[sdiy] Bad offset on VCA output

[Naoki Iwakami]

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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