[sdiy] Filter problem

[Neil Johnson]

Hi all,

Well, the weekend proved slightly productive, as I made some progress on
my 3-pole filter.  I've got it tamed down to something vaguely usable,
with both Q and cut-off controllable with sensible control voltage ranges.
I've set the upper cut-off frequency to around 24kHz, limited by the size
of integrating capacitors.  And it goes right down to sub-audio (I
measured 4Hz on 'scope but couldn't be bothered to dig out my freq counter
to go any lower).  It is made with an SSM2164 and a TL074.

Now, the problem.  Occassionally, and related to noise spikes on the mains
(evidence: turning on my sig-gen triggers the fault, even if not connected
to anything), it goes mad and oscillates at full amplitude -- it normally
sits at a couple of volts pk-pk, but goes +/- 15V fullswing.  My guess is
that one of the op-amps is being overloaded, perhaps an input exceeding
limits, and so going into phase reversal, with the resulting honking.

So, firstly, has anyone seen an TL07x op-amp phase-reverse??  Perhaps it's
the SSM2164??

I think tonight I'll try reverse-parallel diodes across the op-amp inputs
to see if that has any beneficial effect.

Any sage words of advice gratefully received.

Cheers,
Neil

--
Neil Johnson :: Computer Laboratory :: University of Cambridge ::
http://www.njohnson.co.uk          http://www.cl.cam.ac.uk/~nej22
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