WHY DO MIC. LEADS HAVE THREE PINS?
A normal 2-conductor line (from a cd player or an electric
guitar) is usually referred to as unbalanced. This is where
the negative is “Tied to Ground” or “Earthed”. The reason for
this is so any extraneous signals or interference, drain to
earth. To achieve this the negative is usually a braided
shield wire surrounding the positive wire so that any
interference lays itself on the negative only and is not able
to reach the positive wire.
A 3-conductor Mic lead is a balanced line. this is where the
negative is the exact opposite of the positive because neither
are tied to ground and both come from opposite sides of the
The main advantage is that any interference that manages to
get past the shield (which is still connected to earth) would
affect both the positive and negative wires to the same degree
and hence (because the two are opposite) cancel out.