My plasma TV causes my subs to hum - AVS Forum
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post #1 of 19 Old 03-09-2014, 03:37 PM - Thread Starter
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This doesn't appear to be a ground loop issue but please correct me if i'm wrong.

When I power on my 65" Panasonic VIERA TC-P65VT50 plasma TV, it causes both my subs to have an very noticeable oscillating hum. It seems worse with a bright / white screen. With a black screen, the hum is not noticeable. No other part of system needs to be powered on or connected. The amps, pre-pro, BD player are all unplugged from the wall. I don't have a cable box. The subs do not have their XLR cables connected to the pre-pro. All i am talking about here are the power cords for TV and subs. All I have to do is power on the TV and pick a white screen and the sub hum starts. One complete cycle of the audible hum takes around 8 seconds. There are about 2 seconds out of the 8 seconds where the subs are silent. Eight seconds is a bit weird for cycle time but i'm not a plasma expert.

The TV has a three prong power cord and so do the subs. If I plug both the TV and the subs into a power strip so that they all use same wall outlet, hum is still there. If I plug TV into an outlet across the room, far away from wall/outlet with sub but still same circuit, hum is still there. It seems that if I plug TV and subs into same outlet or different outlets in the room, the hum is always there. All I need to do to get rid of hum is turn TV off or make the screen black.

If I put 65" plasma TV on a different circuit in the house than the subs in HT room (i had to go out of HT room to an adjacent room with a really long extension cord), the very loud oscillating subwoofer hum that is audible with a white screen completely disappears and is replaced with a constant hum that is only barely noticeable with my ear right up against the subwoofer. The subwoofers appear completely silent when i'm at the MLP. It is a huge improvement.

So apparently, the plasma on white screen draws enough power for the subs to generate a hum.

The HT room is on a normal 20 amp circuit.
The breaker has never tripped while playing loud bass heavy movies but i am curious how close to 20 amps i am getting.

Possibly dumb question:

Can the 65" plasma TV on a white screen cause a big enough voltage drop in the room circuit that the subs "see" it and produce a hum? Why is the hum cycle time 8 seconds?

thanks in advance

B&W 802 Diamond mains, B&W HTM2 Diamond center, B&W 804 Diamond surrounds, Parasound HALO A 51, Krell KAV-300i , Integra DHC-80.3 pre-pro, two JL Audio Fathom f113 subs, Panasonic TC-P65VT50 65" plasma
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post #2 of 19 Old 03-09-2014, 05:27 PM
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I suspect that the signal cables to the subs are picking up some EMI from the TV. Take a look at them, you may want to replace with better shielded cables. Quad shield RG6 with solid copper core would be my fist pick.


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post #3 of 19 Old 03-09-2014, 09:15 PM - Thread Starter
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Quote:
Originally Posted by BIGmouthinDC View Post

I suspect that the signal cables to the subs are picking up some EMI from the TV. Take a look at them, you may want to replace with better shielded cables. Quad shield RG6 with solid copper core would be my fist pick.

I don't think so:
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Originally Posted by bao01 View Post

No other part of system needs to be powered on or connected. The amps, pre-pro, BD player are all unplugged from the wall. I don't have a cable box. The subs do not have their XLR cables connected to the pre-pro. All i am talking about here are the power cords for TV and subs.

B&W 802 Diamond mains, B&W HTM2 Diamond center, B&W 804 Diamond surrounds, Parasound HALO A 51, Krell KAV-300i , Integra DHC-80.3 pre-pro, two JL Audio Fathom f113 subs, Panasonic TC-P65VT50 65" plasma
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post #4 of 19 Old 03-09-2014, 09:21 PM
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if the cables are connected to the subs they can act like antennas, even if they aren't connected to the pre-pro. If you haven't tried yet disconnect the cables at the subs and see if you still have the issue.


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post #5 of 19 Old 03-09-2014, 10:03 PM - Thread Starter
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They are not connected.

B&W 802 Diamond mains, B&W HTM2 Diamond center, B&W 804 Diamond surrounds, Parasound HALO A 51, Krell KAV-300i , Integra DHC-80.3 pre-pro, two JL Audio Fathom f113 subs, Panasonic TC-P65VT50 65" plasma
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post #6 of 19 Old 03-10-2014, 05:34 AM
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have you tried lifting the grounds on the sub amps?


