[sdiy] Question about ASM-1 state variable VCF

[Magnus Danielson]

From: Scott Bernardi <sbernardi at home.net>
Subject: [sdiy] Question about ASM-1 state variable VCF
Date: Tue, 15 May 2001 06:04:14 -0700

> I breadboarded a state variable VCF very similar to the ASM-1;
> schematics are at:
> http://members.home.net/sbernardi/elec/og2/og2_svfilter1.gif
> http://members.home.net/sbernardi/elec/og2/og2_svfilter2.gif
> 
> 1. I would like a nice sustained oscillation when the Q is turned up. I
> get oscillation, but it is very frequency dependent and runs into
> clipping. I notice the ASM-1 VCF has a Q limiter built around R32, D1,
> D2, R31, and R30. There are two pins labeled Qpot and Qout.  How do you
> connect the Q pot?

Oh... I forgot to draw that in... in the ASM-1 original drawings the
pot is on a separate schematic, which is kind of confusing now I
see...

You take a 100k pot and use it as a fader of the Qout downto ground
and pot Qpot on the tapper. If I recall correctly you should run the
pot in reverse...

> 2. I was using TL074's; I notice Gene uses the CA3140, and Electronotes
> recommends it also, for low phase shift across the integrators. Do these
> make a difference in stability compared to TL074's?

I haven't dugged into this detail... haven't the foggiest...

> 3. Has anybody used the CEM3320 in a state variable connection (I've got
> a few extra of these)?
> 
> Any tips you gurus have in getting stable Q and oscillation will be
> appreciated.

I think the limiter is part of it, but you might want to experiment
some with it so that it get you sufficiently into self oscillation but
not into such extremes that you changes the frequency alot.

Cheers,
Magnus