[sdiy] Bad offset on VCA output

[Roman Sowa]

Isn't it simply oscillating at high frequency, and you see that detected 
as DC offset? Hook p an oscilloscope to every spot and see if it really 
does.
Also notice if the offset goes down to 0 the moment you touch the probe 
to certain points.

Roman

W dniu 2025-08-27 o 13:34, synth at schmitzbits.de pisze:
> 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 <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/ <https://gaje.jp/2025/08/26/7810/>
> 
> Thank you,
> -- Naoki
> 
> 
> 
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