[sdiy] Bad offset on VCA output

[Naoki Iwakami]

Hi Magnus, thanks for the reply

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.

This might be the issue. I removed a part of the plated ground from the
PCB. The offset changed then. Strictly symmetric routing may be necessary.

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.
>
BCM847 are matched pair transistors.  I first tried BC847DW1 that are
not-particularly-matched pair transistors and picked up mached ones.  Only
1 out of 10 was usable.  BCM847 are reasonably matched and I can pick up
better-matched ones if necessary.  It's a kind of trimming though :-)

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.

Actually, I'm planning to use this for CVs, too.  So DC capability is a
must.  This VCA is meant to be used in a larger circuit. When a
coupling capacitor is needed, I'll put it externally. I can put a trimmer
terminal as a last resort, but will try trimless a little more.  I'm pretty
sure it's doable if the PCB is larger.

-- Naoki

On Tue, Sep 2, 2025 at 3:13 PM Magnus Danielson <magnus at rubidium.se> wrote:

> 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/
>
> 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
>
>

-- 
岩上直樹
Naoki Iwakami
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