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Sub Hum

Scott Wilkinson  |  May 20, 2011

I have a Pioneer VSX-1019 A/V receiver and a Paradigm subwoofer. When the receiver is turned off, I hear a slight hum from the sub when a certain volume is selected. I have to turn the sub volume all the way down to get rid of it. The sub is supposed to have an auto-on feature, but I'm not sure if that is working or not.

I have tried different cables from the receiver to the sub. I have used a 3-conductor AC power cord. I have tried different power strips, but I still get the hum when the receiver is turned off. How do I get rid of the hum?

Douglas Dye

Is the sub plugged into the same power strip as the receiver? Do you hear the hum when the sub is not connected to the receiver?

I suspect it could be one of two things. First, if the hum disappears when the sub and receiver are powered from the same outlet, or when the audio is disconnected, the problem is something called a ground loop. This happens when two connected devices are powered from different outlets and the ground of each outlet is at a slightly different voltage. (They should both be at 0V, but there is often a slight difference between them.)

Professional audio equipment sometimes includes a ground-lift switch, which disconnects the ground to eliminate ground loops, but few if any consumer products have this feature. Some people eliminate ground-loop hum by using a "cheater" plug, a 3-prong plug with its ground disconnected, but this can be dangerous and is definitely not recommended. Another potential solution is an isolation transformer, which is connected in-line between the receiver's output and the sub's input. However, such a device can distort the audio signal, especially the bass.

If the problem really is a ground loop, the best solution is to simply power the two devices from the same outlet.

Otherwise, it's probably the circuitry of the sub's power amp. If the hum is truly slight, you won't hear it when playing audio, so I'd ignore it. If it's more than slight, or it gets worse over time, I'd have a technician look at it. The worst-case scenario would be to replace the entire sub.

If you have an A/V question, please send it to askscottwilkinson@gmail.com.

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