The most troublesome problems encountered in sound reinforcement are unwanted hums and buzzes. On hearing this, engineers will often give a knowing look and mutter under their breath about 'ground loops'. In fact, they are usually wrong in making this diagnosis, as a ground loop is but one of several causes of noisy audio.

Fig 1 is a diagram of a ground loop condition, which arises when two (or more) pieces of equipment are connected together but also have a link through the 'ground' connection of the electrical supply. You will be familiar with this connection as the 'Earth' connection in a three-pin mains plug. In the 110V (in the US) or 240V (in the UK) mains supply, there is a connection made for safety purposes to earth or ground.
This is at a very low voltage (not necessarily 0Volts) so that if a fault develops, current has a path to flow. Now if it happens that the two pieces of equipment are connected to two earth or ground points that are at different voltages or potentials but still have a physical connection, for example, one 'ground' may be a large metal pipe going into the earth and the other may be directly into the earth, then a current will flow and a complete circuit, or 'loop' is created as shown in Fig 1.

The important thing to remember is that a current will only flow if the two grounds have different potentials of Voltages. If both are connected to the same ground then there would be no differences in the voltages and so no current flow.

You will notice from the diagram that the current flows through the screen of the audio cable that links the two units, and from there passes to the earth connections. The current never gets to the audio signals that are passed via the +ve and -ve conductors in the cable between the audio electronics of both units. Thus in this example, although there is a 'ground loop' condition, it would not be heard in the audio signal.

This brings us to the most important statement in this discussion -

Most hums and buzzes in audio systems are preventable and are caused by poor design of systems and equipment.

Thus, in the following pages of this 'How to…' guide, I shall be outlining ways to implement good system design to minimize the effects of noise currents, whether they arise from ground loops or any other source.