[sdiy] Bad offset on VCA output

[Magnus Danielson]

Hi Naoki,


Den 2025-08-27 kl. 09:28, skrev Naoki Iwakami via Synth-diy:
> 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/>

In my experience, any diff-pair OTA like this needs triming, because 
reality does not give us very well-matches components and also one needs 
to be quite protective about diff-pair traces and keep them away from 
other sources, potentially use guard-rings.


Temperature will be a factor, so just heating/cooling becomes an issue. 
The BCM847 is a pair of transistors in one package, this helps to 
thermally connect them, but for precision work like the MAT series of 
transistors, you actually have 4 transistors in parallel for each, and 
they interconnect such that temperature gradients cancel first degree. 
This is why such transistors is prefered in expo-converters, but it is 
really just he same diff-pair or "long tailed pair" as you do an VCA.


A challenge you also get is a hidden one, the input audio and output 
audio both convey DC and thus DC offsets is not blocked. This put 
requirements on the design, and the electrolytic caps is the easy way to 
cheap around from trimming DC, but here you try to avoid both and that 
is a higher challenge.


I don't think you can avoid trimmers, not with these components at 
least. You really have to choose to either have trimmers or DC-blocking 
caps in the audio path, you really can't have both unless you spend more 
on the components, and even then you would improve with trimmers.


Cheers,
Magnus

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