Feedback on VCA...

[Marc Nostromo [M-.-n]]

So, as an exercise, I'm trying to build a vca circuit to hook up to a
Teensy. It's a good exercise for me to get pieces together and start
messing about with OTA's.

My path has been to look at Oliver Gillet's description of the shruti's VCa
from

http://mutable-instruments.net/static/documentation/smr4mkII_analysis.pdf

and move it to different chips (TL072 / CA3080) and single supply mode
(since I want to power it directly from the .

I've finally got some schematics that seems to work and it looks like this:

https://dl.dropboxusercontent.com/u/323652/vca_iteration1.png

There's one thing that is not completely right though:

When I apply a voltage at the gain knob, the circuit only starts amplifying
when the voltage goes above 2.5v. I thought I could cope with this by
adding some offset voltage from the 5v rail (like olivier describes) but it
doesn't seem to add up.

as anyone an idea on the right way to look at this ? I can post the Ltspice
project if that helps.

Other feedback is of course welcome !

Thanks
Marc.

-- 
http://marc-nostromo.com
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