By Ethan Winer • August 30, 2018 The mic cable and AC extension cord bundled together in the author's studio. For years we’ve been told never to place AC power wires near audio wiring, especially cables carrying mic-level signals. The warnings claim that the electromagnetic (EM) radiation from the power cable will couple into the signal wires to create hum. When AC and signal wires must be adjacent, the standard suggestion is to cross them at 90 degrees to minimize coupling. But professional audio wiring is balanced to reject hum and other common-mode signals. Even microphone cables should be able to reject hum adequately. In my home studio I have many mic cables, all standard-issue stuff that didn’t cost a lot. I don’t own a single Star Quad cable and I’ve never had a problem with hum due to radiation from nearby power wiring. A similar concept was recently being discussed in an online audio discussion group, highlighting the importance of keeping XLR cables away from AC power wires, so I figured this was worth testing. No surprise, it turns out to be a myth, at least for balanced wiring. Figure 1 I bundled a 25-foot microphone cable with an AC extension cord and wrapped them together in a circle (like a transformer) to maximize any potential transfer of 60 Hz hum into the mic cable. I used twist ties to ensure both wires stayed close together. Then I connected an Audio-Technica AT4033 microphone to my aging Mackie 1202 mixer and set the preamp gain to its maximum 48 dB. The main setup in Figure 1 shows the AC power wire and microphone cable bundled together in a coil. Going Further Perhaps simply having an unused power wire near a mic cable doesn’t tell the full story because there’s no current flowing through the power cable. To test this, I connected a 600-watt halogen light that I use for video shoots, and switched it on and off while recording audio through the mic. There was no hum or power-related clicks. (An MP3 file of the recording is available here, where the only sound you’ll hear is that of the light’s power switch being turned on and off.) I also turned off the forced air heat in my studio for this test in order to avoid any possible room rumble in the background that might mask low-level AC power hum. Note that the mic was several feet from me, pointing away in the opposite direction because the preamp gain was at maximum and would have distorted as I spoke. Now, before I get a dozen angry letters telling me I’m full of it, I’m not saying that it’s never possible for AC hum to be picked up in nearby mic- or line-level wiring. Maybe with a very long run of hundreds of feet, with the wires bundled tightly together, some hum might possibly leak through the signal wire’s shield. And obviously unbalanced wiring could pick up hum more easily, especially if the shielding is poor. Devices like dimmers and motors that cause harmonics and noise on the power line are probably more likely to cause higher-frequency noise problems. Further, a lot of audio devices that use unbalanced wiring are high impedance, and that too will more readily receive radiated hum. But I’m confident that my test is valid, at least for runs of 25 feet or less, even when the wires are tightly coupled like windings in a transformer. About Ethan Ethan Winer Ethan Winer has been an audio engineer and professional musician for more than 45 years. His Cello Rondo music video has received nearly two million views on YouTube and other web sites, and his book The Audio Expert published by Focal Press is available at amazon.com and through Ethan’s own web site ethanwiner.com. Ethan is also a principal at RealTraps, a leading manufacturer of high quality acoustic treatment. http://ethanwiner.com Comments Have something to say about this PSW content? Leave a comment! Cancel reply Scroll past the ”Post Comment” button below to view any existing comments. Your email address will not be published. Required fields are marked *Comment Name * Email * Website This site uses Akismet to reduce spam. Learn how your comment data is processed. Jay says A large part of preventing a 60Hz Hum depends tremendously on how the Service Panel is wired. A 60hz hum is a problem when the Neutral Bar and Ground Bar are "Tied Together" instead of being separated. A larger issue today with 60hz hums being produced by "Dirty" fixtures and appliances made in China that blatantly violate FCC Part 15 rules. I've even seen faulty Hum Producing items in adjacent buildings where they backfeed through a faulty item and/or neutral, (but you'll also see other issues present in this situation). An easier way to stop the propagation of a 60Hz hum in audio systems is to add an APC unit in the supply power to feed the audio system. Tagged with: AC Cabling electricity electromagnetic radiation Ethan Winer hum Interconnect Interference Live Sound International Noise Power · all topics Subscribe to Live Sound International Subscribe to Live Sound International magazine. Stay up-to-date, get the latest pro audio news, products and resources each month with Live Sound. Subscribe Today!