Subwoofer humming

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. ?

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!

Hmm. Maybe I shouldn’t say perfect. A better phrase would be: ‘assumes that its route to ground is the same as every other piece of equipment’...

Of course, the whole topic of ‘Dangerous ways to stop ground loops’ Is huge!

Hi Lisa I'm no audio expert but I do remember a friend who had a mobile disco suddenly got hum problems only on the left channel turned out that the braid in the lead from a tape deck to the amp was corroded and had almost completely parted company from the plug solder tag I know yours is a new system so it's unlikely to be faulty but get your trusty DVM out and check mite be worthwhile. A relative of mine had a system where the speakers whistled slightly  if the amp was off turned out that at nite especially the speaker wires were picking up broadcast stations and the amplifiers transistors were rectifying the signal and it could just be heard with an ear right up to the speaker!! It drove his Alsatian crazy!!

What happens if the audio cable to the sub-woofer is disconnected as opposed to switching it off? (Or is it Bluetoothed?)

What sort of system is it, and which components (if any) have an earth connection? (thinking of the system I used to have just about everything, was double insulated or SELV powered, so all the audio "grounds" where probably floating, limiting the scope for traditional earth loops).

  - Andy.

In modern systems, to a subwoofer especially, I would not expect a ground loop, isolation is usually ensured.

They used to occur between low level input devices (not CD players) and amplifiers being separately grounded.

(OK, a subwoofer has it's own amp - but with a relatively high input - like a CD player not an old record deck)


I would therefore suggest that 2 more likely causes are

a) Faulty signal cable - where the shield is not properly/well connected  (simple mass manufacturing fault)

- so replacing what is hopefully a replaceable [connectors at both ends cable - standard "RCA" phono-phono] cable. might eliminate this as a cause..

 (if someone has accidentally used un-screened speaker cable for a low level signal - that would cause the same issue)

 I am assuming at this point, that it is NOT being connected to the speaker terminals to pickup the sub-woofer signal.


b) a genuine internal power supply fault - insufficient smoothing/faulty smoothing capacitor

-> if you turn off the mains input (at the power socket/not pull the plug), does the hum immediately stop for example. 

     In a subwoofer there will be lots (several seconds) of reserve power.. so the amp will still be powered and working,

     Killing the input mains, and the hum going would indicate a power supply fault.


.As for the neighbours.. .yes... 

be aware. that bass sound travels very well with very little loss in power for a long way - especially in concrete and walls.

it may not sound loud to you./ merely be light background atmosphere... to you...

.. but someone some way away will just be getting the thud/thunder/rumbling..for the 1hr plus of your film.

  Try going to another room whilst it is on...


But enjoy anyway.. just not after 11pm, perhaps - or invite them around  ;)

Thank you all ever so much for your replies and thanks especially to mapj1‍  and  Martin Russ‍ for their expert and helpful explanations! ?


We've managed to stop the hum by earthing the amp. ? It looks as though the sub is earthed but the amp isn't and when earthing the amp the hum stops.


The Tibetan Monks have now, thankfully, moved out... ? ?

 

PS LISA  is this post in the right bit of the fourm. ?

mapj1‍ I posted this in the WR category as it was a question initially related to domestic wiring (i.e sockets) so was wondering if any Sparkies had come across something like this before or had any insight to provide... ?

Andy88:


.As for the neighbours.. .yes... 

be aware. that bass sound travels very well with very little loss in power for a long way - especially in concrete and walls.

it may not sound loud to you./ merely be light background atmosphere... to you...

.. but someone some way away will just be getting the thud/thunder/rumbling..for the 1hr plus of your film.

  Try going to another room whilst it is on...


But enjoy anyway.. just not after 11pm, perhaps - or invite them around  ;)

Yes I live in a semi-detached cottage so any type of sound can travel around the structure which we obviously share! There's quite often some kind of electrical hum that resonates around the walls and I can hear quite clearly in certain areas of the house. I think it's either from their bathroom fan running or perhaps their washing machine as it can go on for hours... ?