[sdiy] Bad offset on VCA output

[Tim Stinchcombe]

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

 

 

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