Avoiding Mains Hum
Mains hum is a low frequency hum or buzz which increases in volume as the PA system is turned up louder.
In the context of PAs the most common reasons for mains hum is when you have two pieces of mains powered equipment which are connected via an audio cable (e.g. jack plug). For example, mains powered monitors connected to the mixer. Unless one of the pieces of equipment has been 'ground lifted' a mains loop will be present and a hum will result.
If you have a mains hum problem then firstly identify which piece of equipment is causing the problem.
The best way to do this is disconnect all the equipment from the mains and also all the connecting signal cables between the equipment. Now start introducing the equipment piece by piece until you notice the problem. The best sequence to do this in is power amp and speakers, mixer and then connect them together via a signal cable. Next introduce the monitors (if you use them) and then each piece of equipment.
Once you have identified the offending piece of equipment then you will probably need to seek qualified advice on solving the problem.
Never, ever remove an earth wire from a mains plug to avoid the problem!
Most powered speaker systems and monitor systems now have a 'ground lift' switch which avoids the problem when connecting them to a mixer.
Many pieces of kit such as mixers and keyboards are 'double insulated' i.e. they do not have a earth wire connected to the mains plug - therefore mains hum can not occur.