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.

Thought I'd 'resurrect' this thread as I'm having the hum/buzz from my 7500. Never noticed it before, but now that I've moved my amp I notice it. Starts as a cyclic hum/buzz (softer to louder) then after the amp's on for a couple hours it's at a constant level that you can hear when the volume is muted. Amp is cool as can be, and no noise from the speakers. Is the only 'fix' a ~$300 filter (either PS Audio Humbuster II or the Channel Island XDC-2)?

That is what I had to do to take care of the same problem with my 7500.
I tried different outlets, unplugging the fridge and other electrical devices but to no avail.
I went with the CI XDC-2 and all of the problems went away.

That's what I figured I might end up going with, but when I showed my wife a pic of the CI filter and told her it was $300 she about stroked. I was hoping there was another 'cheaper' possiblitiy for a fix out there. I didn't have this issue with the 7125 or the 7075. Apparently it can be a common issue with amps and residential A/C service, but it's kind of sad that you have to spend another $300 to 'fix' a $1,500 amp.

I feel your pain.
Outlaw was going to start selling a DC filter but I am not sure if they ever did but you might want to call them and ask.
You would think they would have a better price for their customers.

I will give them a call and check, but I'd guess if it was a product they offered it would be on the website. Probably one of those things that would be nice to offer, but not a big seller, so it was never developed. At this point, looks like I'll either have to live with it, pony up the $300 for a filter, or sell it and move on with something else.

Sounds like a ground loop hum. probably 60hz. Look at all your grounds and make sure they are all referenced to the same point. This includes you antenna or cable. Try disconnecting them one at a time to see it the hum goes away. If you have two different grounds of slightly different values it can set up a current which generates the hum. Plug into a dedicated power receptacle or get a power conditioner whick you plug all of your equipment into so a common ground is guaranteed.

More than likely it is standing DC on the line rather that a ground loop but that is worth investigating.
I have been told that this can come from a number of different sources including your neighbors that are on the same transformer.
When my 7500 started to do the cyclic hum I thought it was from a new fridge that we bought so I unplugged it and nothing changed.
So I do not know where my problem actually came from.

I thought ground loop hum was audible through the speakers? I don't have that at all...it's just right from the amp. I've had the amp (and everything else) plugged into a Belkin surge protector, but I tried plugging the amp directly into the wall outlet and no change. The only connection I have with my satellite receiver to the preamp is via an optical connection. It's connected to the TV via HDMI. I guess I could try a 'cheater' plug on the amp to see if that's the cause. I'm leaning more toward it being he standing DC that's causing it at this point, as the hum/buzz is just at the amp.

og33 - PM sent.

Interesting development..had the amp on yesterday for about 4-5 hours playing mostly 2 channel music, but some multichannel too, and no hum/buzz at all. The amp was dead quiet. Guess I will see what it does over the weekend.

Well, my hum/buzz is apparently not a ground loop hum. I installed a cheater plug yesterday after the hum/buzz came back, and it's still there. Must be the standing DC or there's some other issue with the amp.

Sorry the cheater plug didn't work.

Yeah, it seems to come and go. Used it all day yesterday and had a bit of buzz after it was on for about a half hour but then it went away. Hard to figure. Don't know if it's worth spending a couple hundred bucks for a line filter. I may just put up with the occasional buzz.

Blue Jeans has a Calrad transformer for 25 bucks. I've never used one, but it may be worth a try.

Could the noise be mechanical? Loose screw or a vibrating cooling fin? Use a mechanics stethoscope and list directly to the amp. Does the noise come from a specific part of the amp? Touch various areas to see if you can make it stop.

Maybe someone could check with Furman to see if any of their products could help this situation.

http://www.furmansound.com

Just thought I would share, might help someone?

I own a 7500. Recently I replaced my 970 with an Onkyo 807 for pre/amp duties. In doing so I used the same 3 pair of Blue Jeans analog cables to power the 5 speakers. After 4 weeks in the set up I noticed a very audible hum (ground loop) going through all 5 speakers. I tried to diagnose with the amp in the system and couldn't get it to stop (checking all cables for solid connections, etc). I never figured it out so I pulled the amp from the system about a month ago and the hum went away.

Only last night did I get some time to play with it some more. To troubleshoot, I tested all of the channels of the 7500 using my Outlaw RR2150 as the preamp and everything worked great, so I decided to hook it back into the 807 fearing that I had a possible faulty pre/out on the Onkyo. As I was connecting all of the connections I figured out the problem even before testing it - the extra analog cable (3pair but only using 5 channels - 1 extra cable hanging loose). The unused cable had to have been crossing another connection somewhere causing the issue - so I split the pair and no longer have the extra cable back there.

One stray cable, simple fix, problem gone.

Last May I purchased a 7500 to replace some older amps that I had built myself. Don't get me wrong these amps I built were awesome, just that they were too big to fit in the newly acquired equipment cabinet. In the process of doing the upgrade, I decided to replace the old audio cables. I acquired these cables from a major midwest distributer and spent a fair sum of money on them. When the 7500 arrived I hooked it up to the 990 and I had a constant low level hum from the speakers. If I muted the 990, the hum went away. I called Scott after trying to fix this for some time with no luck. He said that he would be willing to help me out, but I wasn't ready to do that yet and decided to try a number of things. Since I work in Seattle, this was limited to the occasional weekend that I was home and had no other chores to do. Long story short, I replaced the audio cables with Blue Jean's cables, and the hum went away. Out of curiosity, I did an autopsy on these other cables I purchased and found that they had a red wire connected to the center connector and a white wire to the common. The shielding was not connected at all. Go figure!