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Thread: Sub Woofer started humming

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    Default Sub Woofer started humming

    Hi all,
    Hope everyone is well

    I have a active sub woofer connected to a Amp via RCA plugs.
    A few weeks ago, it would just start a deep rumble humming whilst watching the TV.

    So I turned it off and back on and it was ok.
    Then it did it again after a few days. This time I unplugged the RCA leads (which stopped the humming), put them back in and the humming was gone.
    All the times have been when watching TV - with no ceiling light on.

    This morning 3:00am, I awoke to hearing humming.
    I went into the dark lounge, turned on the lounge light and the humming stopped!!

    The lounge light is a 28w LED ceiling light.

    I would have thought the 240v supply to the AMP was different to the 240v supply to the Light - or maybe not...?

    Any idea as to what's going on guys?

    Cheers,
    GT250.



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    In car audio amps that sort of problem is normally an input shielding/grounding problem, I would think possible capacitor failure around the low level input.
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    Seems likely, if 'twere one of the leads I'd expect it all of the time.
    I'm out of my mind, but feel free to leave a message...

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    It's a bit odd for a light to cause that problem while it's off.
    I don't think the bulb would be the source of the humming but more likely the sudden spike on the mains when turned on has affected what ever fault the subwoofer has.
    Next time it hums try removing the bulb from the socket instead of turning it on, just to see what happens.

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    Quote Originally Posted by lsemmens View Post
    Seems likely, if 'twere one of the leads I'd expect it all of the time.
    You'd think that, right....but I've seen the contra-condition -- bad solder connections on the PCB at the input jack. If you get the right sort of nasty joint, variations in temperature and/or vibrations are enough to set it off.

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    And the problem is as soon as you put it anywhere near a test bench it will not fail for days and no amount of heating, cooling, poking, prodding will get it a to play up.
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    I asked this same question about 7 years ago on this site and got the answer, only problem is i cannot remember the answer
    But i now switch the sub woofer off after using it, think the position of the unit had some thing to do with it in relation to the power point and a power bank?
    There is a fine line between "Hobby" and "Madness"

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    Hi All,
    Many thanks for the replies.

    Loopy, it's not a bulb, it's a LED light.

    Well, last night watching TV (no ceiling light on), it started humming again!

    I got up, flicked the ceiling light on and it stopped!!

    I'm sure that the lighting circuit in the house is separate to the utility Mains. When I get back from work, I'm going to turn off the light breaker and see if the SUB still has power...

    Last thing I want to do is open the SUB up

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    I hope by now you have tried other RCA cables to rule out intermittent shielding hum. Why are you talking about RCA cables in plural? A subwoofer is mono or are there also the satellite speakers connected, so is it a multi channel amp?

    This could be parasitic oscillation either within the subwoofer, an interaction between the subwoofer and source or maybe even in the source itself.
    The actual oscillation could be above audible range and what you are hearing is the heavy load on the subs power supply where the filter capacitors can't keep up, assuming this is not a switched mode supply. This would be a 100Hz hum.

    Assuming this is a more modern class D amplifier, a parasitic oscillation could overlay the already existing oscillation and create a subharmonic that you can hear and that could be any frequency even much lower than 100Hz.

    A spike coming from any where could trigger the oscillation and any changes with the cables or more spikes could cancel it.

    Books could be filled with what causes these oscillations so I suggest a DuckDuckGo search on the matter.

    Try to see first if you can get the sub to hum with no cable(s) connected. If its stays quiet try with the cable connected to the sub but not to the source, just leave it open.
    You will probably have to do this over a longer time period including the night, regularly switching on and of your LED light.
    If it still hums you will need to fix or replace the sub.

    If it is a multichannel amp then the wiring of the satellite speakers may be too close to the input RCA cables, so try avoiding that.
    Last edited by Uncle Fester; 02-06-20 at 11:48 AM.
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    Quote Originally Posted by GT250 View Post
    Loopy, it's not a bulb, it's a LED light.
    Ok so is it an led light that's plugged into a socket or some sort of self contained led fixture ?
    The point is, does it have a metal chassis or frame that's connected to earth ?

