Induced Buzz in audio system




I have problems with a buzzing sound induced in the audio signal cables at the theater I work in.

The problem started after I changed the PA system. Now I have powered speakers and I had to run a long signal cable from the left side to the right ( 60 feet or so ). The problem is that the cable runs one feet paralel to the lighting power cables and I have almost no chance of driving it another way. Today I came to work and powered my gear and nothing could be heard apart from a light hiss coming from the mixer ( that's another problem altogether ). After some time the light guy came and powered his dimmers and the buzzing started. I could hear the buzzing modulating as he ran through his different light cues.

Now the problem beeing placed on the table how can I deal with it?


sometimes powered speakers will work better if the power plug is lifted from ground (earth)
or different parts of the audio system like the mixer can be lifted to cut down on the noise.
Sometimes you can get the hum out by using balancing transformers in the audio line,
either just at the start or the end, or at both ends. this gives you the option to cancel an induced voltage that is common to both phases, or lift the screen (shield) at the speaker end.
In any case good transformers are expensive and cheap ones do not pass low frequencies.
Connect lighting to one phase of the power, exclusively, connect audio to the other phase, or if you have 3 phase, the lights can use two and sound can use one.
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