<div dir="ltr">Good point about the main filter caps David. Two other things to watch out for to make low frequency self oscillation happen with filters:<div><br><div>(*) 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</div><div>(*) make sure you use good buffers between stages to not drain out the current from your caps. </div><div><br></div><div>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.</div><div><br></div><div>Cheers,</div><div><br></div><div>Andy</div><div><br></div><div><br></div></div></div><br><div class="gmail_quote"><div dir="ltr">On Mon, 12 Nov 2018 at 02:17, David G Dixon <<a href="mailto:dixon@mail.ubc.ca">dixon@mail.ubc.ca</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><u></u>
<div>
<div dir="ltr" align="left"><span class="m_5891091620987099791815431418-11112018"><font color="#0000ff" size="2" face="Arial">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.</font></span></div>
<div dir="ltr" align="left"><span class="m_5891091620987099791815431418-11112018"><font color="#0000ff" size="2" face="Arial"></font></span> </div>
<div dir="ltr" align="left"><span class="m_5891091620987099791815431418-11112018"><font color="#0000ff" size="2" face="Arial">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.</font></span></div><br>
<blockquote style="BORDER-LEFT:#0000ff 2px solid;PADDING-LEFT:5px;MARGIN-LEFT:5px;MARGIN-RIGHT:0px" dir="ltr">
<div dir="ltr" lang="en-us" class="m_5891091620987099791OutlookMessageHeader" align="left">
<hr>
<font size="2" face="Tahoma"><b>From:</b> Synth-diy
[mailto:<a href="mailto:synth-diy-bounces@synth-diy.org" target="_blank">synth-diy-bounces@synth-diy.org</a>] <b>On Behalf Of </b>Pete
Hartman<br><b>Sent:</b> Sunday, November 11, 2018 9:20 AM<br><b>To:</b> Tom
Wiltshire<br><b>Cc:</b> <a href="mailto:synth-diy@synth-diy.org" target="_blank">synth-diy@synth-diy.org</a> DIY<br><b>Subject:</b> Re:
[sdiy] AS3320 VCF chip - some queries<br></font><br></div>
<div></div>
<div dir="ltr">
<div class="gmail_quote">
<div dir="ltr">On Sun, Nov 11, 2018 at 10:32 AM Tom Wiltshire <<a href="mailto:tom@electricdruid.net" target="_blank">tom@electricdruid.net</a>> wrote:</div>
<blockquote style="BORDER-LEFT:#ccc 1px solid;MARGIN:0px 0px 0px 0.8ex;PADDING-LEFT:1ex" class="gmail_quote">2) The resonance response is tilted to the high end.<br>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?<br></blockquote>
<div><br></div>
<div>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.</div></div></div></blockquote></div>
_______________________________________________<br>
Synth-diy mailing list<br>
<a href="mailto:Synth-diy@synth-diy.org" target="_blank">Synth-diy@synth-diy.org</a><br>
<a href="http://synth-diy.org/mailman/listinfo/synth-diy" rel="noreferrer" target="_blank">http://synth-diy.org/mailman/listinfo/synth-diy</a><br>
</blockquote></div>