[sdiy] AS3320 VCF chip - some queries

Tom Wiltshire tom at electricdruid.net
Sun Nov 11 19:47:31 CET 2018

My stripboard prototype has Wima poly film caps in it, so I don’t think that’s it.

Thanks though.


       Electric Druid
Synth & Stompbox DIY

> On 11 Nov 2018, at 18:17, 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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