I have been having what appears to be the same hum as in this post

http://ubb.outlawaudio.com/ubb/ultimatebb.php?/ubb/get_topic/f/41/t/000156.html

I noticed this hum years ago but since the amp was in an open rack and I was using a RPTV I rarely noticed it.

Now I have a plasma tv and I have moved the amp into a more enclosed rack the sound is much more noticeable. I am certain it has something to do with power or filtering ( all of the components and tv are plugged into a monster power conditioner) as when I plugged my new tv directly into the wall socket the humming completely stopped. I am assuming I need to buy some other power conditioner. But which ones besides the one mentioned in the post above have people found will get rid of this hum?

Does the hum come and go or is it constant?
My 7500 had a hum that would come and go constantly and I bought a Channel Islands Audio XDC-2 AC filter and it completely stopped the hum.
Apparently my problem was with standing DC on the line for some reason.
I didn't have this problem for the first few months and I don't know what changed but this little unit took care of the problem.

The "DC" is caused by high amounts of 2nd order harmonic distortion on the AC line. The even harmonic components are asymmetrical, and as such, cause an offset voltage which is like DC.

Altec what is the best means for removing or minimizing the 2nd order harmonics?

There is no device or filter which will remove low order harmonic distortion from the power line short of regeneration of the AC (and even this has it's problems).

The best way to solve the DC problem is to run the AC hot lead through two back-to-back electrolytic capacitors (negative-to-negative) of around 10,000uF (or more) and about 200VAC rating. I wouldn't try this at home though unless you know what you're doing because these voltages can be hazardous.

The hum definitely comes and goes. It is not constant.

It sounds exactly like what I had going on.
The Channel Islands AC filter did the trick for me.
It was $300.00 but it was worth it.
You might contact Outlaw because I think they also have an AC filter available.

I have all my A/V gear going through this:

http://www.mitcables.com/?option=com_vir...9&product_id=94

From there, a PowerMax bar is connected to the TV, DVD, VCR, HD/DVR box, and a Rotel bar is connected to the 990, 7500, LFM1Plus, CD player.

I don't get any hum from my 7500, either through the speakers for from the unit it self.

I would try repowering the system from different receptacles in the house via an extension cord to find a circuit which is clean. If nothing improves try looking for a florescent light or other device which might be causing the hum. Since it comes and goes I would start with the house circuits then the system.

I bought the Channel Islands Audio XDC-2 AC filter and that completely fixed the problem. All kudos to channel islands audio. Whatever they are doing in that little box is working perfectly.