Subwoofer humming

Question

Well the OH finally installed the surround sound system he's been wanting to do for many months now (God help our neighbours...) but when the subwoofer is turned on, it's rather audibly humming like a Tibetan Monk!

A quick look on Google for a solution mentions that it may be because it's plugged into it's own socket outlet?? ?

Can anyone explain why this would cause the sub to hum?

As an Audio expert Martin Russ‍ can you provide any insight?

mapj1 · Accepted Answer

It presumably stops humming if you unplug it, so in a sense correct. When folk come up with this sort of thing there is usually a grain of truth behind  it, but not well explained How does the signal that is to be amplified get to it ? I assume via a screened audio cable, and if so, are the earths of the equipments at the two ends at the same voltage -  every microvolt counts here - if one end puts out  an audio signal of say 10mV RMS relative to its earth, and the  other amplifies and plays the voltage it sees relative to its earth, but the two nominal zero volt earths are actually some mV apart, then the ampifier sees the signal and the extra ground slope voltage as being in series. The chances of the two chassis voltages being different is increased if they are plugged into different places, so putting them onto a common 4 way block in a single socket tends to help, by reducing the voltage difference between the '0V ' ends of the link. PS LISA  is this post in the right bit of the fourm. ?

Former Community Member · Answer

This effect is more commonly called a ‘ground loop’. What that means is that there are different paths to ground (the earth - that the yellow and green wire in the plug is ultimately connected to...). Now the equipment (amplifier, etc) assumes that its connection to ground is perfect, but if the equipment is connected to ground via different routes, then that assumption isn’t necessarily true. So the amplifier ends up amplifying the frequency of the mains and you hear this as a 50 Hz ‘mains hum’ tone. There are various cures with varying levels of dangerousness - but the best solution is to plug both bits of equipment into the same socket.

As an analogy, imagine two people stood on top of rotating square platforms. The platforms rotate at different speeds. Everything is fine (apart from the people getting dizzy) until they try to hold hands - at which point the different rotation speeds suddenly matter.

Electronics is all about understanding what the actual circuit looks like - not the theoretical idealised simplified abstraction that it might appear to be without considering reality. There are all sorts of traps when you believe the abstractions, and don’t consider the reality!

Subwoofer humming

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