[sdiy] Bad offset on VCA output

[H.Sergio Alves]

most of the times capacitors are to blame but depends on your circuit
design.


On Wed, 27 Aug 2025 at 08:33, Naoki Iwakami via Synth-diy <
synth-diy at synth-diy.org> wrote:

> 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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