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    Ok,
    Been a big day

    Anyway, I turned off the Light breaker and the SUB turned on. So not connected via physical cabling.

    Loopy, the ceiling light is a standard hardwired Bunnings Circular LED saucer shaped 'thing' that I just replaced with an old existing Two bulb unit that was mopunted int he middle of the lounge room. From memory, I can't recall if it has an earth.. Maybe I'll have to check that lol!

    As Uncle Fester has asked... The Amp I use has a Single RCA connection for output. I use a 2 into 1 adaptor from the AMP that goes to the SUB.

    The Sub has two RCA inputs...!?

    I include a photo that might help;




    My oldie goldie Amp...


    Uncle Fester, thank you for your in-depth problem solving, I hope the solution isn't going to be that involved..

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    First non-invasive thing I'd try is ditching the RCA inputs and trying the speaker inputs instead to see if behaviour changed. (unit obviously has mixer/low-pass built in for both line & speaker level signals)

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    Quote Originally Posted by GT250 View Post

    As Uncle Fester has asked... The Amp I use has a Single RCA connection for output. I use a 2 into 1 adaptor from the AMP that goes to the SUB....
    I'm puzzled....

    1/. Why are you using a 2 into 1 adaptor?

    Yes, I know that you have said that "The Amp I use has a Single RCA connection for output", but what else is connected to the amplifier output necessitating the use of the two into one adaptor.

    The only thing that should be connected to the amplifier speaker output is a loudspeaker.

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    Subs are typically mono.

    I don't know this particular sub, but most that have two RCA inputs, they are intended for amps that have a stereo sub output.

    If the amp only has a mono sub output, it usually only connects to the left input of the sub.

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    Quote Originally Posted by mtv View Post
    Subs are typically mono.

    I don't know this particular sub, but most that have two RCA inputs, they are intended for amps that have a stereo sub output.

    If the amp only has a mono sub output, it usually only connects to the left input of the sub.
    I'd actually heard it expressed as the 2 RCA inputs (and 2 speaker inputs) are there to facilitate connection to amps/receivers, that didn't/don't have a dedicated sub-woofer output. In such cases, you would use the amp/receiver line-out/tape-monitor outputs and use a standard stereo RCA cable to hook things up -- if you didn't have that, you'd connect the speaker outputs to the sub speaker inputs (as well as your speakers), and it derived the signal from there (resistive droppers to attenuate signal).

    Then a lot depends on the input circuitry (as to whether or not the speaker inputs go through another op-amp stage or not), but basically the left/right signals are mixed (summing amp) and fed to a variable low-pass filter, then onto the sub power amp itself. In both of these cases, it was important to provide both left&right signals to the inputs, because the sub-woofer pre-amp section was doing all the signal processing & filtering.

    As mtv says, in the case where your existing equipment *does* have a dedicated sub-woofer output signal, that output has already been summed/filtered, and you'd use a single RCA lead to connect this, to the left RCA input of the sub (which is typically wired to be the non-inverting input of the op-amp signal path).

    In this case, just because it's a powered sub box, you know the amp board has been subjected to a fairly harsh, almost industrial duty environment (constant, strong vibrations in the 20-100Hz range), and for that reason if I can discern the sub-amp PCB is the problem, I do something 'heretic' like not even attempting to debug it, and just resolder the *entire* board and retest.

    As said, to do that, I would investigate whether or not connecting via the speaker inputs (instead of RCA) has any sway on the situation and/or connect another signal source to the unit, to see if causes any change, or the problem persists. I should point out, if this has an builtin SMPS section, it's possible the phase discriminator in the SMPS chip is being stupid (because I've seen that before, and it drove me nuts...damn knockoff tl494).

    Fortunately, these things are easy to work on typically speaking =)

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    Quote Originally Posted by GT250 View Post
    Ok,
    Been a big day

    Anyway, I turned off the Light breaker and the SUB turned on. So not connected via physical cabling.

    Loopy, the ceiling light is a standard hardwired Bunnings Circular LED saucer shaped 'thing' that I just replaced with an old existing Two bulb unit that was mopunted int he middle of the lounge room. From memory, I can't recall if it has an earth.. Maybe I'll have to check that lol!

