[sdiy] CA3080 VCF and Mysterious Op Amp Behaviour

[Gavin Russom]

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From: "Gavin Russom" <elmystico at earthlink.net>
To: EFM_Synth at egroups.com
Subject: vclf1a
Date: Sat, Dec 8, 2001, 3:15 AM


Hey all, I built the VCLF1a that used to be on Tom G's EFM site in the
cookbook section way back
when.  This is the 2 pole OBX type state variable (LP,BP) filter.  It is
similar to the state variable VCF in Electronotes #76, but it usses a 10uF
cap delay back into the signal input for resonance.  Actually I built two. 
One works great.  The other one doesn't.  I think I've
narrowed down the problem to the two op amps working as followers with
direct feedback (no resistor).  Even with the CA3080's out and the outputs
from these op amps cut, they put out all kinds of wacky signals!  Sometimes
they pin near supply, sometimes the oscillate VERY SLOWLY, like .1 Hz. 
Sometimes the voltage level jumps around.  This is all readings taken
without changing anything!  Sometimes I get the feeling the probes are
somehow charging the circuit.  So my question is... what is causing this? 
I've checked over my work a bunch of times and the two circuits are
connected exactly the same... and both match the schematic.  Do I need to
put a small value resistor in the feedback loop of these op amp followers?
And (of course) WHY?