My plasma can cause my subs to hum. - AVS Forum
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post #1 of 17 Old 03-09-2014, 11:53 AM - Thread Starter
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When I power on my 65" Panasonic VIERA TC-P65VT50, it causes both my subs to have an oscillating hum. It seems worse with a bright / white screen. With a black screen, the hum is barely 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 hum starts.

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, hum is still there. It seems that if I plug TV and subs into same outlet or different outlets, the hum is always there. All I need to do to get rid of hum is turn TV off or make the screen black. How is this possible? Why only with a white or bright screen?

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 17 Old 03-09-2014, 12:09 PM - Thread Starter
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For a plasma TV, does a white screen consume a lot of power? If so, is it possible that the large power consumption by the plasma can cause some sort of power fluctuation that is "seen" by my two powered subs? I guess if I powered the TV via some sort of UPS battery backup system instead of a wall outlet in same room as subs and the hum goes away, i'd have my answer.

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post #3 of 17 Old 03-09-2014, 12:15 PM
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Try an automatic voltage regulator first.

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post #4 of 17 Old 03-09-2014, 12:18 PM - Thread Starter
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can you explain what is happening - just for my own edification - how can a white screen cause a sub to hum?
thanks

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 #5 of 17 Old 03-09-2014, 12:19 PM
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Yes. An all white screen uses the most power. But it's also not very common during normal content.
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post #6 of 17 Old 03-09-2014, 12:24 PM - Thread Starter
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Does the hum oscillate because the white screen is being "scanned"? One complete oscillation (the audible part anyway) takes more than one second - probably two or three seconds to do one complete oscillation of the sound coming from sub.

thanks

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 #7 of 17 Old 03-09-2014, 02:56 PM - Thread Starter
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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 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. Strange.

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 17 Old 03-10-2014, 03:36 PM
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This is most likely a grounding problem. You usually see this kind of issue in vehicles and in that situation you would use a ground loop insulator. In the home most people don't ground there equipment properly. People have a tendency to use a three prong plug and assume that there equipment is grounded. I would first start by checking the outlet to see if it is wired properly then connect a true earth ground directly to the equipment. This should resolve the grounding issue, and stop the hum. Over the years I have found that there are many electronic devices that create RF interference, however there are even more that become susceptible to it. Amps in particular are very susceptible to this. A prime example of this was several years ago when was setting up a new stereo had an issue with what appeared to be a ghost as you could literally hear someone speaking. It turns out the person next door had been using a ham radio and was sending out a huge amount of RF energy. My amp was not connected to a source but was plugged into a three prong outlet. As soon as the amp had been properly grounded the issue went away.
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post #9 of 17 Old 03-10-2014, 04:23 PM
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Let me know if you ever find a solution to this. My ZT and JL Fathom exhibit the same issues. I have tried cheater plugs and the issue remains.


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post #10 of 17 Old 03-10-2014, 04:49 PM - Thread Starter
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Quote:
Originally Posted by Travis Smith View Post

This is most likely a grounding problem. You usually see this kind of issue in vehicles and in that situation you would use a ground loop insulator. In the home most people don't ground there equipment properly. People have a tendency to use a three prong plug and assume that there equipment is grounded. I would first start by checking the outlet to see if it is wired properly then connect a true earth ground directly to the equipment. This should resolve the grounding issue, and stop the hum. Over the years I have found that there are many electronic devices that create RF interference, however there are even more that become susceptible to it. Amps in particular are very susceptible to this. A prime example of this was several years ago when was setting up a new stereo had an issue with what appeared to be a ghost as you could literally hear someone speaking. It turns out the person next door had been using a ham radio and was sending out a huge amount of RF energy. My amp was not connected to a source but was plugged into a three prong outlet. As soon as the amp had been properly grounded the issue went away.

It's possible it is ground loop issue but i don't think so.

Yesterday i found this out.

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. But this is no good because when i connect everything back up (TV, pre-pro, amps, subs), my speakers (NOT the subs) make a HUGE amount of hum/buzz. This is because now i DO have a ground loop problem since i am using one 20 amp circuit for the TV and a different 20 amp circuit for everything else.

So, back to using same 20 amp circuit in the HT room.

Today I tried a 3 prong to 2 prong power cord plug adapter (cheater plug) from Radio Shack and it didn't work. I tried it on TV and the sub power cords.

I don't think it is ground loop related because it appears when a white/bright screen is on plasma.
If plasma is powered on but on a dark or black screen, the hum disappears or is barely audible.
Also, why is the cycle time of audible hum 8 seconds or so. The sub is completely silent for about 2 seconds at end of 8 second audible cycle.

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?

Again, doubt it is ground loop related. I can plug both the TV and sub into SAME power strip and it makes no difference.
Somehow the power consumption of a white screen can be "seen" by my JL Audio subs.

I will try the voltage regulator suggested by cardinal4.

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 #11 of 17 Old 03-10-2014, 05:00 PM
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+1 for ground problem

How is your electrical panel grounded? Ground rod...water pipe?

"Man, I told you we shouldn't have shot Niedermeyer"

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post #12 of 17 Old 03-10-2014, 05:02 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 it?

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post #13 of 17 Old 03-10-2014, 05:03 PM - Thread Starter
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Quote:
Originally Posted by xfett View Post

+1 for ground problem

How is your electrical panel grounded? Ground rod...water pipe?

ground rod i bet. it was new construction 3 years ago

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post #14 of 17 Old 03-10-2014, 06:02 PM
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Perhaps play an all white signal and then turn down the intensity of the tv. Do they still hum? Does the hum volume go down? If the volume of the hum goes down then there may be a problem with the power supply of the TV.
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post #15 of 17 Old 03-10-2014, 06:35 PM
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I still recommend a separate earth ground for the stereo equipment.
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post #16 of 17 Old 03-10-2014, 07:35 PM
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Quote:
Originally Posted by Travis Smith View Post

I still recommend a separate earth ground for the stereo equipment.


I would try to ground the sub. Take a piece of wire and connect it to a good ground then just touch it to the sub amp (any place on the metal) to see if that eliminates it.

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post #17 of 17 Old 03-10-2014, 09:18 PM - Thread Starter
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I will try and report back.
Thanks

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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