    As Uncle Fester has asked... The Amp I use has a Single RCA connection for output. I use a 2 into 1 adaptor from the AMP that goes to the SUB.

    The Sub has two RCA inputs...!?

    I include a photo that might help;




    My oldie goldie Amp...


    Uncle Fester, thank you for your in-depth problem solving, I hope the solution isn't going to be that involved..

    Yes the photo helps a great deal
    That is an older conventional sub amp and will have a mains transformer.
    Plug it through one of those little power meters to mains and make a note of it's power consumption when it is on but not making any hum or sound.

    If it reads a lot higher when it starts to hum then we have oscillation. If it is only a little higher then it would be a (lack of)shielding hum or maybe an intermittent fault in a filter capacitor.

    Plug in an RCA cable into the sub, turn the high cut to 150Hz and volume up a bit and touch the inside pin of the other end with a finger without touching anything else. Memorise that sound. It is a deep 50Hz with a bit buzzing noise. A shielding issue would sound very similar.
    Next time the fault occurs try to compare it with that noise.
    If it sounds less deep 100Hz but more pure, what you are hearing is a sawtooth wave, then filter cap or oscillation could the culprit. This is what I call the scary hum. It also occurs if there is short somewhere and I always race to pull out mains if I hear it, anywhere really.

    In any case that sub looks very fixable.
    Last edited by Uncle Fester; 03-06-20 at 01:11 PM.
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    That is an older conventional sub amp and will have a mains transformer.

    Yep, seems so...found a wider image online -- big toroidal mains transformer at the bottom, so -not- smps related, which is a good thing usually =)



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    Just got back in...

    Ok, ok, ok...
    Jeeze, lots to tak in there...

    As MTV has said "..If the amp only has a mono sub output, it usually only connects to the left input of the sub.."
    I never knew that!
    After many moons in all this 'tech', I never knew that..!

    I will try and put the Black RCA lead - Only into the Amp - and I will disconnect the RED.

    Wotnot: Very good find there with that full length pic

    Tristen: I use a 2 into 1 adaptor going into the Amp, as the Amp has only a single RCA output to a Subwoofer. And whereby the Sub I have (anyway), looks as though it requires 2 RCA inputs..

    I know my gear is very old by todays standards, maybe 17yrs ago.
    From memory that Sub cost me $1000 and the Amp about $1200. The Amp actually does 7.1 surround, so not bad for the old tech

    Most ankle biters would probably be asking "What's a RCA lead grandad..."?

    So guys, I will do what MTV has said and put the AMP output Black to Sub Left and see what happens.
    Funny, how it's just shown up in the last few weeks...


    As Uncle Fester has suggested the cause could be a lot deeper.

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    Quote Originally Posted by GT250 View Post
    Just got back in...

    Ok, ok, ok...
    Jeeze, lots to tak in there...

    As MTV has said "..If the amp only has a mono sub output, it usually only connects to the left input of the sub.."
    I never knew that!
    After many moons in all this 'tech', I never knew that..!

    I will try and put the Black RCA lead - Only into the Amp - and I will disconnect the RED.

    Wotnot: Very good find there with that full length pic

    Tristen: I use a 2 into 1 adaptor going into the Amp, as the Amp has only a single RCA output to a Subwoofer. And whereby the Sub I have (anyway), looks as though it requires 2 RCA inputs..

    I know my gear is very old by todays standards, maybe 17yrs ago.
    From memory that Sub cost me $1000 and the Amp about $1200. The Amp actually does 7.1 surround, so not bad for the old tech

    Most ankle biters would probably be asking "What's a RCA lead grandad..."?

    So guys, I will do what MTV has said and put the AMP output Black to Sub Left and see what happens.
    Funny, how it's just shown up in the last few weeks...


    As Uncle Fester has suggested the cause could be a lot deeper.

    GT250, I think you may have missed my second post, probably because your large image was in the quotation. I should have deleted it.
    It is important that you identify the type of hum as described, so we can get to the next step.
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    Be good to put a scope on it to see what freq the hum is which will relate it to 50hz or not then half the job is done ????
    are you a self appointed hypocrite sheriff or just a nasty person

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