Version 3 of the A-131 is based on the THAT2150 and the exponential scale is
predefined by the THAT2150. The module is a plain standard application of
the THAT2150 and there is nothing special.
The trimming potentiometer "GAIN OFFSET/P6" is used to define the max.
amplification (of amplification offset). In the factory it's adjusted so
that with Gain control fully CW (and without external CV) the
"amplification" is about 1.
Another trimming potentiometer "OUTPUT OFFSET/P7" is used to minimize the CV
feedthrough of the VCA.
The control scale of the module is defined by the control scale of the
THAT2150. I can send you the THAT2150 data sheet and schematics if you tell
me your email address.

I suppose that your "problem" has to do with the exponential scale. As a
rule it's always a problem with exponential VCAs because an exponential VCA
does theoretically never reach zero amplification (but in reality the signal
disappears eventually in the noise floor). On has to find a compromise
between the exponential scale and the minimum amplification (i.e. the
amplification with manual GAIN control fully CCW):

If the exponential scale is too "flat" the amplification with GAIN fully CCW
is not small enough (i.e. there is still some feedthrough @Gain=0).

If the amplification with manual GAIN control fully CCW is very small
(e.g. -80dB or 1/10000 with Gain=0) so that no audio feedthrough is
noticeable it takes a while until the audio level becomes audible because at
center position (5) of the gain knob it's still about -40dB (or 1/100), and
even at 75% of the rotating angle (about pos. 7-8) it's still -20dB (or
1/10). In the range 8...10 then the level increases suddenly from 1/10
(-20dB) to 1 (0dB).

Hope this helps to understand the exponential VCA
Dieter Doepfer

> -----Ursprüngliche Nachricht-----
> Von: Doepfer_a100@yahoogroups.com [mailto: Doepfer_a100@yahoogroups.com ]
> Gesendet: Donnerstag, 21. Februar 2019 13:08
> An: Doepfer_a100@yahoogroups.com
> Betreff: 1 Re: Doepfer A-131 exponential VCA - calibration
> procedure
>
>
> I have a degree in maths, and I studied electronic engineering
> for a while. I also own more synthesizers than I probably should
> - so I understand the concepts involved.
> What I mean is, the response curve of this one, for gain vs CV,
> is far from desirable. I get a very gradual increase, then a
> very sudden one. I haven't attempted to graph it.
> I have seen that website page before, and read the manual, but I
> haven't seen anything official about adjusting or calibrating
> one. I have read that there are preset pots to adjust, but only
> vague details.
> It seems the one I have is version 3, and there are only two
> presets / trim pots, labelled Gain Offset, and Output Offset. Is
> there anything to be gained by adjusting those
> I own multimeters, basic oscilloscopes, soldering irons, and a
> large heap of resistors, capacitors and various other components.
> I am quite prepared to modify it, if necessary. Is there a
> schematic available
>
> - Andy
>