Wiring a Studio: THE GROUND RULES

5.  Ground Loops

5.1  What is a Ground Loop?

A ground loop is created when an item of equipment is connected to ground by more than one path.

This situation can occur very easily.  For example, if two items of equipment are both connected to the mains ground and then interconnected with an audio cable which also interconnects the equipment common connections, a ground loop is created.

5.2  How Ground Loops Work

Suppose that two items of equipment are connected to a central ground, as would normally happen when you plug them both into a power point if they both have 3-wire power leads:

Two grounded items

Ideally, you would expect that the common connection of the first item (ground "A"), the common connection of the second item (ground "B") and the central ground would all be at the same potential, since they are all connected together.  However, due to the effects of electromagnetic interference and common path interference described in the previous chapters, there will be differences in potential between ground "A" and the central ground and ground "B" and the central ground.  Let's think of these differences as being very low impedance ac generators, represented by the symbol:

Ground interference

Now, let's assume that the two items of equipment have unbalanced inputs and outputs and we interconnect them with a shielded audio cable:

Ground loop

Now grounds "A" and "B" are also connected together by the shield of the audio cable.  In reality all wires have some resistance (albeit very low) and we end up a fight between the ac generators, the ground wiring and the shield of the audio cable.  Although joining grounds "A" and "B" together with the audio cable shield will reduce the difference between grounds "A" and "B", it can never eliminate this difference, since there is no such thing as a perfect conductor.

With two small well designed items of equipment plugged into the same power point, these differences should be negligible.  With large groups of equipment and items containing large conventional power transformers, these differences can become highly significant!  The voltages involved are normally very low but the current can be relatively high.

If there is a ground loop and part of this loop is formed by an audio signal path as in the unbalanced wiring example above, we've got some serious problems!  This is explained in the following chapter.

In the previous chapters, we have focused mainly on mains-derived interference in ground wiring and we can think of this as being predominantly the mains frequency (50Hz in Australia) with harmonics, commonly known as "hum".  There could also be traces of stray audio signals and supersonic frequencies from things like switched mode power power supplies and digital equipment present in ground wiring.

5.3  The Golden Rule of Grounding

As we stated in 2.3  Grounding to reduce Electrostatic Interference, all equipment should be connected to ground (which is either mains ground or a technical ground) so that it is held at the same potential as its surrounding environment to minimise electrostatic interference and for safety reasons.

As we stated in 4.2  Grounding to reduce Common Path Interference, all equipment should be connected in star fashion to a central ground point to minimise common path interference.

Now we are saying that joining the audio common connection of two items of equipment together when they are already connected to ground by some other means causes a ground loop.  Ground loops are bad because they cause relatively heavy currents to flow in ground wiring, producing differences between the grounds of different items of equipment.

We can summarise these grounding requirements with one simple rule:

"All audio equipment must be grounded via one path only to a single central point."

This rule may sound simple enough, but can be surprisingly hard to implement!