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Discussion Starter #1
I have a subwoofer which hums. I have tried various scenarios to see where the hum is coming from. I'd like some help with a cause and solution.

Cable in question is a coaxial rca cable.

Gain of subwoofer is at 50% during test. Hum is audible until around 25% gain.

No Hum


  • Off. Cable/No Cable
  • On. No Cable
  • On. Cable connected, on the other end of cable the rings are shorted

Hum

  • On. Cable connected with other end connected to receiver or not = Hum

The Hum is constant through all tests when present.

Since it is perfectly silent when the cable is connected and the rings shorted I believe this is telling me that the subwoofer is fine and its ground connection is fine. This confirms the cable is picking up something as it runs through the wall. It is running about 1 inch from USB, HDMI and some 16 awg speaker wires for another speaker. The nearest ac electrical wire is 6 inch away.

Any solutions aside from running the cable again?
 

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Search 'Ground Loop'.
 

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Hum

  • On. Cable connected with other end connected to receiver or not = Hum
Any solutions aside from running the cable again?
This makes me think the cable is acting as an antenna, picking up external noise.
Run a temporary cable outside the wall and away from any possible sources of noise, if the problemis solved, you have your answer.
 

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Search 'Ground Loop'.
...and get ready to blow your brains out trying to track it down. :)

You can try connecting the cable to a battery powered device to see if the hum goes away which would confirm a current circulating in the ground. Start disconnecting things from your AVR. Cable TV feeds are often a problem.
 

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Discussion Starter #5
So some more details might help:
- The subwoofer is not plugged into the same outlet as the AVR.
- The Hum is present with AVR on or not

So I disconnected everything AVR related, absolutely all of it, Hum still present. This includes disconnecting every cable running near the subwoofer RCA cable. Disconnecting the Cable TV Cable. I even switched the breaker off for the room with all the equipment after physically disconnecting all of it. The only live outlet is the subwoofer outlet which it is plugged into alone.

The only time the hum goes away is if I SHORT the rings on the other end of RCA input cable.
 

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The only time the hum goes away is if I SHORT the rings on the other end of RCA input cable.
Try a different cable of the same length, but not inside the wall. If the hum isn't there then you know something's wrong with the cable in the wall.
 

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Discussion Starter #7
So I brought the sub to an isolated area of the house again.

Tried 3 different coaxial RCA cable.

They all HUM when just merely plugged in. Exact same hum as the cable going through the wall.

The hum also went away when I shorted the cable rings on the other end.

So now what is wrong. Problem with the subwoofer itself? It is perfectly quiet with no cable or with the cable shorted. Did I just go through 3x bad cables?

I notice if I just touch the cable where it is connected to the sub the hum changes, quiets or goes boom. This occurred with all 3 cables. Perhaps the subwoofers input connection is broken or all the cables I tried are not making a good enough connection?
 

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I was getting hum on my fathom. I was using a cheap unshielded cable thinking it wouldn't matter. For the sake of it I purchased a mediabridge shielded cable and the hum went completely away. Lesson learned, shielded RCA cables are a must.
 

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I notice if I just touch the cable where it is connected to the sub the hum changes, quiets or goes boom. This occurred with all 3 cables. Perhaps the subwoofers input connection is broken or all the cables I tried are not making a good enough connection?
I'd say the RCA connector solder connection inside the amp is loose.
 

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Discussion Starter #11
What happens when you connect the sub to a battery operated device like your phone for source?
Interesting. Took an RCA -> 3.5mm cable. It hummed when connected like all other cables. Connected it to my phone and the hum is gone. I tried this on both the subs unfiltered and filtered ports, same results all around.

So connecting the other end to a battery powered device shut it up completely.

What does this tell me?
 

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2 things...
1. There is nothing wrong with your sub
2. The hum that you are experiencing is a ground loop or something that is injecting noise on the ground wire of the house power.

Your battery powered device's ground reference is isolated from your house mains ground so there is no way for a current to circulate in the system causing the noise. The cheater's way out of this problem is to use an isolation transformer to connect your sub to whatever (AVR?). Cheap ones will have rolloff down low which may or may not be a problem and good ones that don't distort the signal are not cheap. There are scenarios where even this will not fix the noise. You mentioned that your sub and AVR(?) are on separate power circuits. Have you tried hooking both up to the same circuit?

