If you record sounds or music from a cassette deck to a Power Macintosh 8500 series computer by way of RCA jacks from the cassette deck connected to the computer's RCA audio input jacks, you may notice that there is a humming noise in the recording. This kind of noise can be avoided.

When connecting external audio devices (such as a CD-ROM player, cassette deck, mixing boards, and so on) to the RCA audio input jacks on the Power Macintosh 8500 computer, you may create a ground loop between the audio device and the computer.

A ground loop occurs when you connect two devices together with different ground points. Both devices must have the three-prong plug for this to happen. To avoid ground loops, many consumer electronics only have two-prong plugs.

The loop most commonly goes from one device to the other along the audio cables and then returns through the ground wire. The difference in potential between the two grounds in the ground loop results in an audible noise or hum.

To eliminate this noise, disconnect the audio device from the computer. If the noise disappears, plug the power cords from the computer and the other devices into the same power strip. This places all of the equipment at the same ground potential and eliminates the loop.