Subwoofer hum without a source (LFE cable) - AVS Forum
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post #1 of 28 Old 02-13-2010, 08:15 PM - Thread Starter
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After a tremendous amount of research and searching to no avail, I am hoping to find some help from the AVS Forum community.

My subwoofer has been producing a very noticable hum. For easier reading, here's the setup of the problem:
  • Both the subwoofer and receiver are double prong, so from what I understand this excludes me from the "ground loop" problem
  • The volume of the hum is directly affected by modifying the volume and Hz controls on the subwoofer
  • The hum only occurs when the receiver it is connected to is off - when the receiver is on, the hum completely disappears.
  • The subwoofer is connected to the receiver via LFE
  • Removing or disconnecting the coaxial cable from the cable box has no affect
  • And perhaps most interestingly, with the subwoofer plugged in without any inputs (just the power cord), there is no hum - however the moment I plug in the LFE cable (even with NO connection on the other end) the hum presents itself. To recap that point, with the subwoofer plugged in to power and the LFE cable going from the sub to absolutely nothing, the hum is produced.

From what I've read, no one has quite had a problem like this. I have my immediate assumptions, but for the sake of not wanting to "taint" the potential results from the community, I would rather not post them. If you have any ideas what might be causing this kind of behaviour, please share your insights!
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post #2 of 28 Old 02-13-2010, 08:53 PM
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It hums when the receiver is off but not on? Weird. Have you tried a different cable to the sub yet?

Stephen.

Chances are very good that I was drinking when I posted the above.

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post #3 of 28 Old 02-13-2010, 09:17 PM - Thread Starter
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EDIT: Misread what you originally typed. Yes, it is unusual - the subwoofer hums only when the receiver is off. When the receiver is turned on, the hum disappears. I have ordered a new cable from monoprice in attempt to see if that rectifies the issue. Shipping date is apparently the 15th.
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post #4 of 28 Old 02-13-2010, 09:22 PM
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Here is something you may want to check. If your sub cable has ends on it that are threaded on. Check to see that the ends are on tightly. If they have come loose this may cause a hum.
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post #5 of 28 Old 02-13-2010, 09:31 PM
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My bet is that your amplifier is bad.
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post #6 of 28 Old 02-13-2010, 11:04 PM - Thread Starter
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Thanks for your suggestion, SecretSquirrel. I tried tightening the ends on the cords, and although they were not originally as tight as they could have been, upon tightening the problem still persisted, even without a source on the other end of the cord.
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post #7 of 28 Old 02-14-2010, 02:00 AM
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Have you tried a different cable? Might still be an electrical issue if swapping the cable doesn't help.

Try plugging the power from the sub into the same outlet as the receiver and power up the sub with the receiver off. Still hums?



Also, as mentioned, could be an amp issue as well.

ala Yoda...

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post #8 of 28 Old 02-14-2010, 02:32 AM
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Are your sub and AVR on the same power circuit?

Are they on the same phase of the same power circuit?

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post #9 of 28 Old 02-14-2010, 04:23 PM - Thread Starter
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Quote:
Originally Posted by cavu View Post

Are your sub and AVR on the same power circuit?

Are they on the same phase of the same power circuit?

For the sake of isolating the problem I took the sub and lugged it to my second floor on the opposite side of the house and plugged it in up there. Just like downstairs, when plugged in with power itself, it does not produce the hum, but immediately upon plugging in the LFE cable, even without a source, the hum starts right up again.

I think being that I have done this, plus the fact that the receiver and sub are, as mentioned, only double pronged, we can safely rule out any power related issues.
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post #10 of 28 Old 02-14-2010, 05:24 PM
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i had a somewhat similar issue that was solved by using rg6 quad shield cable to the sub.

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post #11 of 28 Old 02-14-2010, 07:39 PM
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Quote:
Originally Posted by JamesLai View Post

the fact that the receiver and sub are, as mentioned, only double pronged, we can safely rule out any power related issues.

You didn't answer my question: are they plugged into the same outlet, on the same phase??

The fact that there is no safety ground is irrelevant to the sort of symptoms you are experiencing!

