Xpander VCF

[Hairy Harry]

Hmmm... beats me.

I tried a spice simulation of this, using several models for the
4-pole filter as a basis (including the JH 2040 clone). I did not
note where self oscillation occurred.  The cutoff frequency was not
always the same, so probably the self oscillation point moves also

OTOH, in use you probably select a mode and tune to suit...

H^)  harry


>From: Haible Juergen <Juergen.Haible at nbgm.siemens.de>
>To: harrybissell at prodigy.net
>CC: synth-diy at node12b53.a2000.nl
>Subject: RE: Xpander VCF
>Date: Thu, 26 Oct 2000 14:48:12 +0200
>
>	>in some modes, one stage is bypassed. That stage will be unstable
>	>without ANY cap at all, so it is disabled by using a cap that is
>way too
>	>small (that pole tuned all the way up...)
>
>Yes, that's an important point !
>It's like an externally compensated opamp which must be stable over
>decades of varying bias current of its input stage.
>
>But ... isn't there a problem with resonance then ? Self
>oscillation should occur later (more gain) in these "bypassed modes".
>Is this compensated for in the XPander ?
>
>JH.
>

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