What's the real story on balanced lines?  Are they really so much better than 'normal' unbalanced ones?  Balanced lines were originally only seen in professional audio systems such as used in recording studios.  They were hardly ever seen in Hi-Fi systems except for a few high-end units which were built to professional standards.  

Over the last decade or so, balanced lines have become more common in domestic Hi-Fi with many components now offering balanced input/output options.

To properly appreciate the advantages of balanced lines, it is helpful to understand a little about how they work.

We like to think it's music going through the wires, but it's actually just an electric current flowing in a circuit between say, your CD player and your amp.  It's called a circuit because electric currents always follow a circular path (logically, not physically).  The electric current in this circuit is the 'analogue' of the music signal it represents.

The standard for unbalanced connections in Hi-Fi is the RCA plug.  With the exception of a small handful of European brands, just about everything connects with these.  The centre pin carrys the music signal which travels down the centre of the cable.  The outer ring completes the circuit by providing the return path, via the 'shield' which usually takes the form of a wire braid wrapped around the signal wire.

In unbalanced circuits, the return path is always the unit's 'Ground' - which means that the outer ring is always connected to the metal chassis.

This is called "Unbalanced" because there is only one signal line which is referred to the Ground which (in a perfect world) represents 'zero volts' or silence.  The significance of this will become much clearer when you compare it with the Balanced circuit (below).

The signal can be represented graphically like this...

The line through the middle represents zero volts, which is the same potential as the RCA plug's outer ring and the Ground (or chassis) of the equipment.  The wave shape represents a sound-wave as positive and negative motion of electrical current.

The same signal in a balanced circuit looks like this...

The signal is not referred to the chassis, instead, it is referred to its own mirror-image.  The music signal is carried by the difference between the two identical, but opposite electric currents... as opposed to the difference between one current and the unit's chassis.

Obviously, this 'balancing act' will require an extra signal line & therefore a different plug.

The standard connector for balanced lines is the XLR (sometimes called a "Canon plug") which has two signal pins plus a shield pin.

Unfortunately, XLRs are quite a bit bigger than RCAs but they're rugged, reliable and relatively inexpensive.

So, why does this extra line make such a difference?  Well, there are several reasons...

For one, the signal does not 'sit' on the Ground as it does in an unbalanced circuit -- remember, the Ground is also connected to the unit's chassis, which is also connected to the Earth pin of the power plug, so it's also connected by the house-wiring to your TV, the fridge, the microwave, the computer, the house next door etc. etc.  

As mentioned above, "in a perfect world", the Ground is meant to represent SILENCE... but the problem is... it ain't a perfect world!

But the really BIG bonus is a thing called "Common Mode Rejection" - do read on!

Common Mode Rejection is all about how the circuit behaves in the presence of unwanted signals.  Unwanted signals?  Consider this: Every wire is also an antenna for interference and no shield is ever 100% effective.

Remember, The music signal is carried by the difference between the two identical, but opposite lines... which implys that: if there is no difference, there is no signal.

Now, if one line sees some interference, the other one will too...  The graphic on the left shows what happens when a balanced circuit picks up a 'spike' of interference.  As there is no difference in the spikes, they have virtually no effect on the signal.

Another huge bonus that comes from the balanced connection is the abscence of ground-loops.  These are the major cause of hum in Hi-Fi systems, and probably the most widely mis-understood and common of problems.

As mentioned above, the connection between say, your CD player and your amp is a circuit because electric currents always follow a circular path.

The music signal which travels down the centre of the cable and the outer shield completes the circuit by providing the return path... or does it?

Electricity ALWAYS follows the line of least resistance -- and we know that the shield that is supposed to be the return-path for the signal is also connected to the chassis and the chassis is connected to the Earth pin of the mains plug.

So... what happens if the mains cable has less resistance than the shield of your RCA cable?  Well, it becomes part of the signal path - that's what!

The signal current returns, not via the shield, as it should - but via the mains cables which are connected together by your power-board - and in the process, it picks up interference from the mains, resulting in the noise that we know as HUM.

Because the balanced connection does not rely on the shield to return the signal current, ground-loops are not a problem.