[sdiy] Buchla 291 analysis?

Sat Oct 10 08:29:04 CEST 2015

So I'm still beating my head on this.  I found some changes that have tamed
the buzziness etc, actually mostly to do with better grounding practice
(originally was grounded only through the panel), and a couple of component
tweaks.  Also matching expected audio levels of Buchla gear more closely
helps as well.

The remaining problem is that the lowest setting of the cutoff knob comes
nowhere near the "spec" value of 30Hz.  It's more like 700Hz in the
original circuits I was working with.  I've also since gotten another
single 291J pcb that I can play with a bit more easily, and I find that if
I increase R22 to 4M and C2 & C5 to ~55nF, I can easily get my cutoff down
where I expect it to be.... but it's at the expense of the resonance.  High
bandwidth/low resonance settings get a realllly soft knee that is a few
steps higher in cutoff frequency than when I have lower bandwidth/higher
resonance; the peak / knee shifts quite a bit (from 150Hz at max resonance,
to 200Hz at moderate high resonance to 400Hz at no resonance... though that
latter is squishy because of how soft the knee is).

http://elmegil.dynathome.net/~elmegil/IMG_20151009_223326015.jpg

(obviously that highest resonance setting is oscillating too)

I tried simulating this in iCircuit, but unfortunately it seems their op
amp models (or something) are crap for this kind of circuitry, I've found
that it doesn't even come close to resembling what goes on in the real
circuit.

I've tried experimenting (on my own board) with various other component
values, but I can't seem to find anything that will let me get both the
reasonably good resonance curves of the "stock" configuration and the
expected bottom cutoff frequency.  When I'm more awake again tomorrow I
think I may try simply running the CF pot from +V to a resistor going to -V
so I can find a lower voltage than ground that will take me all the way
down to 30Hz without mucking up the other circuit curves.  Using a resistor
as a voltage divider with the pot to set a bottom to it.

On Fri, Aug 28, 2015 at 12:21 PM, Walker Shurlds <walkershurlds at gmail.com>
wrote:

> 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.)
>
> -Walker
>
>
>
> 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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