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<div class="moz-cite-prefix">I agree with Mattias' analysis of the
pitfalls, but still think it's worthwhile to experiment with. In
practical use I'm not using the HP modes much. I do really like
combinations that include a notch, like LP1 and LP2 + notch. I
also find 1P LP more useful than expected.</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">Also, important to remember that there
are other feedback types. Bandpass might have less side effects
on the HP modes. It's proven useful to prevent bass drop-out for
LP modes.</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">Dave</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">On 5/4/26 12:09, Tom Wiltshire wrote:<br>
</div>
<blockquote type="cite"
cite="mid:FAE2E753-D963-4817-94A0-276F4764A57E@electricdruid.net">
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Just a reminder of this excellent page for experimenting with
pole-mixing filter combinations:
<div class=""><br class="">
</div>
<div class=""><a
href="https://rnd.expeditionelectronics.com/Diy/Polemixing"
class="moz-txt-link-freetext" moz-do-not-send="true">https://rnd.expeditionelectronics.com/Diy/Polemixing</a></div>
<div class=""><br class="">
</div>
<div class="">This can show you how sensitive different modes are
to tolerances. Shift a mix value very slightly and see what
happens. Sometimes it's a lot. 1% precision is probably the
*minimum* required for certain responses.</div>
<div class=""><br class="">
</div>
<div class="">I don't think I agree about Mattias' comment about
the 4-pole HP. At least, I can't get the sim to match what he
describes, so at least in theory it shouldn't do that.</div>
<div class=""><br class="">
</div>
<div class="">Tom</div>
<div class=""><br class="">
<div><br class="">
<blockquote type="cite" class="">
<div class="">On 4 May 2026, at 08:15, Mattias Rickardsson
<<a href="mailto:mr@analogue.org"
class="moz-txt-link-freetext" moz-do-not-send="true">mr@analogue.org</a>>
wrote:</div>
<br class="Apple-interchange-newline">
<div class="">
<div dir="auto" class="">Hi,
<div dir="auto" class="">Pole mixing is fascinating, but
from my experiences from employing it on a four-pole
cascade lowpass filter I'd recommend to think twice.
Some drawbacks are not obvious from the start - in the
order of decreasing obviousness:</div>
<div dir="auto" class=""><br class="">
</div>
<div dir="auto" class="">- Precision. Especially the
resistors need a certain precision, which can be less
fun if looking for those deep notches or if you need
matching stereo pairs or synth voices.</div>
<div dir="auto" class=""><br class="">
</div>
<div dir="auto" class="">- Noise. The more complex pole
mix modes rely on mixing many taps very loudly and
then cancelling out most of their signals except for
the desired ones. The catch is that noise is not
cancelled out, and adds together with lots of gain.
Noisefloor is then much higher on some modes.</div>
<div dir="auto" class=""><br class="">
</div>
<div dir="auto" class="">- Resonance. The pole-mixed
filter slopes are correct when the resonance is turned
down, but not when you turn it up. Resonant peaks can
then appear in less desirable ways. Also the stopband
suffers: The 4-pole HP mix for instance is then not 24
dB/oct anymore since an increasing resonance pulls up
a flatter response. I guess it would need some
resonance switching scheme or resonance-dependent pole
mixing to get this right.</div>
<div dir="auto" class=""><br class="">
</div>
<div dir="auto" class="">So in general I'd recommend to
stay away from it and create the esoteric 4-pole
responses in other ways, unless you want to try it for
fun. :-)</div>
<div dir="auto" class=""><br class="">
</div>
<div dir="auto" class="">But the few pole-mixing
possibilities on 2-pole filters work very well!</div>
<div dir="auto" class=""><br class="">
</div>
<div dir="auto" class="">/mr</div>
<div dir="auto" class=""><br class="">
</div>
<div dir="auto" class="">PS: I've seen some DC offset
issues as well!</div>
</div>
<br class="">
<div class="gmail_quote gmail_quote_container">
<div dir="ltr" class="gmail_attr">Den fre 1 maj 2026
17:12Jeff Whitman 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>>
skrev:<br class="">
</div>
<blockquote class="gmail_quote"
style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div style="line-break:after-white-space" class="">
<div class=""><br class="">
</div>
<div class=""><br class="">
</div>
<div class="">
<p class="">Hello there,</p>
<p class="">I’ve been working on an analog synth
project for several years using Sound
Semiconductor parts. Their data sheets and
design notes have been incredibly helpful,
especially for those of us who are not primarily
analog designers. I’m a firmware engineer by
profession - now retired, with an EE background
from a long time ago, so I understand the basics
but would really appreciate input from people
with more analog design experience.</p>
<p class="">For this project, I’ve built a “voice”
of three oscillator boards using the SSI2131, a
filter board using the SSI2140 in the suggested
pole-mixing configuration to create a 16-mode
multimode filter, a low-pass filter using the
SSI2144, and linear-controlled VCAs using the
SSI2164.</p>
<p class="">My questions are specifically around
using the SSI2140 for pole mixing. From what I
can tell, this design is very similar to the
filter used in the Matrix-12, and I’ve spent
some time studying that schematic. I’ve also
reviewed the AN701 filter design document on the
Sound Semiconductor website. I’m including my
schematic in PDF form in the hope that some of
the very smart analog folks here can offer
suggestions about something I’m seeing.</p>
<p class="">I’ve been using <a
href="http://claude.ai/" target="_blank"
rel="noreferrer" class=""
moz-do-not-send="true">Claude.ai</a> to help
analyze each of my filter configurations and
determine whether they are behaving as expected.
