[sdiy] Using SSM2164 in stereo

[cheater cheater]

But a true virtual current source does not need to be limited, since
it is going to supply a constant (or controlled) current, right?

Don't resistors only limit current from voltage sources? With current
sources, we have U = IR, so that we're 'limiting' voltage. I
understand that the losses in current would be due to the resistor
heating up which is not a very efficient way of limiting current.

D.

On Thu, Jan 21, 2010 at 05:32, David G. Dixon <dixon at interchange.ubc.ca> wrote:
> 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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