Basically the problem is that ground is supposed to be 0V as a reference for your electronics to work properly. Something in your house either on one of the circuits on your sound system or something in your sound system itself is injecting a voltage on the ground. Try to find what that is by disconnecting/swapping components or power things from different circuits in the house.
 

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Discussion Starter #13
Very interesting.

Can I narrow the search space of this bad device by flipping breakers off around the house until the hum goes away? Then I at least know which devices to check.
 

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Take LONG extension cord and try an exterior outlet to see if it hums. If it does ask your neighbor if you can plug into one of his exterior outlets for 5 minutes to see if it hums.
Oh have you tried a cheater plug yet for a quick test? It is a $0.99 plug adaptor to take it from 3 prong to to prong (no ground)
 

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Can I narrow the search space of this bad device by flipping breakers off around the house until the hum goes away? Then I at least know which devices to check.
Unlikely this will work unless the noise injected onto the ground is from a non-local source like your HVAC or fridge or something like that. Like Haas said, cheater plug is a good tool to help track it down. If it was me and I had an AVR, I'd plug that and the subwoofer into the same outlet and disconnect everything else from the outlet and the AVR and see if the hum is still present using the AVR's internal tuner as the source. If it is, try to find something else in the house that you can turn off that might be responsible. If you can't stop the hum with just these two things connected, you're pretty well screwed and I'd look into an isolation transformer at that point.

If you get no hum with only those 2 things hooked up, move them to separate outlets to see if that changes anything and keep connecting things 1 at a time until the culprit reveals itself.
 

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Discussion Starter #16
2 things...
You mentioned that your sub and AVR(?) are on separate power circuits. Have you tried hooking both up to the same circuit?

So I went ahead and tried this. Connected the sub to the same outlet as the AVR. With the rca cable connected from sub to avr the hum went away!


So I guess I have two options, somehow get the sub and avr on the same breaker or find the source of the ground loop in the other breaker?
 

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Discussion Starter #17
Unlikely this will work unless the noise injected onto the ground is from a non-local source like your HVAC or fridge or something like that. Like Haas said, cheater plug is a good tool to help track it down. If it was me and I had an AVR, I'd plug that and the subwoofer into the same outlet and disconnect everything else from the outlet and the AVR and see if the hum is still present using the AVR's internal tuner as the source. If it is, try to find something else in the house that you can turn off that might be responsible. If you can't stop the hum with just these two things connected, you're pretty well screwed and I'd look into an isolation transformer at that point.

If you get no hum with only those 2 things hooked up, move them to separate outlets to see if that changes anything and keep connecting things 1 at a time until the culprit reveals itself.

Replied without seeing this.

The Sub and AVR connected to same outlet fixes the HUM completely.

Tthe AVR and Sub are in different rooms. Not sure what is the best solution to get them onto the same outlet, same breaker count?

I will get a cheater plug and give it a shot before doing anything drastic.
 

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Try plugging AVR/Sub into the same outlet like you just did, then try switching to the other outlet to see if one or the other is the problem. If they are both clean of noise, you might have to keep them together. If one is dirty and the other is clean, track down what else is on the dirty circuit and unplug those devices to see if it cleans it up.
 

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Keep in mind, I can't recommend a cheater plug as a permanent solution because it is potentially lethal in a worst case fault scenario.
 

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Basically the problem is that ground is supposed to be 0V as a reference for your electronics to work properly. Something in your house either on one of the circuits on your sound system or something in your sound system itself is injecting a voltage on the ground.
The ground will never be at 0v due to the resistance of the wire. The longer the length of wire between the two devices the greater the resistance and the higher the hum level.
So I went ahead and tried this. Connected the sub to the same outlet as the AVR. With the rca cable connected from sub to avr the hum went away!
That works because you made the length of ground wire between the two devices shorter.
There are a lot of well intentioned but inaccurate replies here, because those making the replies do not have a good understanding of what a ground loop is. Required reading:
http://www.rane.com/note110.html
 
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