveable pole and one pole at zero.

There's one path that lacks capacitors--from the output through the 6.8k to the middle stage through the 10M||X to the third stage, so *most* of the loop gain depends on that. I haven't done the math yet. All of the other paths only really affect the loop gain at particular frequencies. Looks like the output to 22nF to trimpot path is to boost the loop gain of frequencies above some minimum only ... and the 470p was added so that above some maximum frequency it isn't boosted *too* much.

I'm completely ignoring the fact that the capacitors in the loops influence the phase margin / gain margin to change the gain needed for resonance anyway.

Anyway, that's my take on it, so far.

(Your most recent message just came in. As for your problems ... we could put it in spice and see if you het the same results. I've had suspicious amounts of success with making analog filters in spice reveal things.)


On Fri, Aug 28, 2015 at 11:52 AM, Richie Burnett
<rburnett at richieburnett.co.uk> wrote:

> 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!!!
> -Richie,
> -----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.
> Thanks
> Pete
> On Fri, Aug 28, 2015 at 10:12 AM, Walker Shurlds <walkershurlds at gmail.com> 
> wrote:
> 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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