[sdiy] Bad offset on VCA output

[Naoki Iwakami]

Thanks, Tim, for the input.

Yes, some hidden capacitance seems to be affecting. The offset disappeared
once by putting bypass capacitors, but it was hard to repeat. I still am
not sure if it's oscillation though.  I observe very small high frequency
noise, but the amplitude is about 10mV p-p.  It's not clear if it's
oscillation or external noise.

-- Naoki

On Sun, Aug 31, 2025 at 4:06 AM Tim Stinchcombe <tim102 at timstinchcombe.co.uk>
wrote:

> Hi Naoki,
>
>
>
> This seems an interesting problem, so I whacked the whole lot into my
> simulation program, and after checking that it performed approximately as
> my calculations suggested (at -5V CV, +/-5V input gives o/p swing of
> +/-3.27V, so gain of x0.65; unity gain at approx CV = -7.7V), I then stuck
> a largish cap, 12pF, from the diff amp o/p back to the positive input:
> immediate ~1MHz oscillation (and reducing the max time step in SPICE
> brought this down to about 570kHz, along with much 'smoother' curves). No
> sign of any large offset though, but this could well be down to the 'lack
> of real world' nature of SPICE. That you have discovered by physically
> separating Q3 from the board that the offset disappears, I'm thus going to
> go with the 'something is oscillating' hypothesis as suggested already,
> though perhaps it may not be the op amp.
>
>
>
> I'd say that if it *were* the op amp there would be a fair chance of
> actually being able to *see* the oscillations on an oscilloscope (do you
> have access to one?); if it is one of the *transistors* oscillating, then
> (in my experience at least) it can become *much* harder to work out what
> is going on, as the frequencies involved can be so much higher, and probing
> around the circuit can alter things to the extent that the problem changes
> as you do so, and you can't observe it. (An example I have seen, and which
> I have yet to analyse fully, is a suspected transistor oscillating in an
> exponential converter of a VCO, and initially the only way to 'see' it was
> the effect it had on the '1V/oct' response; later I used a spectrum
> analyser with a 'sniffer' coil of wire, which suggests it is oscillating at
> something like 37MHz.) To this end, running a finger lightly around the PCB
> to identify which part is causing the problem by making things better or
> worse (rather like your bamboo stick), may help, especially if you can rig
> things up so that you can *hear* the difference of 'problem' vs 'no
> problem'.
>
>
>
> Obviously if you cannot identify which part of the circuit is to blame it
> becomes a bit hit-or-miss as to whether re-laying out the PCB will make it
> go away or not. But if you *do* suspect a certain region of the PCB is
> causing problems, it is quite possible to actually *measure* the
> capacitance between tracks, down to just a handful of picofarads, using a
> signal generator that will output a triangle wave (at units to tens of MHz)
> and a scope – I've done so on a few occasions, and it is *very* useful to
> have some hard data to back any theoretical basis for whatever problem you
> have!
>
>
>
> Tim
>
> __________________________________________________________
>
> Tim Stinchcombe
>
>
>
> Cheltenham, Glos, UK
>
> email: tim102 at timstinchcombe.co.uk
>
> www.timstinchcombe.co.uk
>
>
>
> *From:* Synth-diy <synth-diy-bounces at synth-diy.org> *On Behalf Of *Naoki
> Iwakami via Synth-diy
> *Sent:* 27 August 2025 08:29
> *To:* synth-diy at synth-diy.org
> *Subject:* [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/
>
>
>
> Thank you,
>
> -- Naoki
>
>
>
>
>


-- 
岩上直樹
Naoki Iwakami
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