[sdiy] SVF problems

[Stephen Begin]

I tried moving r5's connection to the non-inverting in, but now i'm getting
nothing at all...I think all my poking around may have upset one of the
IC's...
I might have to drag this out of the archives again when I have a few more
bucks :)
-Steve

----- Original Message -----
From: "patchell" <patchell at silcom.com>
To: "Scott Bernardi" <sbernardi at attbi.com>
Cc: "Synth-DIY list" <synth-diy at dropmix.xs4all.nl>
Sent: Friday, May 02, 2003 11:32 PM
Subject: Re: [sdiy] SVF problems


>     If you look at the below circuit at the Q feedback loop, you will note
that one is
> a negative feedback loop...becasue there is an OTA in the circuit with the
signal going
> through the negative input, makes this one negative feedback, the previous
circuit, had
> only two opamps in the loop with the signal going through the negative
inputs...making
> positive feedback.  For the SVF, this loop does need to have negative
feedback...the
> more feedback, the higher the damping, thus lower Q.
>
>     I can't really talk, back in 1980 I had a major mistake published in
> Polyphony...real embarassing.
>
> Scott Bernardi wrote:
>
> > Ummm... isn't that the idea of the resonance feedback loop? It's a bit
of frequency
> > dependent positive feedback, and that's what causes the corner peaking.
And that's
> > why you also often have to place some sort of limiters like back to back
diodes
> > parallel to the pot (for example, see the ASM-1 VCF
> > http://home.swipnet.se/cfmd/synths/friends/stopp/asm1vcf.new.pdf, which
has the
> > same polarities as the SVF but has a limiting diodes also).
> > That may be the problem. Question: if you turn the Q pot all the way
down so it
> > feeds no signal back, do you still get the distortion?
> >
> > patchell wrote:
> >
> > >     That circuit won't work as drawn...there is a positive feedback
loop around
> > > U1 and U2 through the Q pot....Move R5 from the Negative input of U1
to the
> > > Positive input, and it should work....
> > >
> > > Stephen Begin wrote:
> > >
> > > > I'm trying to put together this little circuit from Magnus's page
> > > >
> > > > http://home.swipnet.se/cfmd/synths/schematics/svf1.pdf
> > > >
> > > > and I've tried twice on breadboard, and both times i end up with a
signal
> > > > that is so distorted it's hard to tell if the filters are even
working, and
> > > > for about 9/10th of the range of the pots, i get no signal at all.
> > > >
> > > > I'm really not very experienced here so i'm wondering if maybe there
is
> > > > something that is assumed in this schematic that i don't know
> > > > of...basically, I got the pinouts for the two chips, and made the
> > > > appropriate connections (or tried).  Are there supposed to be power
supply
> > > > bypass caps or anything like that?
> > > >
> > > > Also, forgive me for even asking, but does the p1a and p1b mean it's
a dual
> > > > pot?
> > > >
> > > > Thanks,
> > > > Steve
> > >
> > > --
> > >  -Jim
> > > ------------------------------------------------
> > > * Visit:http://www.silcom.com/~patchell/
> > > *-----------------------------------------------
> >
> > --
> > Scott Bernardi
> > sbernardi at attbi.com
>
> --
>  -Jim
> ------------------------------------------------
> * Visit:http://www.silcom.com/~patchell/
> *-----------------------------------------------
>
>
>