Originally Posted by Travis Smith
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.