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post #12 of 28 Old 02-14-2010, 10:23 PM - Thread Starter
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Quote:
Originally Posted by cavu View Post

You didn't answer my question: are they plugged into the same outlet, on the same phase??

The fact that there is no safety ground is irrelevant to the sort of symptoms you are experiencing!

I believe the fact that I disconnected the sub, moved it to an entirely other part of the house, and plugged it into an outlet on a completely different breaker should answer this question, but if I'm wrong please let me know.
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post #13 of 28 Old 02-14-2010, 10:29 PM
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Quote:
Originally Posted by JamesLai View Post

I believe the fact that I disconnected the sub, moved it to an entirely other part of the house, and plugged it into an outlet on a completely different breaker should answer this question

Is the question so hard to answer?? Either they are or they are not. I'm not sure what moving the sub to another room has to do with that.

The sub and the AVR SHOULD BE plugged into the same circuit/phase to eliminate the potential for ground loop effects.

If your AC outlets are split plugs, ie. three wire where the upper and lower sockets are connected to opposite phases, that could cause your problem. If they are plugged into different receptacles on different circuits/phases then that could cause the problem.

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post #14 of 28 Old 02-14-2010, 11:59 PM - Thread Starter
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Unfortunately not being terribly familiar with electrical, I have no idea how to answer your question. Thanks anyhow.
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post #15 of 28 Old 02-15-2010, 01:07 AM
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Quote:
Originally Posted by cavu View Post

are they plugged into the same outlet?

Quote:
Originally Posted by JamesLai View Post

I have no idea how to answer your question.


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post #16 of 28 Old 02-15-2010, 08:56 AM - Thread Starter
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Quote:
Originally Posted by cavu View Post


For christ's sake, if I have moved the subwoofer to the other side of the house and the problem still persists, do you honestly need me to answer if they're plugged into the same outlet? I assumed since you continued to press the issue that your question somehow did not involve this point.
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post #17 of 28 Old 02-15-2010, 02:55 PM
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Quote:
Originally Posted by JamesLai View Post

For christ's sake, if I have moved the subwoofer to the other side of the house and the problem still persists, do you honestly need me to answer if they're plugged into the same outlet? I assumed since you continued to press the issue that your question somehow did not involve this point.

He wants you to plug the sub into the same outlet that your AVR is in.
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post #18 of 28 Old 02-15-2010, 03:09 PM - Thread Starter
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Quote:
Originally Posted by Fanaticalism View Post

He wants you to plug the sub into the same outlet that your AVR is in.

Alright - this is the normal setup of the subwoofer and AVR and the problem is still produced.

As a side note, being that the subwoofer produces the hum completely independently of the AVR it seems this would be a bit of a moot point, but perhaps I'm missing something.
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post #19 of 28 Old 02-15-2010, 04:15 PM
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Trust me - it's the cable - it is acting like an antenna picking up the 60 cycles from your house wiring along its lenth.

With the cable disconnected at the receiver you get the hum right? - now disconnect it at the sub and it stops right?

Have you at least tried a different/better cable?

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post #20 of 28 Old 02-15-2010, 04:26 PM - Thread Starter
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Quote:
Originally Posted by TJ7 View Post

Trust me - it's the cable - it is acting like an antenna picking up the 60 cycles from your house wiring along its lenth.

With the cable disconnected at the receiver you get the hum right? - now disconnect it at the sub and it stops right?

Have you at least tried a different/better cable?

Yes, you've described the scenario accurately - I'm hoping you're right!

I have ordered a new cable from monoprice, apparently is arriving today, though perhaps tomorrow is a bit more likely. I didn't specifically order anything with special shielding, though perhaps I should have? This is the item I ordered as a replacement:

http://www.monoprice.com/products/pr...seq=1&format=2
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post #21 of 28 Old 02-15-2010, 05:40 PM
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Good move, that should do it.

The only other possibility is some sort of grounding issue, but doubtful based on what I read.

Long runs of unshielded and unbalanced cables can wreak havoc.

