[sdiy] CA3080 VCF and Mysterious Op Amp Behaviour

[Gavin Russom]

The outputs of the CA3080's go both go into follower/buffer op-amps across a
300pF that goes to ground cap and goes into the + input of the op amp.  The
- input is connected to the output directly with no feedback resistor on
both these op amps.  The output of OA-2 which is the BandPass output pins
near supply at @ +13V.  The output of OA-3 which is the LowPass output
starts at 0 when power is applied, and very slowly drops (I think it
oscillates but as I said the oscillation would be about .1Hz so it's
difficult to measure) to -15 and then starts going back up again!
----------
>From: "RFAHL" <8brain at spiritone.com>
>To: "Gavin Russom" <elmystico at earthlink.net>, " synth"
<synth-diy at dropmix.xs4all.nl>
>Subject: Re: [sdiy] CA3080 VCF and Mysterious Op Amp Behaviour
>Date: Sat, Dec 8, 2001, 6:27 PM
>

>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?
>