(from Apple Support, original Link here
Ground loops, a common phenomenon in computer-based sound systems, occur when two paths to ground are available . A ground loop allow current to pass on the ground wire of the cables connecting two pieces of audio equipment together. The result is a buzzing or humming sound. Common ways to reduce the hum or buzz introduced by ground loops include:
Plug the your audio equipment into the same electrical outlet or power strip. This simple procedure may help reduce or eliminate hum and buzz by reducing the difference in electrical potential between paths to ground.
Use an isolation transformer. An isolation transformer isolates the electrical grounds of two pieces of audio equipment and only allows audio signals to pass through. Ground loop issues are usually completely eliminated if a stereo isolation transformer is used. The line output of the computer, USB audio, or FireWire audio interface is connected to the input of the stereo isolation transformer, and the output of the transformer is then connected to the audio equipment. The following outline can help you determine which isolation transformer is right for your application.
Isolation Transformers for Balanced and Unbalanced Inputs and Outputs
There are two types of inputs and outputs. Professional equipment uses balanced inputs and outputs. Consumer equipment uses unbalanced inputs.
Most consumer audio and video equipment has unbalanced inputs that use either coaxial (RCA) connectors or 3.5 mm connectors. A coaxial cable has two wires. One wire carries the audio signal, and the other is the shield or ground wire, which carries the ground. This means that a stereo signal requires two cables, one for right and one for left. 3.5 mm connectors usually contain a pair of wires and one shield, providing left and right stereo plus ground.
Apple computers are designed to integrate with existing consumer multimedia equipment, including most stereo equipment, televisions, VCRs, and portable speakers. Consequently, Apple computers use unbalanced inputs and outputs. If you are connecting your computer's sound output port to another piece of equipment with unbalanced inputs, consider using the following unbalanced isolation transformer:
Radio Shack Ground Loop Isolator (Stereo) Catalog #270
Professional multimedia equipment, with balanced inputs, uses XLR connectors (1/4-inch TRS or tip-ring-sleeve). It consists of three wires or pins for each channel. Two of the wires contain an equal-voltage audio signal, while the third wire contains the ground. This specialized connector is most often found on professional USB audio interfaces, FireWire audio interfaces, mixers, and amplifiers. If you are connecting the ports of two of these balanced professional devices to each other, any of the following balanced isolation transformers may be used:
Audio Kwik Fix ITX
Unbalanced to Balanced
For connecting unbalanced output (such as your computer sound output port) to a balanced professional input, a third type of isolation transformer is used. This isolation transformer has unbalanced inputs and balanced outputs. Consider using this transformer:
Important: Information about products not manufactured by Apple is provided for information purposes only, and does not constitute Apple's recommendation or endorsement. Please contact the vendor for additional information.
Document 17159, "Locating Vendor Information" can help you search for a particular vendor's address and phone number.