[sdiy] 2 queries -- SVF self oscillation

[Scott Gravenhorst]

patchell <patchell at silcom.com> wrote:
>
>
>Gavin wrote:
>
>> hey all,
>> i've got 2 totally unrelated ?'s
>> 1. anyone know have understand the tuning procedure for the yamaha CS-40m.
>> I know there are some CSersout there!
>> 2. AmI correct in thinking that in the basic state variable filter (2 3080's
>> etc.)  MORE resistance in the resonance feedback path equals MORE resonance,
>> LESS resistance equals LESS resonance?
>> thanks
>
>    Yes, in a standard SVF, more resistance (which corresponds to less gain)
>decreases the damping (which is what that feedback path really controls), or
>increases resonance.
>
>    If you use an OTA for the resonance control...this translates more current,
>more damping (lower resonance) less current, less damping (higher resonance),
>zero current = oscilator (well, in theory anyway).

Ah yes, "in theory"...

I've always wondered about an SVF and self oscillation.  I have 2 FatMan synths
which use an LM13600 SVF which looks pretty much like the schematic in the data
sheet.  I've never been able to make either one self oscillate, even when I
break the feedback path open (unsolder one leg of the rez pot).

However, a while ago, I built an experimental SVF with nonlinear feedback.  The
nonlinear circuits I was playing with needed more signal than the feedback was
providing, so I added gain.  In this case, it's pretty easy to make it self
oscillate.  The circuit is basically the same LM13600 SVF with noninverting gain
in the feedback path.

Is there something other than adding gain to the feedback path I can do to get
self oscillation to occur (near the end of the rez pot rotation) on an LM13600
SVF of this type?


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