[sdiy] Bad offset on VCA output

[Naoki Iwakami]

Hi Sergio, Rene, Roman,

Thanks for your advices.  I'll look for any hidden capacitance and double-check feedthrough.  I checked if any oscillation is happening, but it's not observed.  For testing, I also put a 100n capacitor in the feedback loop of the diff amp at the final stage where oscillation may happen, but it didn't change the offset value.  Flux probably is not an issue since washing the PCB did not change the behavior.

Feedthrough may be relevant although I'm not sure what is affecting.  I made two versions of the VCAs.  The first one has an asymmetric design regarding traces and vias around the gain unit. The offset is around 0.6V.  The second one is designed as symmetric as possible around the gain unit, then the offset is around 0.3V.  Both are bad but the second one is a bit better.  This VCA is a typical diff amp type where a transistor pair acts as the gain unit.  The emitters are connected by a cupper trace on the front side; the current is supplied at the middle point from the back side using a via.  Bases are connected to GND through 150 Ohm resistors. The distance of their grounds is about 1.5mm through a plated ground.  No vias for connecting collectors to the diff amp, but the traces have asymmetric routes and lengths.  I put a shunt regulator on the back side.  It bothers me but cannot figure out the theory that it affects the gain unit.

-- Naoki
________________________________
From: Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of Roman Sowa via Synth-diy <synth-diy at synth-diy.org>
Sent: Wednesday, August 27, 2025 6:14 AM
To: synth-diy at synth-diy.org <synth-diy at synth-diy.org>
Subject: Re: [sdiy] Bad offset on VCA output

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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