[sdiy] 3280 VCA question

[Jim Patchell]

    That is just a very simple linear voltage to current converter.  When the input
voltage is at ground (the input side is the one with the emmiter), no current will
flow.  There will be just a little bit of dead band at zero, because the voltage will
have to come up to a certain point before current will start to flow.  This is a very
handy and simple converter to use, although it is of lower performance than some (but
in this case, it is probably a very good choice, a few millivolts of offset voltage
will not make the VCA come on, so it will have maximum attenuation when you want
it.).

    -Jim

Kenneth Martinez wrote:

> I was breadboarding a 3280 VCA, copied from the final voice VCA of the
> Prophet-5...see this link, top right of the page:
>
> http://aupe.phys.andrews.edu/diy_archive/manuals/sequential/prophet5/p5_03_17.gif
>
> I've got it working, but I don't understand the function of Q410 in controlling
> Iabc - what does it do, since its base is connected to ground?  How does this
> resistor-transistor-resistor setup differ from just connecting a single
> resistor?