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A-132 Dual VCA changing DC to AC coupling???
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author A-132 Dual VCA changing DC to AC coupling???
bryan
So like the title says, I'd like to change the coupling on the A-132 VCA from DC to AC. Has anyone done this before, is it worth it and/or possible? I dunno how to go about this. THanks much!
Sound
You just need add a capacitor. That the signal pass by this capacitor.
See the atached picture. It is easy.
You can do it in the Input or in the Output, maybe have more sense in the output.
SO it is very simple: Cut the trace in red (input or output) and connect the capacitor as marked in blue.

I have seen very different values in different schematics.
In the outputs since 2,2μf, 10μf and 47μf. Also the capacitor must fit there.

Regards
bryan
Just to make sure that changing the coupling is the solution...

Unmodded, if I use an AC source in the CV input The VCA only responds to the positive side of the AC, and the negative component of the modulating wave results in silence.

What I want is for the full cycle of AC waveforms to affect the VCA, with the moment of silence only occurring at the very bottom of the negative part of the wave.

Will the mod posted above do the trick? (thanks Sound, for the photo!)
urbanscallywag
AC coupling won't achieve what you're after. what you want to do is add DC offset to your LFO signal. try taking a look at something like:

http://www.analoguehaven.com/doepfer/a138c/

"control in1 works as a dc offset generator (about -5v...+5v) provided that no patch cord is plugged into socket in1."
Sound
Ah, I didnt understand what you wanted, yes in this case I think you need to add offset, as said Urban .

Other module is the attenuaverter of fonitronik http://www.modular.fonik.de/Page40.html#Link276. All the chanels have an offset if are not pluged. +7.5V to -7.5V

By the way, I reversed enginered that A-132 for fun, in order to found a litlle modification that add an permanent offset in the input and divide by 2 the amplitude. In order that an incoming signal 5V to -5V be converdted in a signal 0V to 5V. Its in the breadboard lets see if i achieve something.
grin
Regards.
bryan
Thanks guys, I actually have the A-138C and forgot about the offset, which I had disabled. And come to think of it, I could also use my little gate/trigger box for an offset voltage source as well.

Sound, a problem with the A-132 VCA is that it distorts with too much CV input. I got around that by adding potentiometers onto patch cables to attenuate signals. Permanent offset would be nice but I'm guessing you might still need to attenuate the CV in.

Yup, I like my A-132 VCA's because they save so much space but lack of attenuators on that module (and so many others) is a big problem.
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