[sdiy] AS3320 VCF chip - some queries

Andrew Simper andy at cytomic.com
Mon Nov 12 03:13:37 CET 2018

Good point about the main filter caps David. Two other things to watch out
for to make low frequency self oscillation happen with filters:

(*) make sure any DC blocking caps in the resonance signal path are well
out of range of where you want the filter to self oscillate down to,
otherwise the phase and gain changes can throw off normal resonance
(*) make sure you use good buffers between stages to not drain out the
current from your caps.

With the 3320 the buffers are on chip so this makes things more difficult
if they are causing the issue. Do you know what they are? If they are a
part of the problem then one way to mitigate it somewhat would to use
larger valued caps, and then likewise increase the cutoff current so the
relative drain of the buffers is lower, but this may impact the highest
cutoff you can get.



On Mon, 12 Nov 2018 at 02:17, David G Dixon <dixon at mail.ubc.ca> wrote:

> You might find that the filter will oscillate easier if you use good film
> caps.  I had problems getting a four-pole filter to oscillate at low
> frequencies with monolithic ceramic caps, but it oscillated strongly down
> to the minimum frequencies with WIMA polyester caps.
> I just built an SVF with silvered mica caps (which I usually use in
> oscillators) and it is being finicky with self-oscillation below about
> 50Hz.  Maybe I'll swap those out for WIMA polyester and see.
> ------------------------------
> *From:* Synth-diy [mailto:synth-diy-bounces at synth-diy.org] *On Behalf Of *Pete
> Hartman
> *Sent:* Sunday, November 11, 2018 9:20 AM
> *To:* Tom Wiltshire
> *Cc:* synth-diy at synth-diy.org DIY
> *Subject:* Re: [sdiy] AS3320 VCF chip - some queries
> On Sun, Nov 11, 2018 at 10:32 AM Tom Wiltshire <tom at electricdruid.net>
> wrote:
>> 2) The resonance response is tilted to the high end.
>> It seems to require a lot more resonance CV (ok, current) to make the
>> chip oscillate at the bass end than at the treble end. Similarly, the
>> amplitude of oscillation increases as the frequency rises. Why is this? Is
>> there anything I can do about it? Should I even care?
> If I compare this with the behavior I've seen in other lowpass filters
> that self oscillate, even without CV over the resonance, it seems
> consistent.  Typically I need the frequency at 50% or higher to get it to
> start oscillating -- I can then commonly turn it down and get lower
> frequencies, but my experience is that if the filter is going to oscillate,
> it's always easier to start the oscillation at a higher frequency.  And
> typically if I turn it "too low" then it stops oscillating again.
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