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<p>Firstly I'm impressed with your resourcefulness. It was a
pleasure to read your post.</p>
<p>My background is just about the same as yours (EE then FW eng,
retired) so my answer should be taken with some doubt. In theory,
DC blocking caps should have no negative effect if there is no
need for any DC bias. You'll need to make sure the AC impedance
doesn't kill the low end of your desired frequency spectrum. In
other words, calculating the right capacitance is important. The
theory is easy. An experienced EE should be able anticipate any
practical problems. I'm sure there are some.</p>
<p>Good Luck - John Speth</p>
<div class="moz-cite-prefix">On 5/1/2026 8:08 AM, Jeff Whitman via
Synth-diy wrote:<br>
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<p data-start="35" data-end="47">Hello there,</p>
<p data-start="49" data-end="468">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 data-start="470" data-end="738">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 data-start="740" data-end="1171">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 data-start="1173" data-end="1660">I’ve been using <a
href="http://claude.ai/" 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 data-start="1662" data-end="2010">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 data-start="2012" data-end="2068">Here is the table Claude
generated from my measurements:</p>
<pre class="overflow-visible! px-0!" data-start="2070"
data-end="2237"><div class="relative w-full mt-4 mb-1"><div
class="relative"><div class="h-full min-h-0 min-w-0"><div
class="h-full min-h-0 min-w-0"><div
class="border border-token-border-light border-radius-3xl corner-superellipse/1.1 rounded-3xl"><div
class="h-full w-full border-radius-3xl bg-token-bg-elevated-secondary corner-superellipse/1.1 overflow-clip rounded-3xl lxnfua_clipPathFallback"><div
class="relative"><div class="pe-11 pt-3"><div
class="relative z-0 flex max-w-full"><div id="code-block-viewer"
dir="ltr"
class="q9tKkq_viewer cm-editor z-10 light:cm-light dark:cm-light flex h-full w-full flex-col items-stretch ͼ5 ͼj"><div
class="cm-scroller"><pre class="cm-content q9tKkq_readonly m-0"><code>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 data-start="2239" data-end="2427">Claude’s current
explanation is that the increasing weighting factors in the
pole-mixing network are directly amplifying the DC component.</p>
<p data-start="2429" data-end="2471">Things that appear to have
been ruled out:</p>
<pre class="overflow-visible! px-0!" data-start="2473"
data-end="2741"><div class="relative w-full mt-4 mb-1"><div
class="relative"><div class="h-full min-h-0 min-w-0"><div
class="h-full min-h-0 min-w-0"><div
class="border border-token-border-light border-radius-3xl corner-superellipse/1.1 rounded-3xl"><div
class="h-full w-full border-radius-3xl bg-token-bg-elevated-secondary corner-superellipse/1.1 overflow-clip rounded-3xl lxnfua_clipPathFallback"><div
class="relative"><div class="pe-11 pt-3"><div
class="relative z-0 flex max-w-full"><div id="code-block-viewer"
dir="ltr"
class="q9tKkq_viewer cm-editor z-10 light:cm-light dark:cm-light flex h-full w-full flex-col items-stretch ͼ5 ͼj"><div
class="cm-scroller"><pre class="cm-content q9tKkq_readonly m-0"><code>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 data-start="2743" data-end="3059">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 data-start="3061" data-end="3400">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 data-start="3402" data-end="3589">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 data-start="3591" data-end="3813">Thanks,</p>
<p data-start="3815" data-end="3836" data-is-last-node=""
data-is-only-node="">Jeff</p>
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<div>Sent by voice-transit conductor</div>
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