[sdiy] Using SSM2164 in stereo

[David G. Dixon]

I'd like to retract my last posting on this topic.  Unfortunately, I suffer
from a rare disorder known as "Homer's Syndrome" whereby, occasionally, a
small piece of jelly donut lodges itself in my carotid artery, temporarily
suspending the flow of blood to my brain.  Unfortunately, it seems to have
happened again this afternoon.  Of course the 2164 can take currents
directly from current sources!  It sinks any excess current to the rails.

I stand by my first post on the topic.  I think the 2164s can be driven just
fine directly from the current source, with no intervening resistors.  In my
experience with the 2164 Expo VCO, the 2164 input is basically a summing
node which is held at virtual ground, just like the - input of an opamp,
except that it admits current.  Then again, input resistors wouldn't hurt,
especially as current limiters, provided that the current source is a true
(Norton) current source, and therefore insensitive (within limits) to the
downstream voltage.  I just don't think it matters.  Personally, I'd leave
them out and use just the one stabilizing RC network.



> Upon reflection, I don't think it is possible to drive a 2164 VCA with a
> current source without frying it.  Consider: When you convert a voltage
> into
> current through a resistor, the current is not predetermined, since the
> voltage source (in the Thevenin sense) will source or sink as much or as
> little current as is required to maintain its output voltage.  This means
> that the VCA inputs can sink or source as much or as little current as
> they
> want.
> 
> Charge (unlike potential) is conserved.  The current must go somewhere.
> Remember: When feeding a fixed current to an opamp (as with a I-V
> converter
> or an integrator) the current flows around the opamp, through the feedback
> loop.  There is no feedback loop around the VCA.  The current has to go
> through the VCA, but the VCA has its own ideas about how much current to
> pass.
> 
> Bottom line:  2164 VCAs must be driven with voltage sources which are
> subsequently converted into currents with I-V resistors.  Hence, each
> input
> requires its own stabilizing RC network.
> 
> 
> 
> > > 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.
> 
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