Polyfusion VCA building

[Tony Allgood]

Hi Carlos and all,

I thought someone might have answered this one first:

>..I noticed that the first voltage divider at the audio input (100K and
470R) only lets very little audio into the 3080, when this chip is able
to cope with 10V audio signals.Any explanation, please?

The 3080 and other OTAs cannot cope with a differential voltage of over
+/-50mV or so without sizeable waveform distortion occuring. Some OTAs,
the 13700, 6110 and others, offer a diode/transistor linearizing circuit
at the front end to compensate for this non-linearity. This works very
well, except that when you exceed the linearising circuit's limits you
tend to get horrible clipping which sounds unpleasant. The 3080 does not
offer any form of linearising, although you can do this externally if
you wish. Do download the CA3080's datasheet from the Harris or National
Semi website. In there you will find a graph of distortion/non linearity
and differential input voltage.

Do check the DIY archives for comments on suitable values of the input
resistors. Secondly check out Jorgen's VCA shoot out on the Bergfotron
webpages:

http://www.idg.se/personal/bergfors/bergfotron/index.htm

Personally I like the BA6110, although getting hold of this IC in the UK
can be tiresome sometimes. See below for my simple linear VCA.

http://www.techrepairs.freeserve.co.uk/vca.htm

Regards,

Tony Allgood  Penrith, Cumbria, UK

Modular synth circuits, TB303 clone and Filter Rack

http://www.techrepairs.freeserve.co.uk/projects.htm