… for Codecs & Media

Tip #958: How Do Audio Cables Prevent Hum?

Larry Jordan – LarryJordan.com

All cables pick up hum. Shielded, balanced cables cancel that hum in the mixer.

OK, so this is a bit off topic, but… I’ve known for years and years that audio cables with XLR connectors don’t have hum, while cables with RCA connector so. Today, I wondered: why?

Here’s what I learned at
ProSoundWeb.com.

“One cause of hum is audio cables picking up magnetic and electrostatic hum fields radiated by power wiring in the walls of a room. Magnetic hum fields can couple by magnetic induction to audio cables, and electrostatic hum fields can couple capacitively to audio cables. Magnetic hum fields are directional and electrostatic hum fields are not.

“Most audio cables are made of one or two insulated conductors (wires) surrounded by a fine-wire mesh shield that reduces electrostatically induced hum. The shield drains induced hum signals to ground when the cable is plugged in. Outside the shield is a plastic or rubber insulating jacket.

“Cables are either balanced or unbalanced. A balanced line is a cable that uses two conductors to carry the signal, surrounded by a shield (Figure 1). On each end of the cable is an XLR (3-pin pro audio) connector or TRS (tip-ring-sleeve) phone plug.

“Hum fields from power wiring radiate into each conductor equally, generating equal hum signals on the two conductors (more so if they are a twisted pair). Those two hum signals cancel out at the input of your mixer, because it senses the difference in voltage between those two conductors—which is zero volts if the two hum signals are equal. That’s why balanced cables tend to pick up little or no hum.”

XLR cables (called “balanced”) use two wires and shielding. RCA-type cables (called “unbalanced”) do not.