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Humming sound in subwoofer after it's been playing for a while

Hi!

My Sunfire True Subwoofer Signature Mk II is starting to behave a bit strange. After the subwoofer has been playing relatively loud a humming sound sometimes remains and won't go away. If I remove the signal cable it goes away, and returns when I reconnect it.
If I turn the volume up and down the humming sound changes in volume, and sometimes when the volume is close to max a thumping sound is heard and the humming stops.
If I unplug the power and plug it in again the humming disappears and doesn't come back until I play loudly with the subwoofer again.

Could you please advise? I have tried changing the subwoofer signal cable without any effect, and I have tried removing all other cables from my reciever, also without any effect.

I bought the subwoofer used about half a year ago, and I think it's about 7 years old in total and during the time I've owned it, it has worked perfectly until for a few days ago when this started happening.

BR
Henrik Larsson
sigma@home.se

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  • Anonymous Jan 11, 2010

    I have now tested the earth loop possibility, and from what I have found this doesn't cause the problem.

    I've connected a galvanic isolator (is that the correct english name for the component?) that's supposed to separate the ground the device gets from the signal from the one it gets from the power. I connected it to the signal cable between subwoofer and reciever. With it connected the humming still starts, and when it has started it continues even if I remove the signal cable from the reciever (with the isolator still connected to the subwoofer).

    If I remove the galvanic isolator or disconnects the signal cable at the subwoofer it stops, and resumes if I reconnect.

    So it seems something is broken inside. My options now are either to hand it in to some electronics person, or to open it myself and see if there's anything obvious I can do without much problem.
    Would you consider this an option, and have any suggestions on what to look for, and what to do if I find it?

    First problem then is to get inside it. It seems to be some kind of special tools required to remove the panel. Any tips regarding how to get in?

  • Anonymous Jan 11, 2010

    I have now tested the earth loop possibility, and from what I have found this doesn't cause the problem.

    I've connected a galvanic isolator (is that the correct english name for the component?) that's supposed to separate the ground the device gets from the signal from the one it gets from the power. I connected it to the signal cable between subwoofer and reciever. With it connected the humming still starts, and when it has started it continues even if I remove the signal cable from the reciever (with the isolator still connected to the subwoofer).

    If I remove the galvanic isolator or disconnects the signal cable at the subwoofer it stops, and resumes if I reconnect.

    So it seems something is broken inside. My options now are either to hand it in to some electronics person, or to open it myself and see if there's anything obvious I can do without much problem.
    Would you consider this an option, and have any suggestions on what to look for, and what to do if I find it?

    First problem then is to get inside it. It seems to be some kind of special tools required to remove the panel. Any tips regarding how to get in?

  • Anonymous Jan 11, 2010

    I have now tested the earth loop possibility, and from what I have found this doesn't cause the problem.

    I've connected a galvanic isolator (is that the correct english name for the component?) that's supposed to separate the ground the device gets from the signal from the one it gets from the power. I connected it to the signal cable between subwoofer and reciever. With it connected the humming still starts, and when it has started it continues even if I remove the signal cable from the reciever (with the isolator still connected to the subwoofer).

    If I remove the galvanic isolator or disconnects the signal cable at the subwoofer it stops, and resumes if I reconnect.

    So it seems something is broken inside. My options now are either to hand it in to some electronics person, or to open it myself and see if there's anything obvious I can do without much problem.
    Would you consider this an option, and have any suggestions on what to look for, and what to do if I find it?

    First problem then is to get inside it. It seems to be some kind of special tools required to remove the panel. Any tips regarding how to get in?

  • Anonymous Jan 11, 2010

    Sorry for the double post above. I don't know how to remove it.

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It sounds like you are describing an earth loop problem. I won't go into detail in this post, but try an isolation test first. disconnect everything from the reciever except the sub and try connecting say a BATTERY powered mp3 player or cd player to minimise connections to the electrical mains earth. you could also isolate things by taking your sub to a friend's house and trying it on their system.
If it is an earth loop use a process of elimination to find out which two pieces of equipment causes the earth loop. then apply filtering.
you can buy earth loop isolation transformers for about AU$26 and I've used one on my sub myself. Theyhave RCA (phono)plugs on each end and they connect inline with the signal cable to the sub amp.

as for the thumping sound it might be the speaker protection circuitry cutting off the amp to protect the subwoofer speaker from damaged. This might be triggered if the earth loop sound is causing the sub amp to overload and clip.

Failing this there might be a dried out overheated electro capacitor in the amp circuit causing the hum. Hav a chat to your local electronics person, who should be able to spot it!
Hope this puts you on the right track
Happy Hunting!

Posted on Jan 07, 2010

Testimonial: "Thank you for your explanation of the possible causes for this error. I will investigate with the methods you suggested! :-)"

  • robroycabler Jan 13, 2010

    Good that you narrowed it down to the one piece...

    If you open it(the amp), look for Dried up electrolytic capacitors, they are the large cylindrical cans that vary in size, they have voltages and microfarad ratings on them (uF). The larger ones (bigger than 470uF e.g) and ones located near a heat source i.e transformers and output transistors may be more susceptible to drying out. when they dry out they expand slightly and the top or bottom pops open or bulges. you can notic a split in the plastic covering or stretching as a minor fault to the top bursting open as a major. these can be replaced if you can solder. note down the polarity this is marked with a stripe and/or minus sign they are dangerous if connected the wrong way around. they also have a temperature rating 85 or 105 degrees and a voltage rating 50v, 100v 1000v etc. if the original was 50v and you cant get a 50v one, get a 63volt or 100v one- go over voltage but not under.
    For opening it up if you can remove the speaker driver you might get a clue on removing the amp. the one i've got is a one piece amp which screws onto the box in 8 places. Other than that I have no other experience removing amps from active subwoofers..
    Good luck


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I'm experiencing same issue with my Sunfire True Sub wondering if Henrik Larsson found out the fix?

Posted on Aug 19, 2015

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Posted on Jan 02, 2017

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