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post #7 of 19 Old 03-10-2014, 09:32 AM - Thread Starter
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That was next on my list. That is the 3 prong to 2 prong plug adapter right? Available at Home Depot.

B&W 802 Diamond mains, B&W HTM2 Diamond center, B&W 804 Diamond surrounds, Parasound HALO A 51, Krell KAV-300i , Integra DHC-80.3 pre-pro, two JL Audio Fathom f113 subs, Panasonic TC-P65VT50 65" plasma
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post #8 of 19 Old 03-10-2014, 09:50 AM
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right, basically disconnecting the ground wire.


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post #9 of 19 Old 03-10-2014, 10:01 AM - Thread Starter
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I am trying to find on HD website now

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post #10 of 19 Old 03-10-2014, 02:05 PM
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just a cover my @zz statement. disconnecting the ground can be dangerous and may damage equipment so proceed with caution.


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post #11 of 19 Old 03-10-2014, 04:34 PM - Thread Starter
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it didn't work

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post #12 of 19 Old 03-10-2014, 05:04 PM - Thread Starter
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Is it possible the big amps in the subs can "see" RFI from the plasma when the plasma is working hard at making a white screen?
Maybe the plasma is emitting so much energy through the air that the sub amps are picking it up?
Maybe when the TV is on different circuit, it kind of changes the cycle or something so that the sub amps don't see the RFI?

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post #13 of 19 Old 03-10-2014, 06:11 PM
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You could leave it plugged into the outlet causing the hum and move the sub as far away from the TV as possible, that would answer that question


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post #14 of 19 Old 07-14-2014, 11:37 AM
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Quote:
Originally Posted by bao01 View Post
Is it possible the big amps in the subs can "see" RFI from the plasma when the plasma is working hard at making a white screen?
Maybe the plasma is emitting so much energy through the air that the sub amps are picking it up?
Maybe when the TV is on different circuit, it kind of changes the cycle or something so that the sub amps don't see the RFI?
Were you able to figure out what the issue was?

I have the same exact problem with my ZT60 and JL F112. It has followed me to my new house as well.


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post #15 of 19 Old 07-14-2014, 07:57 PM - Thread Starter
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Unfortunately no. I am living with the hum when screen is white/bright. There appears to be no good answer. I tried a lot of things. sorry

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post #16 of 19 Old 07-15-2014, 09:03 AM
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Originally Posted by bao01 View Post
Unfortunately no. I am living with the hum when screen is white/bright. There appears to be no good answer. I tried a lot of things. sorry
How close are the subs located relative to the plasma TV? If they are close, does moving the subs away from the TV affect the level of the hum?


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post #17 of 19 Old 07-15-2014, 09:31 AM - Thread Starter
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6' and 22'
no

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post #18 of 19 Old 07-15-2014, 09:36 AM - Thread Starter
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i am hoping that when i change to a nice projector and screen instead of plasma, the problem will go away

B&W 802 Diamond mains, B&W HTM2 Diamond center, B&W 804 Diamond surrounds, Parasound HALO A 51, Krell KAV-300i , Integra DHC-80.3 pre-pro, two JL Audio Fathom f113 subs, Panasonic TC-P65VT50 65" plasma
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post #19 of 19 Old 08-30-2014, 12:02 PM
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I have this exact same problem. Panasonic TC-P65ZT60, two Martin Logan Descent-i subs.

Happens when screen is bright (mostly whites), goes away when screen is darker.

Subs are about 5' from panel, and can't be relocated due to room layout.

Full-range left and right RCA from power amp goes to subs' full-range inputs, and XLR .1 channel (Y'd at preamp) goes to subs' .1 input (configuration reviewed and approved by Martin Logan).

Kuro didn't do this--we only heard only very quiet sub hum (and if you look at our system per my signature, you'll see the metal conduit and grounding and Furman we used to minimize the hum).

Having already had to add vents to quiet the fan noise on the Panasonic (Panasonic ZT60 and VT60 Fan/Fridge Noise Production Numbers), this latest problem is particularly disheartening.

My system? Google for: Martin Logan 420 CLX Descent Stage Summit
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