[Engineering Essentials]
What Was That Noise?
Learning the basics of noise in amplifiers against the backdrop of some new ideas on how to cope with it offers fresh perspectives on a plan of attack.
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.
A very common equipment design error, referred to as the "pin 1 problem", causes it to output hum or buzz when a shield (delivering ground current) is connected to it. It is all too common since XLR connectors are mounted on PCBs rather than metal chassis. A paper on this is in the same June 1995 AES Journal as my paper. I'm working on a new paper that finally explains the origin of ground voltage differences among AC outlets. Readers may also be interested in a new IC that, for the first time, truly imitates the excellent CMRR behavior of a good audio transformer (see http://www.thatcorp.com/1200-series_High_CMRR_Balanced_Line_Receiver_ICs.html).
Bill Whitlock -June 22, 2009
As an audio technician I appreciate this article.These are good points to bring up for future designs.I discovered the wonderful world of noise trying to mate a floating ground power supply device with a referenced ground,(actually used 0 ohm resistors to the chassis),supply.It came down to lifting the ground on the inter- connect on just the input side and lifting the ground of the referenced supply,on the cable end,( very dangerous,some musicians like to drink on stage and this was the mixer for the stage sound),oh and did I mention that the input side was transformer isolated.This is common in my industry as they like to think we don't mix and match different brands of gear.Digital technology has helped,but getting it right in the first place can help with compatibility down the line.The audio world is extremely subjective and we will continue to mix and match brands,( with their different design philosophies),with out much thought of noise and compatibility.
Graham Pearson -June 22, 2009
Where is Figure 4?
Anonymous -June 15, 2009
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