[sdiy] Using SSM2164 in stereo

[Neil Johnson]

Hi Tom,

> I'm designing a dual stereo VCA based on this chip.

...

> The VCA will be fed by a current output, so I don't have the 30K  
> input resistor that the datasheet also shows.

I would be more concerned about this aspect of the design - this is only guaranteed to work iff both inputs have *EXACTLY* the same input characteristics so that the input current is perfectly split 50:50 between the two inputs.  Trouble is, you can't rely on this - it may change with temperature, with control voltage, with age, etc.  The result would be audio panning all over the place out of your control.

Adding the V-to-I resistors fixes this problem.  Then you add the little RC compensation network for stability.

Note: SPICE won't show this, unless you have a _very_ good model of the input, and run Monte-Carlo over several parameters.

Cheers,
Neil
--
http://www.njohnson.co.uk