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<div class="moz-cite-prefix">On 13.12.2022 07:54, Donald Tillman
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:23AE9A28-32C8-4A97-9F1F-D525BA20FEB6@till.com">
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<div class="">Hey Aaron,</div>
<div class=""><br class="">
</div>
<div class="">I don't recognize it either.</div>
<div class=""><br class="">
</div>
<div class="">And the filter circuit, as presented, doesn't make
any sense. If you work out the transfer function, it's not a
bandpass filter; it's some highpass bandpass combo with a low Q.
And like you said, the opamp buffer doesn't make any sense
either.</div>
<div class=""><br class="">
</div>
<div class="">Hmmm, y'know... </div>
<div class=""><br class="">
</div>
<div class="">If the schematic was in error... </div>
<div class=""><br class="">
</div>
<div class="">And the filter's output signal was not the opamp
output as shown, but instead tapping off of the RC network that
leads to the opamp's + input, that would make more sense.</div>
<div class=""><br class="">
</div>
<div class="">Something like this:</div>
<div class=""><br class="">
</div>
<div class=""><img apple-inline="yes"
id="42E83385-B8C8-4F87-95D1-7F0C58FAA340"
src="cid:part1.yh3XRNWE.1EUmfQLD@schmitzbits.de" class=""></div>
<div class=""><br class="">
</div>
<div class=""><br class="">
</div>
<div class="">That's a proper bandpass filter. And it would
require a buffer.</div>
</blockquote>
<p><br>
</p>
<p>I had a similar hunch, but couldn't quite figure it it out. <br>
</p>
<p>When I simulate it as above, I find that the gain is way down in
the mud at -50-ish dB, and the peak is very flat and broad. <br>
</p>
<img src="cid:part2.amLumg0q.HS7Y2yAq@schmitzbits.de"
moz-do-not-send="false">
<p>But if you swap + and -, and tap at the -input, then it works
like I'd expect.</p>
<p><br>
</p>
<p>Best,</p>
<p> René<br>
</p>
<p><br>
</p>
<p>PS. "Look ma', no algebra" </p>
<p><br>
</p>
<blockquote type="cite"
cite="mid:23AE9A28-32C8-4A97-9F1F-D525BA20FEB6@till.com">
<div class=""><br class="">
</div>
<div class="">
<div class="">
<div dir="auto" style="word-wrap: break-word;
-webkit-nbsp-mode: space; line-break: after-white-space;"
class="">
<div style="color: rgb(0, 0, 0); font-family: Menlo;
font-size: 12px; font-style: normal; font-variant-caps:
normal; font-weight: normal; letter-spacing: normal;
text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; word-spacing: 0px;
-webkit-text-stroke-width: 0px;"> -- Don<br class="">
--<br class="">
Donald Tillman, Palo Alto, California<br class="">
<a href="https://www.till.com"
class="moz-txt-link-freetext" moz-do-not-send="true">https://www.till.com</a></div>
</div>
</div>
<div><br class="">
<blockquote type="cite" class="">
<div class="">On Dec 11, 2022, at 5:11 PM, Lanterman, Aaron
D via Synth-diy <<a
href="mailto:synth-diy@synth-diy.org"
class="moz-txt-link-freetext" moz-do-not-send="true">synth-diy@synth-diy.org</a>>
wrote:</div>
<br class="Apple-interchange-newline">
<div class="">
<div class="">I’ve been looking at the Serge Resonant
Filter:<br class="">
<br class="">
<a
href="https://electro-music.com/wiki/pmwiki.php?n=Schematics.SergeResonantEqualizer"
class="moz-txt-link-freetext" moz-do-not-send="true">https://electro-music.com/wiki/pmwiki.php?n=Schematics.SergeResonantEqualizer</a><br
class="">
<br class="">
The lower 7th bands are standard bandpass Multiple
Feedback filters. But the 3 top band (see rightmost two
on the bottom row, rightmost one on the top row) are
something else weird I haven’t seen. They vaguely
remember a Wein-bridge-style filter in the Boss SP-1
Spectrum pedal, but instead of having a capacitor in
series in the positive feedback loop, Serge has it in
series with the input. <br class="">
<br class="">
Does anyone recognize this filter? I have an urge to
give things names.<br class="">
<br class="">
I tried to work out the transfer filter under the
assumption that the negative feedback is stronger than
the positive feedback so “golden op amp” rules hold. But
when I plot the frequency response of my transfer
function, I get peaks that are way too broad — they’re
more curves than peaks. So I’m doing something wrong
somewhere. <br class="">
<br class="">
My transfer function has a numerator of<br class="">
<br class="">
c*s*(r+rp*(1+r*c*s))*rn <br class="">
<br class="">
And a denominator of<br class="">
<br class="">
rp*r^2*c^2*s^2 + (rp*2+rn)*r*c*s + rp <br class="">
<br class="">
Where rn is the resistor in the negative feedback look,
rp is the resistor in the positive feedback loop, and
the input resistor and resistor to ground are the name
and denoted by r.<br class="">
<br class="">
Serge doesn’t remember where he got the filter from. He
said he went to this topology for the higher bands
because limitations in the open loop gain bandwidth
product prevented the MFB topology from working well at
the higher frequencies.<br class="">
<br class="">
One interesting thing is he felt the need to add an op
amp buffer at the output of the filter, which suggests
to me that non ideal op amp effects are in play.<br
class="">
<br class="">
Aaron Lanterman, Prof. of ECE, Georgia Tech<br class="">
My blog on Education and Innovation:
<a class="moz-txt-link-freetext" href="https://edupocalypsenow.wordpress.com">https://edupocalypsenow.wordpress.com</a><br class="">
My blog on Electronics and Programming:
<a class="moz-txt-link-freetext" href="https://lantertronics.blogspot.com">https://lantertronics.blogspot.com</a><br class="">
My YouTube channel on Electronics and Programming:
<a class="moz-txt-link-freetext" href="https://www.youtube.com/c/lantertronics">https://www.youtube.com/c/lantertronics</a><br class="">
<br class="">
</div>
</div>
</blockquote>
</div>
<br class="">
</div>
<br>
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