[sdiy] AS3320 VCF chip - some queries

David G Dixon dixon at mail.ubc.ca
Sun Nov 11 19:17:43 CET 2018

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
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>

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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