[sdiy] AS3320 VCF chip - some queries

Andrew Simper andy at cytomic.com
Mon Nov 12 07:54:52 CET 2018

The op-amp is the buffer.

On Mon, 12 Nov 2018 at 13:16, David G Dixon <dixon at mail.ubc.ca> wrote:

> I don't think I have ever put a buffer between two filter stages.  Since
> ever filter I've ever built uses 2164 as the variable resistor, the
> capacitor is always part of an integrator.  I don't believe that what
> follows such a filter stage is at all relevant to its performance, provided
> that the following input impedance is reasonably high (and mine are always
> 30k, which is more than enough).
> ------------------------------
> *From:* Andrew Simper [mailto:andy at cytomic.com]
> *Sent:* Sunday, November 11, 2018 6:14 PM
> *To:* David G Dixon
> *Cc:* pete.hartman at gmail.com; Tom Wiltshire; SDIY List
> *Subject:* Re: [sdiy] AS3320 VCF chip - some queries
> 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.
> Cheers,
> Andy
> 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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