Tech Tip:Avoiding Hum


By Dennis Kambury


The prime reason for hum is the ground loop, caused when the sound system has two or more different ground points. This is easy to do if, for example, you plug your guitarist's amp into the onstage socket, and your mixer into the socket at the back of the house. These separate points will have different electrical potentials, causing electrical current to flow. The result is easy to hear, but how do you fix the problem?


The best solution is to ensure that all AC power is supplied from one single circuit from the power mains. If that's not enough juice, at least make sure that all your circuits come from the same panel with the same ground.


The ground loop can also be broken by the use of a ground lift adapter, but it's not a very good idea, as this method is potentially fatal—breaking the ground means that the signal could potentially find its way to earth directly through YOU!


Poor or damaged cables can also be a source of problems as hum can be picked up from light fixtures, motors, and other common electrical sources. Avoid cheap molded-head connectors, and take time between gigs to check your cables for good solder joints, clean connectors, and undamaged shielding.


There is another cable-based source of hum induced from power cabling into signal cabling. Briefly, when the two types of cables are running parallel, the AC signal can be picked up by the signal cable, amplified, and broadcast for the world to hear! Keep your power and signal cables well separated; and, if they must cross, always cross them at right angles to each other.