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<div class="moz-cite-prefix">A bit hard to follow here, Andrew.
Your main transfer function looks the same to mine if g=1, except
that you have the k term included in the denominator but not the
numerator. In general, my denominator looks like yours with the k
term, and in the numerator the feedback only changes the term on
s^0 which goes from</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">a - b + c - d + e;<br>
</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">to <br>
</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">(a * (1 - k)) - b + c - d + e;<br>
<br>
</div>
<div class="moz-cite-prefix">David Dixon shared his Excel
spreadsheet with me, so verify with that as well.</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">Dave<br>
</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">On 4/11/21 12:21 AM, Andrew Simper
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CAAxOnzYmNdoWdswf-opOouFjDR=qHdLdq8S4KGwoXM7ziARVvw@mail.gmail.com">
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<div dir="ltr">
<div dir="ltr">Hi David,
<div><br>
</div>
<div>I'm not sure you've got the correct frequency response on
your page. <span style="color:rgb(0,0,0)">You may have
better luck with this:</span></div>
<div style="color:rgb(0,0,0)"><br>
</div>
<div style="color:rgb(0,0,0)">(g^4 (m0 + m1 + m2 + m3 + m4) +
g^3 (4 m0 + 3 m1 + 2 m2 + m3) s + g^2 (6 m0 + 3 m1 + m2)
s^2 + g (4 m0 + m1) s^3 + m0 s^4) /</div>
<div style="color:rgb(0,0,0)">(g^4 (1 + k) + 4 g^3 s + 6 g^2
s^2 + 4 g s^3 + s^4)</div>
<div style="color:rgb(0,0,0)"><br>
</div>
<div style="color:rgb(0,0,0)">where g is the cutoff in hz, and
k is the resonance feedback with 4 being self oscillation,
and the mixing terms are as follows:</div>
<div style="color:rgb(0,0,0)"><br>
</div>
<div style="color:rgb(0,0,0)">m0*(in - k*lp4) + m1*lp1 +
m2*lp2 + m3*lp3 + m4*lp4</div>
<div style="color:rgb(0,0,0)"><br>
</div>
<div style="color:rgb(0,0,0)">Note that for the xpander uses
inverting stages, I've used non-inverting stages, so each
other mixing terms needs to be negative, eg:</div>
<div style="color:rgb(0,0,0)"><br>
</div>
<div style="color:rgb(0,0,0)">m0, -m1, m2, -m3, m4</div>
<div style="color:rgb(0,0,0)"><br>
</div>
<div style="color:rgb(0,0,0)">Cheers,</div>
<div style="color:rgb(0,0,0)"><br>
</div>
<div style="color:rgb(0,0,0)">Andy</div>
</div>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Fri, 9 Apr 2021 at 06:24,
David Moylan via Synth-diy <<a
href="mailto:synth-diy@synth-diy.org" target="_blank"
moz-do-not-send="true">synth-diy@synth-diy.org</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex">Hi
All. I banged together a little web app to play around with
filter <br>
pole mixing, of the Oberheim Xpander type. You can mix poles
in varying <br>
amounts and see the output magnitude shape as well as the
transfer <br>
function. Y axis is Db and X axis is log scale based on
normalized <br>
frequency (so basically 1 equals the cutoff frequency).
Haven't done <br>
phase plot yet.<br>
<br>
If you have an interest in this sort of thing check it out:<br>
<br>
<a href="https://expeditionelectronics.com/Diy/Polemixing"
rel="noreferrer" target="_blank" moz-do-not-send="true">https://expeditionelectronics.com/Diy/Polemixing</a><br>
<br>
Cheers.<br>
<br>
-- <br>
David Moylan<br>
Expedition Electronics<br>
sonic adventures!<br>
<br>
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</blockquote>
</div>
</blockquote>
<p><br>
</p>
<pre class="moz-signature" cols="72">--
David Moylan
Expedition Electronics
sonic adventures!</pre>
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