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post #22 of 28 Old 02-15-2010, 07:47 PM
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remember however that extreme circumstances call for extremefixes - consider an rg6 quad shield coax with rca adapters if the "replacement cable" doesn't work completely.

I have a 40 run that was driving me crazy with "hi-grade" subwoofer cable. rg6 quadshield fixed it entirely and that's running right beside 120v feeds to other areas of the house and right past the main electrical panel of the house too.

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post #23 of 28 Old 02-15-2010, 08:48 PM - Thread Starter
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Thanks for the advice - I'll follow up here if the replacement cable fixes the problem (it didn't arrive today, which is understandable...Presidents Day).
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post #24 of 28 Old 02-18-2010, 12:18 AM - Thread Starter
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Just to follow up - got the new cable, and indeed the problem was the cable all along. Crazy! Fixed a $40 cable by replacing it with a $1.22 one.
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post #25 of 28 Old 11-15-2010, 02:29 PM
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Sure others having hum problems with their sub I was having one also and scouring the internet for ideas. Found a good site which I will go back and post to but wanted to give something back here as been helped here for alot of years. By the other post 1st thing I did was disconnect the cable from the mot dvr setup box. Hum stopped, tried the power strip with the in out coax and like the post said sometimes gets worse and it did. Was gonna try the transformer from RS next but that costs money so figured it was the cable no question as also plugged the sub into the same outlet, ran an extension cord still the hum. Went to the receiver after re-connecting the cable to get the hum and I had originally hooked up a coax digital cable for sound from the setup box to my receiver, unplugged it the hum stopped been living with this hum for awhile but finally I was determined to fix it. Luckily I have optical inputs that are switchable so hooked up a toslink cable from the mot box for sound pulling the coax digital cable out and end of the hum so one more thing to try.
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post #26 of 28 Old 11-16-2010, 08:24 AM
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Since you have stated the problem exists when the system is removed from the sub, but only occurs when the cable is inserted to the subwoofer, here are the most likely causes:

1. Your subwoofer's volume is turned up too high. Audyssey and other automatic level systems will often set the LFE gain very low because the will ask you to turn the subwoofer volume up half or all the way. While the receiver is on the signal it is sending drowns out any ambient line noise. Once you power down the receiver or disconnect the LFE cable from the receiver you introduce open gain which will create a hum. Check your LFE settings in your receiver to ensure they are set between 0 and 3dB then adjust your sub volume down accordingly.

2. Your cable is bad. It happens, test with a short inexpensive cable you have laying around to verify.

3. You have RF issues. Have you added any RF light dimmers or lighting control systems to your home? Does a neighbor have a HAM radio set up? Try taking the subwoofer to a friend or family member's home and see if the problem persists.

When in comes to audio there are seldom right and wrong answers.  The opinions of "experts" are just that, and should be used as a suggestion only.  Remember at the end of the day it is your money, your system, and you will need to listen to it.
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post #27 of 28 Old 05-08-2012, 09:48 PM
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Let's see about picking up this topic

I have the same problem - sub hums when receiver is off.

I have the two connected with a 75 foot XLR cable with RCA to XLR Transformer (Balun) adapters on each end. I figured this should 100% eliminate any ground loop problems and with the balanced nature of the XLR- I figured I'd be good... but alas- still have hum.

Thoughts?
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post #28 of 28 Old 05-09-2012, 07:36 AM
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Quote:
Originally Posted by kevinvinv View Post

Let's see about picking up this topic

I have the same problem - sub hums when receiver is off.

I have the two connected with a 75 foot XLR cable with RCA to XLR Transformer (Balun) adapters on each end. I figured this should 100% eliminate any ground loop problems and with the balanced nature of the XLR- I figured I'd be good... but alas- still have hum.

Thoughts?

I have a similar set up. The product I found to eliminate interference and hum was this isolator:

http://www.amazon.com/ART-DTI-Hum-El...sr=1-2-catcorr

I would recommend purchasing from a site that allows a return policy without a reshelfing fee. ...that way if it doesn't solve your problem, you are not out $50 or X percentage penalty when returning it.

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