Whitlock is a treat to read. “More often than not, the reduction or elimination of system hum and buzz is the result of a long series of experiments that stop when someone says, ‘I can live with that,’” he says in the 1995 AES paper. “In an audio system, delivering an audio signal voltage from the output of device A to the input of device B may sound simple, but doing so without adding hum, buzz, clicks, and pops coupled from the ac power line is not easy.”
More seriously, he’s good at presenting noise problems heuristically and step by step. In the paper, he later defines “audio system” as two or more physically separated devices that are connected by audio cables, with at least two devices that are ac-powered. Almost inevitably, one device exhibits a noisy voltage with respect to ground in another device.
That’s because “inside each device, small but significant alternating currents flow from the power line through interwinding capacitances of the power transformer and the capacitors in the RFI filter to the chassis.” Relative to an external reference point, such as the safety ground on the ac outlet, both chassis carry an ac voltage.
If the devices have three-wire power cords, the currents go to ground through the green safety ground wire. But since the green wire isn’t a perfect conductor, the chassis aren’t quite at ground potential. It’s not rocket science, but it’s something that’s easy to forget on the way to “I can live with that.”
The coupling capacitance and the wire resistance and inductance effectively form a high-pass filter, says Whitlock. So, “the resulting chassis voltage will generally be a rich mixture of high-frequency power line distortion components, commonly known as buzz.”
That particular paper is available from the Audio Engineering Society at www.aes.org/e-lib/browse.cfm?elib=7944. The 1.2-Mbyte pdf costs $5 for members of the AES and $20 for non-members. Other Whitlock reading material is available for free on the Web.