Power Interference on an Audio cable
I get asked questions quite often during an installation for advise about cabling techniques. One of the most often asked questions is 'Can't I just run my speaker cables right beside my power cables' or 'can speaker wires run next to electrical cables?' The quick short answer is 'No', with a quick explanation of noise. I want to cover why running speaker wires next to electrical cables is a bad idea. Inductance is an audible transfer of a hum, buzz, noise or interference transferred from one cable to another cable. In quite simple terms, noise on one cable goes to another within a certain distance. Electrical cables have A/C (alternating current) running through them. A typical home in America has 110 Volts A/C at 60Hz (hertz). Some A/C outlets (used for Dryers, Stoves and other limited devices) have 220 Volts A/C at 60Hz. Some countries use 110 at 50Hz or 220 at 50Hz. It the Hertz that provides the buzz or noise. Hertz is the amount of times A/C switches polarity in a second. 60Hz means that 60 times a second, the A/C switches polarity. This when played audibly is a low buzz noise. The closer an audio receiver is placed to the 110/220 VAC (Volts Alternating Current) the more prominent the noise will be. Imagine a power cable with an invisible field around it encompassing 18 inches or so in either direction. Anything within that 18 inches will get a buzz, the closer to the center of that field the louder the Buzz will be. This field is the inductance field of the cable. 18 inches is the approximate area that the inductance will occur. Speaker cable are wires that usually are encompassed by a clear plastic insulator running parallel to each other. There is usually no twisting of the pair involved. This makes the cable a perfect antenna and will easily pick up noise or interference from another cable. Thus, when a speaker cable is placed too close to an Electrical or power cable, the 60 Hz signal easily transfers to the antenna that a speaker cable operates like. Your speakers when powered by an amplifier will play this constant buzz from the electrical circuit. The closest that I would recommend running a speaker cable to an electrical power cable is around 18 inches or so, though there is a good success rate at 12 inches or so. Any closer than that inductance will occur and then your speakers will have a 60 Hz hum play on them any time the amplifier is turned on. Possible solutions if you cannot separate your speaker cables from your power cable by 18 inches vary, I will cover the three most often used. Shielded cable -- A cable covered in a foil shield, this type of cable will have a grounding strip running along the entire length of the cable. Run the cable trying to keep as much distance as possible from the power cable, strip at the ends only the length that you need to splice. At the amplifier strap or twist the the grounds together and tape off or tie to earth ground. You may have to experiment with the last step to see what works best for you. Try twisting the grounds all together first. Try keeping the grounds separate. Finally tie all grounds together and tie to the ground in the amplifier. Twisted Pair - If at all possible find a twisted pair shielded cable. The more twists per inch, the less likely that it will pick up inductance or interference. If you can find it, purchase a category 5 twisted pair, shielded cable (a shielded computer network cable). Ferrite collar - Ferrite is made out of a fragile metal allow, the come in the form of rings or collars. Get collars for both ends of a cable run, at the speaker end and the amplifier end. For a 5.1 surround sound system, you will need 12. At each end of the cable, wrap the cable around the inside diameter of the collar several time and snap it close. Your best solution to reducing or eliminating inductance if you cannot be 18 inches away from a power cable is to use all three methods. Run a twisted pair shielded cable, with the shields grounded to another and place ferrite collars at both ends of the cable run. Doing this seems the be the best solution, though it is not always foolproof. Remember only do these if you have no other choice for routing the cable. Best of luck.