<div dir="ltr">The op-amp is the buffer.<div><br><div class="gmail_quote"><div dir="ltr">On Mon, 12 Nov 2018 at 13:16, 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_4956600541222010789467001105-12112018"><font color="#0000ff" size="2" face="Arial">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).</font></span></div><br>
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<font size="2" face="Tahoma"><b>From:</b> Andrew Simper [mailto:<a href="mailto:andy@cytomic.com" target="_blank">andy@cytomic.com</a>]
<br><b>Sent:</b> Sunday, November 11, 2018 6:14 PM<br><b>To:</b> David G
Dixon<br><b>Cc:</b> <a href="mailto:pete.hartman@gmail.com" target="_blank">pete.hartman@gmail.com</a>; Tom Wiltshire; SDIY
List<br><b>Subject:</b> Re: [sdiy] AS3320 VCF chip - some
queries<br></font><br></div>
<div></div>
<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" target="_blank">dixon@mail.ubc.ca</a>> wrote:<br></div>
<blockquote style="BORDER-LEFT:#ccc 1px solid;MARGIN:0px 0px 0px 0.8ex;PADDING-LEFT:1ex" class="gmail_quote"><u></u>
<div>
<div dir="ltr" align="left"><span class="m_4956600541222010789m_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_4956600541222010789m_5891091620987099791815431418-11112018"><font color="#0000ff" size="2" face="Arial"></font></span> </div>
<div dir="ltr" align="left"><span class="m_4956600541222010789m_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>
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<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></blockquote></div>
</blockquote></div></div></div>