Power Mac G4 (Mirrored Drive Doors): Balanced or Grounded Equipment Has a Humming or Buzzing Noise

This document describes how to eliminate audio buzzing or humming noise coming from the audio sound output port.
Symptom

Balanced or grounded audio equipment connected to the audio sound output port has a humming or buzzing noise.


Solution

Connect the line output of the Power Mac G4 to the input of a stereo isolation transformer, and connect the output of the transformer to the audio equipment. Isolation transformers (also known as "ground loop isolators") are typically available at professional home/car audio retailers.


Additional information

Audio Equipment

There are two types of audio equipment that have two different types of inputs and outputs. Professional equipment, which uses balanced inputs and outputs, and consumer equipment, which uses unbalanced inputs.

Unbalanced Inputs and Outputs

Most of the common audio and video equipment you own has one of two types of connectors; coax connectors, also referred to as RCA connectors, and 3.5 mm connectors, also referred to as 1/8-inch connectors. Both of these types of connectors have only two lines per channel: the wire that carries the audio signal, and the shield or ground. 3.5 mm connectors usually contain a pair of wires and one shield, thus providing left and right stereo in a single cable. However, the 3.5 mm cable still only provides one wire and shield per channel. The shield on these types of cables are shared between the left and right audio wires. These two types of connectors make up most of the audio and video connectors found on common multimedia equipment.

The Power Mac G4 (Mirrored Drive Doors) is designed to integrate with your existing multimedia equipment. As such, it uses unbalanced inputs and outputs. This means it works out of the box with most stereo equipment, televisions, VCRs, and portable speakers.

Balanced Inputs and Outputs

Professional multimedia equipment uses a different connector called an XLR connector. It consists of three wires/pins for each channel. Two wires contain an equal voltage signal, which carries the audio. The third wire contains the ground. In most cases this style connector is not used on your audio and video equipment. In addition to having the specialized XLR connector, equipment with balanced inputs and outputs work on a different signal level than consumer equipment with unbalanced lines.

The Power Mac G4 (Mirrored Drive Doors) does not use XLR connectors, nor does it provide the proper signal level for balanced equipment without additional add-on hardware.

Using the Power Macintosh with Balanced Equipment

When balanced or grounded audio equipment is connected to the sound output port on the Power Mac G4 (Mirrored Drive Doors) computer, you may hear a humming or buzzing noise. This is caused by what is known as a ground loop.

Ground loop issues are usually completely eliminated if a stereo isolation transformer is used. An isolation transformer isolates the electrical grounds of two pieces of audio equipment and only allows audio signals to pass through.

Published Date: Feb 18, 2012