I’ve also been doing sound comparisons against
the Arturia Matrix-12 V modeled synth. One thing
I’ve found is that I’m getting DC offset that
appears to be coming from the pole outputs of
the SSI2140. Depending on the pole-mixing
resistor configuration selected for a given
filter mode, I’m seeing significant DC offset.</p>
<p class="">I’ve been feeding Claude scope shots
to help analyze this, and so far its
interpretation seems pretty consistent with what
I’m measuring. As a side note, this has been a
very useful application of AI for
troubleshooting. For the pole-mixing network,
I’m using 1% resistors and also using 1%
capacitors, as recommended in the data sheet and
AN701.</p>
<p class="">Here is the table Claude generated
from my measurements:</p>
<pre class=""><div class=""><div class=""><div
class=""><div class=""><div class=""><div class=""><div
class=""><div class=""><div class=""><div
id="m_7901873607475970514code-block-viewer"
dir="ltr" class=""><div class=""><pre class=""><code
class="">Mode Mean DC Offset
LP1 -12.5 mV
LP2 -40 mV
LP3 -30 mV
LP4 -30 mV
HP1 -61.9 mV
HP2 -131.1 mV
HP3 -284.4 mV
HP4 -583.6 mV</code></pre></div></div></div></div></div></div></div></div></div></div></div></pre>
<p class="">Claude’s current explanation is that
the increasing weighting factors in the
pole-mixing network are directly amplifying the
DC component.</p>
<p class="">Things that appear to have been ruled
out:</p>
<pre class=""><div class=""><div class=""><div
class=""><div class=""><div class=""><div class=""><div
class=""><div class=""><div class=""><div
id="m_7901873607475970514code-block-viewer"
dir="ltr" class=""><div class=""><pre class=""><code
class="">Power supply asymmetry (-11.93 V vs. +12.02 V) — likely negligible
Input signal DC component — input is about +117 mV, while U1A shows only -8.33 mV offset
TL074 op-amp offset voltage — too small to explain this magnitude
Unity-gain buffers — not populated - I have jumpers.</code></pre></div></div></div></div></div></div></div></div></div></div></div></pre>
<p class="">The suggested fix is to add four 1 µF
film capacitors, such as WIMA MKS2 parts, on the
SSI2140 OUT1–OUT4 pins, placed between the OUT
pins and the 15k mixing resistors. The idea is
that this would block DC before it enters the
pole-mixing network, preventing it from being
amplified by U1B regardless of filter mode.</p>
<p class="">I’m curious whether adding these
coupling capacitors makes sense, and whether
that would be compatible with the intended use
of the SSI2140. I’m a little surprised that the
data sheet does not discuss the need for them if
this is expected behavior. I did find some
discussion about this on Mod Wiggler, but I
didn’t see a clear conclusion.</p>
<p class="">I’d appreciate any advice, and
hopefully I’ll be able to understand it. Also,
if you see any other design issues or possible
improvements, I’d be grateful for the feedback —
pun intended.</p>
<p class="">Thanks,</p>
<p class="">Jeff</p>
</div>
</div>
<div style="line-break:after-white-space" class=""><br
class="">
<div class="">
<div class="">Sent by voice-transit conductor</div>
</div>
<br class="">
</div>
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<pre class="moz-signature" cols="72">--
David Moylan
Expedition Electronics
sonic adventures!</pre>
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