Feedback on VCA...

[Tony Hardie-Bick]

really quick reply, i think the vca is current controlled, and is a pn 
junction,
so it's resistance will vary, could be the cause :)

On 07/03/14 13:43, Marc Nostromo [M-.-n] wrote:
>
> 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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