![]() ![]() The key to understanding how to fix it is the square-of-the-distance rule: the intensity of an electrical or magnetic field diminishes by the square of the distance from its source. ![]() Induced noise, if entering through cabling, is usually fairly simple to solve. Current moving in a cable creates a field around the cable which can cause a similar current to flow in nearby conductors - this is the same sort of thing that's going on when you experience "crosstalk" in telephone lines, bits of signal that bleed over into neighboring wires. We sometimes find that people have routed power cable and audio interconnect cable through the same conduit or cable tray - definitely a no-no not only from a noise point of view, but also from an electrical code point of view. While this can happen internally in your devices, the more common cause is bad routing of cables. Induced 60-Hertz noise is hum that comes into your audio system through contact or proximity to power circuits or cables. If it still hums when there's nothing going in, your issue is probably with the sub, which needs repair or replacement. In most cases, this is pretty easy to detect: disconnect your subwoofer from everything except power (yes, unplug the incoming signal cable), and power it on. Internal failures, however, can mess this up. Any audio reproduction device that runs off of our regular AC power has got to tame that 60-cycle noise in the power supply, convert it to nice level DC voltages, and protect the audio circuitry from the power supply sufficiently to prevent hum from getting from the power supply into the signal path. Unfortunately, sometimes the cause of a humming subwoofer is simply the subwoofer itself. The things that will solve one of these problems will not solve them all and it's entirely possible that you have more than one factor contributing to your problem, so if something helps, but doesn't resolve the issue, keep trying. (4) Noise arising from these causes in or between other components upstream of the subwoofer. (3) Ground loop noise resulting from different ground potential at the receiver and the subwoofer and (2) Induced noise in the audio signal path, most likely around cables (1) Electrical defects in the powered subwoofer The four principal likely causes of hum are: To get rid of hum without having to throw out some of the desired audio at the same time, we need to start by understanding what the various possible causes are. Some parts of a low-frequency audio signal are themselves around 60 Hertz, and a filter doesn't know whether a particular wave is part of the intended sound or is noise - it just strips it out. ![]() The "brute force" method for getting rid of 60-cycle hum is to filter it out, but that's not a particularly desirable solution. There's nothing that can more effectively dampen one's enthusiasm for a nice powered subwoofer than a persistent 60-cycle hum - and since a subwoofer is intended specifically to do a good job of amplifying low-frequency signals, when a sub hums, it can hum very, very loudly. In a perfect world, power hum wouldn't ever get into the audio signal path, but in this respect, our world is far from perfect. Unfortunately, there's another low-frequency signal present in every home, which isn't quite so lovely to listen to: the 60-cycle hum of the AC power lines that power everything in the house. Getting good reproduction of the lower end of the audio spectrum gives sound a more full and realistic quality, and at the lowest audible frequencies and below, a subwoofer adds a tactile quality to home theater - some things are not so much heard as felt. Everyone can appreciate the value of a good subwoofer in a home theater system. ![]()
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