[sdiy] Bad offset on VCA output

[synth at schmitzbits.de]

HI Naoki,

I built similar VCAs albeit with trimming. From memory, I both corrected for the symmetry (= minimal feedthrough) and also residual offset separately. How bad is the feedthrough with your trimmless design?

Could your issue be caused by flux residue? Maybe wash the whole board in iso propyl alcohol.

Best,
 René

--
synth at schmitzbits.de
http://schmitzbits.de
________________________________
Von: Synth-diy <synth-diy-bounces at synth-diy.org> im Auftrag von Naoki Iwakami via Synth-diy <synth-diy at synth-diy.org>
Gesendet: Mittwoch, 27. August 2025 09:28
An: synth-diy at synth-diy.org <synth-diy at synth-diy.org>
Betreff: [sdiy] Bad offset on VCA output

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.

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.).

I posted a blog article about this issue https://gaje.jp/2025/08/26/7810/

Thank you,
-- Naoki


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