Power Mac G4 Humming and Buzzing: Isolated to Mirrored Drive Door Models?

Continuing our coverage of the Power Macintosh G4 audio humming/buzzing problem, which Apple details in Knowledge Base article #75408, we've received a number of different suggestions for resolving the problem, and learned that the problem may not be limited to Apple's Dual Mirror Drive G4 models.

Stephen Hart reports success using the information from yesterday's post to purchase an appropriate ground loop isolator from Radio Shack:

"After reading yesterday's post, I bought a Radio Shack ground loop isolator. The actual catalog number shown on mine is 270-054A. It solved a ground loop problem when recording direct from a telephone by way of another Radio Shack device, 43-1237A and a Griffin iMic. I can now get clean recordings direct to the hard drive of my G4/450."

MacFixIt reader Mike writes that the problem may also affect digital video output:

"I have a similar problem with my Quicksilver tower. But it goes beyond just audio. Oddly, if you look at the video signal coming directly off the FireWire port from Final Cut Pro, you can see the buzz instead of hear it. You would think could be possible since the signal is digital, and any program that you record to DV tape will be perfect. But using your DV camera as just a pass through, you will see the buzz on your analog TV monitor."

David Reyburn says that Radio Shack's ground loop isolator may not meet the necessary specification level for maximum audio performance, and notes another means for eliminating ground loop problems:

"From Radio Shack's support page, I note that the impedance ratio is 1000 ohms to 600 ohms. Transformers have a turns ratio and this means that the turns ratio is sqrt(1000/600). But specifying the turns ratio indirectly in this way may mean that the transformer has low equivalent shunt inductance (at low frequencies) and/or high equivalent shunt capacitance (at high frequencies) and thus is not intended to be used with units of significantly higher source impedance than 1000 ohms. My suspicions increased when I read the frequency response specification: 300 Hz to 4000 Hz, /- 3 dB -- not very high fidelity. Allowable signal level for the transformer also is not specified; as with all audio transformers, if used at too high a signal level, distortion will be excessive. If used at too low a signal level, hum pickup in the transformer from ambient 60 Hz magnetic fields may be excessive.

"Another way to alleviate ground loop problems is to make sure that the computer and the audio equipment are powered from the same wall outlet so that they do share the same ground, and then connect the computer to the audio equipment directly without using an isolation transformer. This may mean a long power extension cord, but the fidelity would be much higher this way. If this is not feasible, or if hum persists despite the use of a common power source, then a high quality transformer may be the only satisfactory solution."