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sub hum only when lfe is plugged in??

3087 Views 8 Replies 3 Participants Last post by mcnizzel, Aug 17, 2011

mcnizzel

Discussion Starter · Aug 17, 2011

I have read a bunch on ground loop problems, but I still don't understand whats happening in my case specifically.

here are the facts.

jbl sub160

does not hum when plugged into the wall on its on and powered on

when powered on but connected to the receiver, using LFE it hums

when plugged in to an outlet and with an lfe cable plugged into the sub, but not connected to the receiver or anything else, it still hums??

do I actually have a ground loop problem? or is there something wrong with the sub?

from what I've read I need the audio cord connected and the power cord connected to get a ground loop problem, but maybe I'm misunderstanding.

I just find it weird that there is no hum if its just plugged in and powered on, but that if you plug a cable into the LFE but don't even connect it to anything it still hums. It does seem like the huming is a touch quieter when the lfe is plugged into nothing than when it is connected to my receiver

any tips or suggestions would be greatly appreciated. I have had no issues like this with anything in my set up including my existing sub, I'm trying to upgrade to the jbl but it gives me nothing but issues in my set up.

thanks in advance

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sinistersix

· #2 · Aug 17, 2011

I am basically in the exact same boat right now. My sub hums only when the LFE cable is connected, and the hum is actually louder when my receiver is turned off. This suggests that the LFE cable I have is picking up some noise on his own, and when the receiver is on the buffers and impedance help reduce it, but its still there. I am going to get a shorter/better LFE cable and see if that fixes the problem.

So far I havent found any cable isolaters that work wit HD satellite directTV, so not sure if I should try that, though I have read a lot about cheater plugs. They seem to fix the hum most of the time, but people say they are dangerous. Would be interested to hear updated info on the idea of permanently installing cheater plugs into a system. I cant turn my sub up higher than 1/4 gain without a bad hum....

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DonH50

· #3 · Aug 17, 2011

A ground loop occurs because the ground of one component is not the same as the other, making a loop through the cable and/or power line. Any time there is more than one ground path (say through the power cord and the signal cable) you have a loop, but whether it is a problem depends upon many other things.

If you are getting hum with the cable plugged into the sub alone it could mean the cable is picking up noise or you have a bad cable. If you short the open end using a clip lead or short wire (short it with the sub off, then turn it on), does the hum go away? If not, bad cable. Could still be a bad cable anyway, and I would try another cable to start with.

Getting both units into the same outlet may fix the problem by shorting the loop path, but it might not. A cheater plug breaks the loop; the issue is without the safety ground there is a potential shock hazard. In practice, that is unlikely to be a problem because ground is established through the signal cable (only), and few people are touching their sub's ground whilst also connected to some other ground (it can happen, e.g. when plugging in the cable -- just unplug the sub, plug in the cable, then plug in the sub). It is non-UL but many folk use cheater plugs on many things with no problem. (Of course, if there is a problem, it can be dangerous.) The (UL-approved, or at last components pass with this) circuit fix is normally to insert a small (low-value, 2 - 10 ohm) power (1 - 2 W) resistor in series with the safety ground, breaking the loop by raising the impedance at the sub but still providing a low impedance overall so you don't get shocked.

Satellite systems feed d.c. to the dish to power the LNB (RF amp). It takes a special isolator to pass d.c. whilst blocking hum. I have seen a few advertised, but they are harder to find and cost more. I seem to recall Part Express having one, and I think Jensen makes them, but I'd have to look. Search for satellite ground isolators.

HTH - Don

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mcnizzel

Discussion Starter · #4 · Aug 17, 2011

thank you so much Don

I'm going to try a ground loop eliminator from radioshack and see if that has any effect. my sub power plug does not have a ground, but it is polarized.

I confess I do have a cheap RCA plug for the sub now, but I've never had any problems before, I'll try a better one and hope that helps.

from what I've read its likely the amp is still in working condition since the hum doesn't happen unless its attached to the reciever there by completing a ground loop?

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DonH50

· #5 · Aug 17, 2011

Hmmm... If your sub does not have the third prong for the safety ground, it is unlikely (not impossible) to be a ground loop. Hopefully it's not a bad amp, but that is a possibility... Get another cable to try, too. It probably does not need to be "better", just different/newer. I have had the wires fail on some older inexpensive cables; does not mean they were bad, just worn out!

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mcnizzel

Discussion Starter · #6 · Aug 17, 2011

will do

is there a way I can verify a bad amp for sure?

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DonH50

· #7 · Aug 17, 2011

Assuming you aren't a techie and want a quick check, plug it into a known-good source (LFE or a preamp output) and see if it sounds OK.

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sinistersix

· #8 · Aug 17, 2011

without a grounding pin it probably isnt a ground loop, I would guess its your cheap RCA laying over power or picking up interference. Change that out and let us know, I am changing mine today and will report back as well.

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mcnizzel

Discussion Starter · #9 · Aug 17, 2011

will do, I also got one of those ground loop islator things from radioshack.

it goes between my sub and receiver. I'm curious to see if it helps at all, or if just a new, better quality sub line will help. I hope one or the other fixes it, I'm not in the mood to go sub shopping again haha

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