Phil Allison wrote:

"Mike Rivers".

Common mode rejection is the ability to reject noise that's common to
the two wires of a balanced input. An example is RF interference
that's picked up by both wires equally.

** That not a correct example. RF interference is defeated firstly by
the use of shielded cable and secondly by the use of filters to reduce such
signals at the inputs of the balanced pre-amp.

Common mode rejection operates across the audio band and maybe a little
beyond but is usually most effective at the lower frequencies since the main
aim is to eliminate ground hum from audio systems. A ground hum voltage will
appear equally on the two wires and so be rejected.

So why is it you think that RF can't exist at audible frequencies?
Even if it's caused by A/C current and at 50 or 60 Hz, it's still
radio noise picked up by the wires because they're acting as antennas,
right?

- Logan