[sdiy] Question about ASM-1 state variable VCF

jh. jhaible at t-online.de
Fri May 18 22:31:07 CEST 2001

> #3  I HAVE used the CEM 3320 in the state variable configuration
> in a ProOne.  After about a year, I took it out and converted back
> to a near stock design.
> The SVF has much less signal handling capacity, and distorts in
> an unpleasent way. The HPF and Notch filters that I used were
> not too useful.

The OB-8 has the CEM3320 switched to state variable configuration
(when 2pole is chosen), and it does a decent - not a perfect job.

Harry is right about overload. High Q means more gain, and overload
is not so pleasant on a 3320. The OB-8 has a little too much gain in
2pole mode. I reduced this (changed resistor divider), and it sounds
very nice now. (You would only notice distortion on a stock OB-8
with very high Q and with a triangle wave from the VCO anyway,
so it's a decent filter even without modification.)

The OB-8-type CEM3320-SVF is not a "perfect" state variable filter
either: I noticed this when I made the HPF output available in my OB-8.
(See my homepage for details.) Apparently the "HPF" mode is a mix of
HPF and BPF, probably because of one gm cell being (ab)used as
opamp. BUT even if it's not a perfect HPF in mathematical sense,
in practice it sounds very good.

I encourage you to try it - start with the OB-8 circuit.


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