Because it doesn't know the words, of course! (boom boom)  But seriously, several things can cause a system to hum, in this article though, I shall address the eternal mystery of Ground-loops (AKA Earth-Loops). These are the major cause of hum (and other problems) in Hi-Fi systems, and probably the most common and widely mis-understood of faults.

Many audiophiles have tried curing hum by adding extra earth wires - sometimes tying them to the cold-water pipe - sometimes even driving copper stakes into the dirt... but in reality the cause is usually TOO MANY earth connections, not too few!

Usually, this comes from the fact that, in this country, the regulation mains-plug has 3 pins, Active, Neutral and EARTH. With respect to any single device, this is a good thing, because it adds an extra element of electrical safety.  

But when you connect several audio components together, they form a SYSTEM and it is important for this system to have a logical SYSTEM GROUND -- otherwise, it will just have a mish-mosh of un-planned earth connections, depending on your particular mix of components and their respective power cables.

Once the simple "Ground Rules" are understood, you will be able to banish ground-loops and their nasty effects from your system forever!  Once again, let's go back to the basics.

We like to think it's music going through our 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 audio 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 ultimately connected to the component's metal chassis.

As mentioned above, the connection between your CD player and your amp is a circuit because electric currents always follow a circular path. This electric current (the analogue of your music signal) travels down the centre of the cable and returns via the outer shield... or does it?

Electricity ALWAYS follows the line of least resistance -- and we know that the shield which 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 noise from the mains, resulting in the sound that we know as HUM.

But hum is not the only potential problem here... remember, the Earth pin of the power plug, is also connected via the house-wiring to your TV, the fridge, the microwave, the computer, the power pole in the street, the house next door, the butcher, the baker... you get the drift.  In theory, in "a perfect world", the Ground is meant to be SILENT... but guess what?... it ain't a perfect world!

A ground-loop can cause your system to pick up all manner of electrical garbage - and in extreme cases, it can even result in instability and catastrophic failure.

So... What's the solution?
Consider the diagram on the right, and notice how it resembles a tree with a root, a trunk and some branches.  

If your system ground is structured in this way, ground-loops are eliminated.  All units are grounded to the pre-amp via their interconnects, and ONLY the pre-amp connects to mains Earth, preferrably via the best power cable you have on hand.

Alternatively, this arrangement will work if you have a power amp with heavy mains cable -- you can use it as the 'trunk' of the tree.

Remember, this is a map of our system ground only, not the power connections or the signal path.

The general tree-shaped map still applys, regardless of your particular setup.

Removing any additional ground wires forces the signal currents to do what they should
i.e. to return via the correct path, the interconnect shields...

BUT

Before you take a hacksaw to the Earth pins on your mains cables...
make sure you know what you're doing

First, be aware that ground-loops are only responsible for hum about 95% if the time.  If the hum you are hearing is coming from inside a faulty component, no amount of mucking about with cables is going to cure it.  Hum can also mean you have a faulty cable somewhere.  So you should first try a couple of tests.

"The Hard Way" (recommended if all else fails)
To test where the hum is coming from, you can try disconnecting EVERYTHING except the power amp and speakers, and then switch on the amp and listen closely to the speakers.  If it has NOTHING ELSE connected to it, the amp should not hum... if it does, then the amp is faulty, pure and simple (so call me, I'll fix it, OK?).

If there's no hum, switch off, connect the pre-amp ONLY to the power amp and once again, switch on and listen to the speakers.  Keep repeating the above procedure, each time adding another component until you find the one that makes the system hum.

"The Easy Way" (try this first!)
You will need a very sophisticated piece of equipment - and you have to make it yourself!

Grab a common old double adapter (the pyramid-shaped ones work best) and a screwdriver.  Undo the screw that holds it together and pull the entire earth-pin assembly out and then put it back together.

Throw the earth thingy into the drawer of bits you're keeping in case they come in handy some day.

Using a CD marker (or similar), write "FLOATER" (or any scary message that'll stop your spouse using it on the kettle and the toaster) on the side.

Keep away from children.

Armed with your Floater and your new 'system ground tree', seek and destroy all ground-loops.  Start by isolating the source components.

If you're hearing hum in the silences between tracks when playing a CD, start by isolating the CD's power plug using the floater.

Always make sure that everything is powered off before unplugging anything or making any changed to the wiring.

Don't forget that your RCA cables must now earth the floated components, so make sure they make good contact. If your interconnects aren't up to the job, then you'll either need to upgrade your cables... OR

If your cables are high-end but the plugs are just worn out and sloppy, talk to us about having your cables refurbished with some high quality plugs, like these gorgeous Furutech rhodium RCAs. The spring loaded centre pins and locking earth rings guarantee best possible connection and rhodium is even better than gold.

That annoying hum will soon be just a bad memory and you won't believe how good the music sounds against a black background of silence!