DISCUSSION

(from Apple Support, original Link here)


Audio Equipment

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.

Unbalanced

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
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=270-054

Balanced

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:

ProCo IT1
http://www.procosound.com/prod02.htm

Audio Kwik Fix ITX
http://www.markertek.com/MTStore/store.cfm?framed=/MTStore/product.CFM?BaseItem=LMX

Sescom IL-19
http://www.whirlwindusa.com/spcint.html

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:

Whirlwind PCDI
http://www.whirlwindusa.com/pcdi.html

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.

From beluga





Thanks for your input, I had to post his for all to benefit by


Sorry beluga if you misread my post, the original post is from Apple.


I don't think it's a "Apple" issue concerning noise in the audio channel, as interference comes from all over, cb radios, police bands, military, hair dryers, poorly grounded equipment etc.

As well as a lack of electrical knowledge by some folks, like mixing power cords and signal cords. (power cords resonate a magnetic field that easily penetrates cables and causes sound problems)

The natural tendency someone has when they experience these problems, is to put their blame on their computers.



However concerning the power supply "chirp" noise issue, which is a OEM defect that materialized upon mass production of the unit.

I can advise folks to try to get the power supply switched hoping they get one that's sufficient, because the only true solution is a new design that eliminates the electrical engineering defect to begin with.



It's my opinion that now that we have very quiet machines, it has raised our expectations even higher, and exposed noise issues that would be normally drowned out by the fans.

Since I run a distributed computing project that keeps my processors, fans and power supply at maximum, I haven't experienced the "chirp" enough to bother me.