Bah! The Subwoofer Hum Bug, and How to Stop It

  Everyone can appreciate the value of a good subwoofer in a home theater system. 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.

  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. 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. 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.

  The “brute force” method for getting rid of 60-cycle hum is to filter it out, but that’s not a particularly desirable solution. 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. 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.

  What Causes Subwoofer Hum?

  The four principal likely causes of hum are:

  (1) Electrical defects in the powered subwoofer;

  (2) Induced noise in the audio signal path, most likely around cables;

  (3) Ground loop noise resulting from different ground potential at the receiver and the subwoofer; and

  (4) Noise arising from these causes in or between other components upstream of the subwoofer.

  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.

  (1) Issues with the Subwoofer Itself:

  Unfortunately, sometimes the cause of a humming subwoofer is simply the subwoofer itself. 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. Internal failures, however, can mess this up. 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. If it still hums when there’s nothing going in, your issue is probably with the sub, which needs repair or replacement.

  (2) Induced Noise:

  Induced 60-Hertz noise is hum that comes into your audio system through contact or proximity to power circuits or cables. While this can happen internally in your devices, the more common cause is bad routing of cables. 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. 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.

  Induced noise, if entering through cabling, is usually fairly simple to solve. 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. So if a power line one inch away from your subwoofer interconnect is inducing a signal in it, that induced signal will be a quarter as strong at two inches, a ninth as strong at three inches, a sixteenth as strong at four inches, and so on — the farther you can keep the two separated, the weaker the effect will be, so it’s time to move some cables and see what happens. If you’ve got power cabling lying directly on or under a sub cable, just a couple of inches of separation can make a profound difference.

  Shielding, too, plays a role in dealing with induced noise. The most effective shielding for low-frequency signals is braid, not foil, and a highly-conductive, high-mass shield will shunt more noise to ground than something more lightweight — see our article on hum rejection in analog audio cable, which inspired our design of the Blue Jeans Cable LC-1 audio cable with its double-braid high coverage shield. It’s important to recognize, however, that the kind of low-frequency, high-energy field set up by a power cord is the hardest thing there is to shield against — all shields are somewhat ineffective against it, and so while a heavy shield such as that on the LC-1 can help, minimizing close contact between power and audio circuits will almost always be the most important thing you can do to solve an induced noise problem.

  Now, induced noise can be trickier than that, not least because the problem can be occurring inside of equipment — bad isolation of power circuits from line-level audio inside a powered sub is something which no amount of work with cable placement or shielding will affect — but with any luck, the problem isn’t internal and these solutions will address it.

  (3) Ground Loop between Receiver and Subwoofer:

  The other common cause of subwoofer hum is completely different — ground loop current flows — and attempts to fix the hum problem that work well for induced noise will be quite ineffective against ground loops noise, and vice versa. A ground loop problem occurs where there are differences in ground potential between pieces of equipment, which causes a small amount of power current to flow along lines which connect the two. This flow, in an unbalanced circuit (that is, where the signal is carried on a single conductor using a ground return path, e.g., a signal carried on RCA-type connections), is in the signal path, and so it gets amplified just as though it were part of the original signal.

  Ground loops can often be resolved without spending any money on the problem. The trick is that one needs all of the grounds on all of the gear to have the same potential. Sometimes this is as simple as getting them all plugged in to the same power circuit; sometimes it’s a matter of making sure that you’re only using modern, three-wire, earth-grounded circuits at all points in your system rather than using old two-prong ungrounded circuits. Changing where things are plugged in, making sure your home power circuits are all properly grounded (an outlet-checker from the hardware store can be handy!), and the like will often resolve the issue. Beyond that, if your home wiring has serious grounding issues, you may need the aid of an electrician.

  There can be ground loop issues that even an electrician can’t solve, though — many home theater devices are not earth-grounded (these will usually have a two-prong, rather than three-prong, power plug), and instead use a kind of pseudo-ground which is tied through resistance to the neutral side of the power circuit. These sorts of pseudo-grounded arrangements can cause ground loop problems that can’t be resolved through ordinary grounding techniques.

  If trying to resolve grounding issues isn’t getting the job done, though, there’s a simple device that can solve the problem for you by breaking the ground loop: an audio isolation transformer.

  The principle that current in a wire induces current in nearby wires — which we’ve just talked about in connection with induced noise — is put to its fullest effect in a transformer. If you didn’t know anything about how induction works, the inside of a simple transformer would be very puzzling indeed — it consists of two wires that aren’t connected to one another, but which are wound tightly together over a core. Without being connected to one another, these wires nonetheless effectively pass current from one to the other. Transformers are usually used in situations where one wants to alter voltage or impedance, which can be done by giving the two wires different numbers of windings — but a transformer with identical windings on both sides can be used to simply pass a signal through, nearly unaltered.

  In many circuits, both the input and the output of the transformer will be tied on one side to the chassis ground, and if this is done, the transformer does not isolate the grounds on the two sides of the circuit. But it isn’t necessary to connect the grounds on both sides, and when this connection is omitted, we get an “isolation” transformer which isolates ground on one side from ground on the other. Now, there’s no path through which current can flow to resolve the difference in ground potential between the two devices, and voila! No ground loop, ergo, no ground loop noise.

  (4) “Upstream” issues:

  If the above attempts have failed to resolve (or have only partially resolved) your problems, it’s time to broaden the search. While subwoofer hum can often be isolated to the sub and its immediate link to a receiver, another distinct possibility is that the hum is getting into the audio stream further upstream. Induced hum, internal electrical defects, and ground loops can occur anywhere in the system.

  First, see if the hum happens only when your receiver is switched to particular sources. If it does, then that’s suggestive of where the problem will be found. But under some conditions, noise can come in even from the source you’re not presently using. Try unplugging all of your sources from your receiver (merely powering them down may not be enough), and reintroduce them one at a time; as you reintroduce sources, see whether you have hum on that source, and on any others you’ve already reconnected. If you can figure out where the issue is coming from, the same sorts of causes referenced above are all potential culprits — internal defects in a source device, induced hum in interconnect cabling, or a ground loop between the source and receiver.

  Silence at Last…

  A good subwoofer should spend a great deal of its time being completely silent, and with the right solutions, a sub that’s humming can usually be made to stop. As always, should you have problems or questions we haven’t addressed here, let us know — we are glad to help!

  Product Links:

  Should you need heavily-shielded sub cables, or isolation transformers, you can find them in our subwoofer cable department — our LC-1 cable has a double dense copper braid shield, and we also offer two isolation transformers. Our subwoofer cables are assembled in the USA in our Seattle shop (using US-made cable which Belden produces to our design at its plant in Richmond, Indiana), and our Blue Jeans Cable brand isolation transformer is manufactured in the USA as well, on subcontract with a California manufacturing firm.