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<DIV dir=ltr align=left><SPAN class=180310209-12112018><FONT color=#0000ff
size=2 face=Arial>Yes. In filter configurations like the internals of the
3320, the filter stages are simply buffered RC tanks. However, when 2164
is used as the variable resistor, the capacitor must be part of a Miller
integrator, because 2164 can only terminate to virtual ground. The point I
was making was that these integrators are not sensitive to what follows unless
an unsuitably low impedance is used to connect the output to the next
stage. This would have to be a very low impedance indeed to overwhelm the
current-sourcing ability of the opamp, and since 2164 filters are typically
connected together with 30k resistors, this is never a problem in such
filters.</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=180310209-12112018><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=180310209-12112018><FONT color=#0000ff
size=2 face=Arial></FONT></SPAN> </DIV>
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<DIV dir=ltr align=left><FONT size=2 face=Tahoma><B>From:</B> Andrew Simper
[mailto:andy@cytomic.com] <BR><B>Sent:</B> Sunday, November 11, 2018 10:55
PM<BR><B>To:</B> David G Dixon<BR><B>Cc:</B> pete.hartman@gmail.com; Tom
Wiltshire; SDIY List<BR><B>Subject:</B> Re: [sdiy] AS3320 VCF chip - some
queries<BR></FONT><BR></DIV>
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<DIV></DIV>
<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>
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<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>
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<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
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