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<div class="moz-cite-prefix">I guess this is a good time to mention
I customized the app to look at the Xpander 3 pole modes:</div>
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<div class="moz-cite-prefix"><a class="moz-txt-link-freetext" href="https://expeditionelectronics.com/Diy/Polemixing/xpander">https://expeditionelectronics.com/Diy/Polemixing/xpander</a></div>
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<div class="moz-cite-prefix">I kept the shifted pole in the analysis
rather than just thinking of it as a buffer. You can see the
effect at higher frequencies on the HP modes, but you'd have to
have the cutoff pretty low to hear it I think. I only included
the 3 pole modes in the drop down as that was what I was most
interested in. The 4 pole modes from the Xpander can be simulated
in the main app.</div>
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<div class="moz-cite-prefix">Dave<br>
</div>
<div class="moz-cite-prefix"><br>
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<div class="moz-cite-prefix">On 4/12/21 9:49 PM, Andrew Simper
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CAAxOnzY+sYCASgEzT54rhsz8xa-vpzf0O2fm2+pngmGCY9N8rA@mail.gmail.com">
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<div dir="ltr">You have two degrees of freedom to place notches
with this structure, and when I worked on this stuff around 15
years ago for the synth Strobe I found the best sounding
position was an octave above and an octave below the cutoff,
but you can place the notches anywhere you want, and in
software land it's trivial to have the notch position as a
filter parameter that can be modulated.
<div><br>
</div>
<div>For those wanting to work out frequency responses and get
it right the first time, I recommend starting with the nodal
/ voltage loop idealised continuous equations, and solve
them directly, instead of introducing confusion by guessing
as to where the feedback terms should be. This also means
you can say use the first g term as 330*g and work out the
correct response for the 3 pole version of the x-pander.
Here is a pdf of the full working from first principles for
the 4 pole version:</div>
<div><br>
</div>
<div><a
href="http://www.cytomic.com/files/dsp/cascade-4pole-freq-resp-working.pdf"
moz-do-not-send="true">www.cytomic.com/files/dsp/cascade-4pole-freq-resp-working.pdf</a></div>
<div><br>
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<div>Cheers,</div>
<div><br>
</div>
<div>Andy</div>
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<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Sat, 10 Apr 2021 at 14:13,
David G Dixon via Synth-diy <<a
href="mailto:synth-diy@synth-diy.org" 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">I
did more or less the same thing more than 10 years ago, but in
Excel. In<br>
that case, the gain factor of the ABCDE are specified as
integer values, and<br>
the resonance is altered with a slider. I found this very
useful for<br>
finding interesting modes, particularly with the various
notches (including<br>
the famous double-notch mode, which has the gain factors of
1-4-12-16-8).<br>
The gain factors translate directly into resistor values for
pole mixing.<br>
This was the basis of the Intellijel Polaris.<br>
<br>
I'd be happy to send the spreadsheet to anybody who wants it.<br>
<br>
<br>
-----Original Message-----<br>
From: Synth-diy [mailto:<a
href="mailto:synth-diy-bounces@synth-diy.org"
target="_blank" moz-do-not-send="true">synth-diy-bounces@synth-diy.org</a>]
On Behalf Of David<br>
Moylan via Synth-diy<br>
Sent: Thursday, April 08, 2021 3:25 PM<br>
To: synth-diy@synth-diy org<br>
Subject: [sdiy] Pole Mixing<br>
<br>
[CAUTION: Non-UBC Email]<br>
<br>
Hi All. I banged together a little web app to play around
with filter pole<br>
mixing, of the Oberheim Xpander type. You can mix poles in
varying amounts<br>
and see the output magnitude shape as well as the transfer
function. Y axis<br>
is Db and X axis is log scale based on normalized frequency
(so basically 1<br>
equals the cutoff frequency). Haven't done 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>
<p><br>
</p>
<pre class="moz-signature" cols="72">--
David Moylan
Expedition Electronics
sonic adventures!</pre>
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