[sdiy] Filter problem

[Sowa Roman]

Hi,
I'm happy user of SSM2164 VCF, so maybe I'll be able to help you.
First - TL074 do phase reverse as I recall.
Second - control voltage of SSM should never go below -0.5V.
Maybe power spike forces your control circuitry to produce short
spike on SSM control input, launching crazy oscillation. Maybe
a shottky there would prevent it?
On my filter I use back-to-back 5V6 zener diodes in one of filter
integrators to limit amplitude at controlled level when Q is set
high. This also prevents from any opamp crazyness and adds a little
dirt to clean SVF sound. I highly recommend doing that.
The schematic is here (actual filter on right hand side):
http://membres.lycos.fr/romcio/m_pics/m_vcf.gif
And I'm using OP482, not TL074. Probably not an issue here, but
I thought I should mention this.

Roman

__-----Original Message-----
__From: Neil Johnson [mailto:nej22 at hermes.cam.ac.uk]
__Sent: Monday, July 08, 2002 12:29 PM
__To: synth-diy
__Subject: [sdiy] Filter problem
__
__
__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
__----  IEE Cambridge Branch: http://www.iee-cambridge.org.uk  ----
__
__
__
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