Doepfer

Hi Roel, Florian,

Confusion abounds! Unfortunately I think Roel got the 130 and 131 
round the wrong way in the second half of his post, the bit snipped 
here...

> > sound as loud. I can tell you the A-131 is sometimes
> > bearely audible, only when I turn the gain to 10, it
> > sounds better, but doesn't come close to the volume of
> > the A-130.
> If you plug a VCO direct to the Audio in of both VCAs and you turn 
the Gain
> on both to 10. You should hear a continous sound now. Has this 
sound the
> same level on both VCAs?
> If yes, everything is ok.
>
> The rest shows you what exponential really means: position 9 on the 
A131 is
> equivalent to position 5 on the A130. Position 8 on the A131 is 
equivalent
> to position 2.5 at the A130 and so on...

... and what Florian suggests falls foul of his own logic, i.e. 
cranking both gains up and expecting to hear roughly the same level. 
Because of the difference in the lin/exp relationship this doesn't 
happen. I have just tried a few quick measurements: A-110 square out 
to both 'In1' of 130 and 131, input attenuation at full, gain at 
full, output at full. The 131 sounds considerably louder than the 
130. The output of the 131 is about +/-8V, whilst the 130 is about +/-
3V. To get the 131 down to the same level as the 130, I needed to 
turn its gain down to about 7. (Using 'In2' on the 131 makes the 
difference even more noticeable, due to the increased sensitivity I 
mentioned before - at full gain it's about +/- 9V, and to get the 
same as the 130, I needed to turn the gain further down, to about 
6.5.)

To be fair, this all really depends on how you look at it. For the 
same value of CV in, whether you need to increase or decrease the 
gain of one versus the other in order to get the same level out, 
depends on the value of the CV. Because of the difference in the 
linear/exponential relationship, they 'cross over' at some point. At 
low CV and gain settings, the linear VCA, 130, should appear louder; 
at higher settings the expo VCA, 131, should win out.

(For any who may not have seen it, there was quite a lot of previous 
discussion about the VCAs, including the 132, leading up to message 
1567, including how to prevent the 131 'chopping off' the top of an A-
140's envelope.)

> BTW: The (service-?)manual explains, that the CEM chip on the 
A130/131
> contains two VCAs; only one of them is used. There is an multipin 
connector
> on the pcb to connect the input and controlling electronics to the 
second
> VCA. Does anyone here use this option?

Well, hadn't taken much notice of this, but so there is! Hence I've 
not tried hooking up to it - you would have to replicate the audio 
input stage (a pot, op amp, couple of caps/resistors), but there 
appears to be a provision on the board (linking 'JOUT' to pin 1) to 
sum the output of both VCAs in the 381/2 to the same output stage, 
and also a to make a simple connection ('JCV') to tie the CV into the 
second VCA.

Tim