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Discussion Starter · #1 ·
Adding a new subwoofer to my HT setup last night seems to exposed a latent hum/buzz problem that wasn't noticeable with the previous placeholder woofer. I'm looking for confirmation of my analysis and suggestions for what to do next.


The relevant parts:

Sony KF60WE610 LCD rear projection TV

Sony STRDA1000ES Receiver

Kef PSW3000 woofer

5x M&K S-85s (center + 4 sides)

Panasonic DVD-S25K DVD player (no optical audio out, only coax)

XBox


The connections:

Receiver->M&K speakers

Receiver->Woofer via RCA plug

DVD coax audio out -> Receiver

DVD component video out -> TV

XBox optical audio out -> Receiver

XBox component video out -> TV

TV->Wall jack via coax

All components -> same power strip/surge protector

Coax is not going thru surge protector

All cables are 'one step up from sucky' - coax audio is Acoustic Research from Fred Meyer, component video is GE from Target, 14AWG speaker wire from Lowes. Tried various Receiver->woofer cables (mono RCA) I had laying around (including the AR coax audio), nothing seemed to make a difference.


The symptom:

When I connect the woofer via RCA plug, the woofer starts humming fairly loudly, and the M&K speakers all start buzzing fairly loudly. If I turn the receiver off completely, the woofer continues to hum.


My initial diagnosis was "unplug the DVD video out, and the symptom goes away". I could play XBox (which included a similar component video connection to the TV) with no problem, and get fine sound. So at first I thought that the DVD player was at fault. Then I swapped in a different DVD player and got the same buzzing as soon as the video was hooked up, so I started looking at the TV. By unplugging various TV inputs, I realized that disconnecting the coax TV cable coming from the wall made the symptoms disappear. Reconnected everything except the coax TV input, and it all runs fine. I tried running the coax cable to a different wall jack, and got the same symptom.


My assumption is this is a ground loop problem.


Question 0) Does the ground loop conclusion sound right?


Question 1) Why didn't the XBox's connection to the TV incur the same symptoms as the DVD players? My supposition: since XBox was connected via optical audio instead of coax audio, this broke the ground loop. I know diddly about the physical properties of optical vs coax, is this reasonable?


Question 2) Why didn't my old subwoofer experience this problem? It was a ~40W placeholder piece of junk, the new one is 300W. Is this increase in wattage a typical thing to expose/amplify grounding problems, or might this indicate a problem with the woofer?


Question 3) Assuming everything above is correct, what can I do to fix the coax input? This is in a ~35 year old house, and previous owners did some of the cable jacks, so I can imagine subpar installation there. I've seen surge protectors with in/outs for the coax input, might running the coax input thru the same surge protector all the other stuff is plugged into 'connect their grounds' and eliminate the problem? I don't have one to experiment with.


Question 4) I'll need to order digital cable soon, and am currently connecting to a jack in another room - will part of the digital cable installation require new wiring/jack anyway?


Question 5) I understand cable shielding can contribute to humming, but I'm pretty sure shielding isn't an issue given that moving stuff around & swapping cables made no perceptable difference. Is it correct to focus on the wall jack as the source of the problem?



Any insight appreciated!

-D
 

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You definitely have a ground loop. Its difficult to diagnose exactly given the complexity of your post but here are some suggestions.


Does your new subwoofer have a 3 prong power plug? If it does try a 3 to 2 prong adapter without the ground wire connected. Often this is enough to break the loop but be advised this also removes the saftey ground from your sub. A minimal risk IMO but not the ideal solution at any rate. If you are uncomfortable with this isolation transfromers are available to isolate the ground provided by the RCA connection to your A/V reciever.


Cable TV connections are the single greatest source of ground loop problems out there next to saftey grounded subs because of the huge area encompassed by the loop. Try inserting the cable in only partially so that only the center conductor makes contact. I think isolation transformers are available for this connection also.
 

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Discussion Starter · #4 ·
More experimentation showed that the cable shield not being grounded adequately was indeed at least contributing to the problem. Adding an ISO-MAX isolater completely fixes the symptoms, however I have only an analog cable hookup right now, so I don't know what'll happen when I get digital and/or HDTV going...
 

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I had the same issue. I connected the cable tv wire to my panamax surge protector and then to the cable box and the problem was fixed. Some how the panamax either filtered the hum or broke the loop. Either way I was happy to have fixed the problem.
 

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Had a similar problem with a sub humming. Replaced an expensive cable with a subwoofer cable from www.bluejeanscable.com and that fixed the problem. I also purchased from them Calrad Isolation Transformer for $25 in case the cable didn't fix the problem. Hum is gone with just the new cable and will return the isolation transformer. BTW, they have a 30 day return policy.
 

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if it is 50 or 60 hz hum you have a grounding problem


if higher pitch probably a frayed wire touching ground
 

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Quote:
Originally posted by cpu8088
if it is 50 or 60 hz hum you have a grounding problem


if higher pitch probably a frayed wire touching ground
I have a persistent hum in my system - HK AVR 510 receiver. The hum sounds whether all the components in the system are plugged into the same outlet (via powerstrip) or not, and when I plug the system into an outlet on a different circuit. It is also audible whether or not any other components are switched on, or even plugged in.


Finally, the OSD on the receiver swims slightly onscreen, though it seems to do this only for the first few minutes after the receiver is switched on.


One other thing. I've had the unit for 3 years, and I swear I only started hearing the hum a month ago - and I have made no changes to the system in the last while.


Any ideas????
 
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