[sdiy] CA3080 VCF and Mysterious Op Amp Behaviour

[RFAHL]

Which input pin on the buffer are you using?

on 12/8/01 2:26 PM, Gavin Russom at elmystico at earthlink.net wrote:

> 
> ----------
> 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?