Hi Roel,
First I went back and checked your original post - I note you bought
your system about 2 years ago. As far as I'm aware, the new version
of the A-131 came out in the autumn of 2001 - thus I suspect yours
*is* one of the originals. This would use the PA382 chip , the
exponential version of the PA381, which is linear and used in the A-
130. New A-131's use a different chip - a CA3080. Thus most of what I
said before doesn't apply, and the fact that you have confirmed the
outputs sound roughly the same at full gain ties in with what Florian
said.

The VCAs don't really 'amplify' - they give you voltage controlled
gain, which at its *maximum* is about 'times 1'. This should explain
why your patch sounds louder directly to the 138, rather than through
the 130/131: generally the output of a 130/(old)131 is less than the
input!

As for the difference between lin and exp, this is not so easy to
explain in words! For the linear VCA, the gain you get is directly
proportional to the gain knob setting (or the CV in) - equal
increments in the gain setting change the gain by the same
increments. for the exp VCA, starting at gain = 0, large changes of
knob setting give small changes in gain, then gradual you work up to
where smaller changes of knob setting give bigger changes of gain. If
you can think of an 'x squared' curve you'll get the idea (it's not
quite right, but good enough to give the idea). The way the circuitry
works out between the two, it appears the exp curve is virtually
always 'below' the linear one, and comes up to meet it near the max
gain setting. This is not the case for the newer A131, where the exp
curve finally overtakes the linear one, hence why it is louder! (I'll
check the PCB of my A-130 to see how the values changed for the old A-
131, and see if I can do some calcs to back all this up...)

After thought: if you have the manual check out pikkies on page 3 of
the A130/131 entry!!!!!

The difference between the 138's is just the potentiometers, being
either 'linear' or 'logarithmic' (which is just the inverse of
exponential - again if you have the manual, in the figures, 'mirror'
the curve line to be the other side of the straight one). Basically
you get a different 'feel' between how much the knob turns for the
effect on the signal. Because of the way the ear hears, things
following the a log or exp relationship should sound more natural
than a linear one.

As for more VCAs, some people find the A132 dual VCA to be very
handy - check out some of the past discussions on this!

Tim

--- In Doepfer_a100@yahoogroups.com , "Roel Steverink"
<r.steverink@w...> wrote:

> Hi Ethan, Tim, Florian and Bakis,
>
> Thanks for all this great info!
> I have been experimenting with the options you gave me.
> I noticed some strange things.
> First I patched the A-110 into the in of the A-131 (Exp.)
> and turning up the gain, nothing was heard until I
> reached position 6. I repeated the same experiment with
> the A-130 (Lin.) I also began to turn up the gain and
> immediately at position 2 I began to hear the continious
> drone.
> So there's the big difference which must be due to fact
> that one is the exp. and the other the linear VCA.
> Now, I'm curious at which position will they be equal in
> volume. Can it be before they reach full gain
> Nop, your right Florian both are equal in volume only
> when they reached full gain (=10).
> So I happily conclude that there's nothing wrong with
> my VCA's!
> Before I forget the both in's were at position 8. And the
> general out also on position 8.
> But when I turn both in's (from the two VCA's) to 10,
> I immediately hear that the linear gives more power!
>
> I also have to say that I directly plugged the A-100 out to
> my mixer in this case.
>
> In the next example I patched 3 waveforms (pulse, triangle and
> sinus) into the A-138 mixer, the out of the mixer goes to
> the audio in of the A-121 Multimode filter. Filter low out
> first into audio in of A-131 and the A-130 (settings of the
> VCA's are the same as above). There's barely any change.
> Same picture the linear gives a slight more power then the
> exponential.
>
> Now, when I plug the low out of the A-121 directly in the
> input of the A-138 mixer (lin) the volume is cranked up
> several levels. How come Using this mixer means a more beefed
> up sound then using one of the VCA's
> I believe this picture will be the same if one uses the
> A-138 mixer (exp)! Or am I wrong here
>
> Can anyone tell me what's the difference between
> the A-138 exp and lin. mixer
> The same goes for both the exp and linear VCA's.
> I have to expand with more VCA's, cause 2 are to little.
> But which one should I take
> Any ideas!
>
> Thanks,
>
> Roel.