To: "David G. Dixon" dixon@interchange.ubc.ca,
From: Tim Parkhurst tim.parkhurst@gmail.com,
cc: synth-diy@dropmix.xs4all.nl,
Subject: Re: [sdiy] filtering out a saw-reset glitch in a triangle shaper
Date: Mon, 7 Sep 2009 21:36:31 -0700
On Mon, Sep 7, 2009 at 8:16 PM, David G. Dixon<dixon@interchange.ubc.ca> wrote:
> I'm designing a triangle shaper for the 2164 expo VCO. This is a new thing
> for me, because all of my previous VCOs are triangle-core. The basis is a
> simple yet elegant precision full-wave rectifier circuit I found on the web
> somewhere. To mostly eliminate the saw-reset glitch at the apex of the
> triangle, I'm thinking of filtering the rectifier output with a passive LPF
> with a cutoff frequency of 50kHz (3.3k + 1nF). That, plus a couple of
> strategically placed 22p caps in the rectifier circuit itself, seems to all
> but eliminate the glitch in my simulations.
>
> So, here's my question: Is there some reason not to do this that I'm not
> thinking of, or is there some other way to reduce or eliminate this glitch
> which is blindingly obvious to everyone but me?
>
A simpler solution might be to try a "crappy" op-amp like a 741 or
1458. Built in slew limiting, and they perform beautifully in
waveshapers like that. You'll also see these also used in tri to saw
shapers and output buffers for older wavetable VCOs. In fact,
sometimes people get in trouble replacing EVERY op-amp in vintage
synths with higher spec parts, only to find that the synth now sounds
horrible!
Tim (sounds horrible, but cute as a buttton and fun at parties) Servo
--
"Sire, the church of God is an anvil that has worn out many hammers."
- H.L. Hastings
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