[sdiy] Equation question

[David G. Dixon]

> So - for an op amp that has V+ supplied by +12 Volts and V- supplied by -
> 12 Volts - would an equation that refers to 'a third of the supply
> Voltage' be talking about 12 or 24 Volts?

That totally depends on the context, but presuming that the opamp is
processing a signal of some kind, it probably means both!  In other words,
the signal will extend upwards to and amplitude of +V/3, and downwards to an
amplitude of -V/3, and the peak-to-peak signal amplitude will be 2V/3.
 
> This is the first time I'll be using an op-amp under dual supply

!!!!!!!!!!..........(gobsmacked, drooling, in shock).........???????????

I've never used an opamp any other way!  In fact, in my (admittedly rather
limited) experience with synth diy, I've never even seen a single-supply
opamp specified in a schematic. (I know: I live such a sheltered life!)

> I could
> just plug it all in and see what happens (which is what I normally do...)
> but I thought it might be interesting to try and calculate/predict some
> results.

It's actually pretty hard to fry an opamp, short of actually switching the
power pins, so you are probably safe there.  However, you really should be
calculating these things.  Provided you don't care about biases, offsets,
bandwidth, etc. (which you hardly ever have to), opamps are the easiest
things in the world to predict.  Two simple rules:

1.  The input pins admit no current.
2.  The input pins share the same voltage.

Everything else is Ohm and Kirchhoff.  As long as the output voltage doesn't
get within two diode drops (about 1.4V) of the supply voltage (+ or -),
these rules are generally rock-solid.

If you need some assistance with this sort of thing, please feel free to
email me the schematic offline, and I'll help you make sense of it.