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<p>Hi Naoki,</p>
<p><br>
</p>
<div class="moz-cite-prefix">Den 2025-08-27 kl. 09:28, skrev Naoki
Iwakami via Synth-diy:<br>
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I'm developing a small trimless VCA of size 7/8" x 3/8", but
suffering from bad offset on output such as 0.4V for zero input
with 5V CV. I tried the same circuit on a breadboard and 3" x
2" universal PCB. Both worked fine (no bad offsets). I
suspected crowded PCB layout did some harm so let the gain unit
transistor pair hang in the air to take some distance from the
PCB — the problem disappeared then. I will redesign the PCB to
eliminate this issue but cannot figure out what to move to take
distance from the transistor pair.</div>
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<br>
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Does anyone have similar experience, such as cupper beneath a
semiconductor changes the behavior, transistors and opamps work
correctly only when they keep certain distances, and so on? I'm
using relatively small SMD components for this project (SOT-363,
TSOT23-8, 0603, etc.).</div>
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I posted a blog article about this issue <a
class="OWAAutoLink moz-txt-link-freetext" id="LPlnk909965"
href="https://gaje.jp/2025/08/26/7810/" moz-do-not-send="true">
https://gaje.jp/2025/08/26/7810/</a></div>
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<p>In my experience, any diff-pair OTA like this needs triming,
because reality does not give us very well-matches components and
also one needs to be quite protective about diff-pair traces and
keep them away from other sources, potentially use guard-rings.</p>
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<p>Temperature will be a factor, so just heating/cooling becomes an
issue. The BCM847 is a pair of transistors in one package, this
helps to thermally connect them, but for precision work like the
MAT series of transistors, you actually have 4 transistors in
parallel for each, and they interconnect such that temperature
gradients cancel first degree. This is why such transistors is
prefered in expo-converters, but it is really just he same
diff-pair or "long tailed pair" as you do an VCA.</p>
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<p>A challenge you also get is a hidden one, the input audio and
output audio both convey DC and thus DC offsets is not blocked.
This put requirements on the design, and the electrolytic caps is
the easy way to cheap around from trimming DC, but here you try to
avoid both and that is a higher challenge.</p>
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<p>I don't think you can avoid trimmers, not with these components
at least. You really have to choose to either have trimmers or
DC-blocking caps in the audio path, you really can't have both
unless you spend more on the components, and even then you would
improve with trimmers.</p>
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<p>Cheers,<br>
Magnus<br>
</p>
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