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Tech Tip - Avoiding Hum
By Dennis Kambury

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

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

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

Poor or defective cables can also be a source of problems. Hum can be picked up from light fixtures, motors, and other common electrical sources through these bad cables. Try to avoid cheap molded-head connectors, and take the time between gigs to check all of your cables for good solder joints, clean connectors, and undamaged shielding.

There is yet another cable-based source of hum induced from power cabling going into signal cabling. Briefly, when these two types of cables are running parallel, the AC 60 cycle signal can be picked up by the instrument or mic cable, amplified, and broadcast for everyone to hear! Keep all your power and signal cables well separated; and, if they absolutely must cross, be sure to always cross them at right angles to each other.

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