resonance with a svf

[danial stocks]

>     One thing I have always wanted to try is based on a circuit based on
>the quadrature sine wave oscilator that you will find in the Analog
>Devices NonLinear Circuits handbook (page 80 in my copy).  This
>oscilator is basically a state variable filter.  If you look at the
>amplifier that controlls the resonance, it keeps the thing oscilating by
>balancing positive and negative feed back.  When the voltage exceeds a
>certain amplitude, the zener diodes begin to conduct forcing negative
>feedback.  When the amplitude is low, the zeners don't conduct, and you
>only have positive feedback.  In the Nov./Dec. 1980 issue of Polyphony,
>you will find the same circuit, except implemented with CA3280's instead
>of analog multipliers.  The article was written by some strange
>character. :^)

Electronics World published a SVF modified to be a sine osc.. no zeners tho, 
it used a trimpot to set the balance..


>
>     If you use an OTA to control the resonance, and the OTA is
>"inverting", if you put a large resistor around the OTA, this resistor
>will provide positive feedback when the OTA is at very low Gm values.


A SVF I made had a dodgy res pot.. which went open circuit.. it took me a 
while to figure out what had happened, but it sounded great! the res was 
soooo high, would have probably done major damage to speakers if it was fed 
by an osc and tuned to a low harmonic, but very nice whistley sound with a 
small amt of noise i/p.. but even this extreme resonance condition could not 
make it oscillate..
Cheers,
Dan
________________________________________________________________________
Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com