[sdiy] Buchla 291 analysis?

Pete Hartman pete.hartman at gmail.com
Fri Aug 28 19:17:02 CEST 2015


Hm, that actually gives some clue about what might be going on....

The issue is 1) it sounds a bit buzzy for a filter when the bandwidth is
relatively narrow (higher resonance), maybe that's normal, but 2) at
certain BW and frequency settings it gets really crazy (no, not self
oscillation, that's a different setting and very obviously what it is)
where the buzz turns into a semi-randomized ticking.

Looking at it on the scope, feeding with a saw wave, when it's buzzy,
there's a very significant spike on the vertical edge; seems possibly tied
to resonance, as I have two filters doing this, and the lower one is both
more highly resonant (self oscillates faster) and more buzzy.  And it of
course isn't nearly so buzzy when you widen the bandwidth.  When it gets
into the ticking state, that spike goes from significant to huge, and
inconsistently so.

There are a lot of these filters out there, but I haven't (so far) found
any simple wave in > result out sound demos, for example the synthcube demo
has a complex processed waveform as the input, so I can't say how much of
the sound is due to the filter and how much is other stuff -- it does
sounds very different than this though.

The question in hand really boils down to "is it broken, or is it just a
combination of settings to avoid unless you want that noisy sound?"

Thanks

Pete





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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