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Sound Advice

Interconnecting Multiple Sound Systems

At concerts, you often see three audio systems in use: house PA, recording, and broadcast. Here are some tips on connecting those systems without creating ground loops and hum.

Consider using a single power distro system, and connect all three systems’ power cables to that distro. Make sure the distro can handle the total current requirements of the three systems.

If you hear hum or buzz when the systems are connected, first make sure that the signal source is clean. You might be hearing a broken snake shield or an unused bass guitar input.

If hum persists, experiment with flipping the ground-lift switches on the splitter and on the direct boxes. If there is no ground-lift switch, insert an adapter that lifts the cable shield at the input of the system you’re feeding. On some jobs you need to lift almost every ground. On others you need to tie all the grounds. The correct ground-lift setting can change from day to day due to a change in the lighting. Expect to do some trial and error adjustments.

Often, a radio station or video crew will take an audio feed from your mixing console. In this case, you can prevent a hum problem by using a console with transformer-isolated inputs and outputs. Or you can use a 1:1 audio isolation transformer between the console and the feeds. Some excellent isolation transformers are made by Jensen (phone (818) 374-5857, www.jensen-transformers.com). Finally, try a distribution amp with several transformer-isolated feeds.

Bruce Bartlett is Senior Microphone Design Engineer at Crown International.

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