[sdiy] State Variable Filter self oscillation

[Tony Allgood]

> Has anyone built a classic state variable that can oscillate?

Out of the current fare, I know the Analogue Systems one does. But I
think the question should be 'how can I get it to go into a nicely
controlled oscillation?'.

Of the older commercial designs out there I think the one in the
Oberheim OB-Xa and OB-Sx is quite good. This is a 3320 based design that
uses a diode limiter to control the amplitude of the oscillations. Most
service notes tell you to back off the Q adjust trimmer so that it does
not oscillate, but it does oscillate nicely, so that seems a bit of a
waste.

Another nice design using 3080 OTAs is the Transcendant 2000 VCF. Nice
sounding filter, and bursts into oscillation. You can drive it harder
than the Tim Orr's original design, and the oscillation is similar in
level to your audio input.

My own Oakley Sound SVF also goes into nice oscillation, but I found the
best sound was given when I allowed the filter not to oscillate at lower
frequencies. I used an 3280 dual OTA and the classic zener and diode
ring limiting used in some state variable oscillator circuits. Oakley
SVF boards are back on sale later this month.

The other thing you need is well matched integrator stages for a decent
oscillation.

Regards,

Tony Allgood                       Penrith, Cumbria, England

Oakley Sound Systems                     www.oakleysound.com
Modular Synthesisers        www.oakleysound.com/projects.htm