Seems like a ground loop to me. Do you have a ground lifter to use for troubleshooting? http://www.amazon.com/45086-3-Prong-2-Prong-Electrical-Adapter/dp/B00166TT3G/ref=sr_1_3?ie=UTF8&qid=1395835417&sr=8-3&keywords=ground+lift+adapter
 
Rocky

Sorry to disagree, but you should NEVER defeat the safety ground, not even for troubleshooting purposes. People, and equipment can get hurt, that's why they call it safety ground!

Here's the thing, ground is a term that is used in a variety of ways, and that leads to confusion. If you defeat the safety ground and there is any kind of fault in the system that was causing current to flow in the safety ground then the current will now hunt for a new path to the source, and it can be you.

If you absolutely must troubleshoot a ground loop problem use a transformer to break the galvanic connection in some other path (typically a single-ended audio path, but can be a data path too.)

The other thing to keep in mind is that the ground loop is not the problem, and breaking the loop is treating the symptom, not the problem!

So again, at the risk of being run out of town on a rail - do not EVER defeat the safety ground - those adapters are intended to provide a safety ground, not defeat it. Misusing them can harm you or your gear.
 

Using ground lifters as a rule of thumb is not recommended. Unless you have a ground fault there is no hazard in using them to troubleshoot.
There are millions of homes in the U.S. wired with old 2-wire romex that have no grounding system without issue. Sure it's better to have a properly grounded system but using ground lifters to provide a ground may give you the false sense of security that you created a ground by connecting it to the device plate screw which may or may not provide a grounding means.
 
That 2-wire metal lamp sitting on the night stand wouldn't have a UL listing if this was so dangerous.
 
In my opinion there is no harm in using a ground lift to temporarily remove one of the paths to ground. This is common in equipment housed in a rack. The plug ground provides a path to ground but the metal case of the equipment that is screwed to the rack bonds the rack providing another path to ground for other equipment installed the rack which can create a ground loop. You can buy insulated washers for the 10-32 screws to prevent this from happening.
 
If a ground loop is the suspect the most effective way to troubleshoot is to remove a grounding path. I respect your opinion and I appreciate all of the information that you provide Bill but I disagree here.
 
Rocky

Appliances that have 3 prong wiring are set up to be safe when the third prong is plugged in.
 
Appliances that have two prong wiring are set up to be safe with 2 prongs plugged in. Some 2 prong appliances have a polarized plug so that the "cold" on the plug is plugged into the "cold" in the receptacle. Some 2 prong appliances are set up so that they are safe regardless of the plug's orientation to the receptacle.
 
 
I'm not saying that I know much about it... heck I shut down my work bench and a part of the house just last week as I was trying to measure my mains side of a power transformer in my new psu project when I slipped with the probe so as to short the "black" and the "white" :-)...  here is some info about appliance classes:
 
http://en.wikipedia.org/wiki/Appliance_classes

Lifting a ground using the adapters.... we've all done it on a gig at one time or another. No, it's never a good idea to defeat the grounding conductor. But hey, you're in the band, you've been hired to play music and the venue's wiring has ground loop problems..... fixing them is not your job or responsibility, and it's 10 minutes to show time......whatcha gonna do?   Yup, same thing I would do.... slap that ground adapter on the amp, and test to see if the mic shocks you.... if so, install a foam windscreen and be careful when singing.
 
In a studio that you will be controlling or owning..... different story. Fix the problem, don't just treat the symptoms.
 
I dare say that there are hundreds of studios, probably home and commercial, where the symptoms are treated and remain so.
 
When hum is caused by grounding issues, it does kinda cross into a safety issue as you start to work on the problem.
 
As far as the 2 prong or 3 prong cords.... all new 2 prong cords are polarized with the larger of the 2 lugs being the neutral.  And all receptacle outlets are polarized now as well.  I'm sure a few non-polarized lamp cords are still around but you'd be hard pressed to find them in a store now.
 
Most hand tools are now manufactured with rounded cords plus they use double insulation. So there are no parts that a user can touch that might be accidentally shorted hot.  
 
A few weeks back, my mother in law asked me and my son in law to look through the tools in her shed and pick out any that we might want. In that cabinet were several electric hand drills. the old kind with the metal casing and a 2 wire non-polarized cord..... Our parents used those old metal cased saws, drills, sanders, all the time.... There's no way I would have plugged those tools in and used them. We left them in that cabinet.

bluzdog
That 2-wire metal lamp sitting on the night stand wouldn't have a UL listing if this was so dangerous.

 
If it is UL listed then there is no way to energize the housing, and a ground path is not required. If your rack mount device is designed such that there is no way to energize the housing then it won't have a three wire power cord, and the point is moot.
 

bluzdogIn my opinion there is no harm in using a ground lift to temporarily remove one of the paths to ground. <snip>

I don't know you, and I don't know how much you know about electronics and electrical power. You might know enough to safely do this. I still think it is a terrible idea, but that is neither here nor there. What I am objecting to is posting this as a suggestion in a public forum where you can't guess at the competence levels of those who read your post. Removing the SAFETY ground is dangerous!
 
And - admittedly a pet peeve for me - ground loops are NOT the problem, nor is ground noise. The problem is poorly designed devices and/or poorly designed systems. I've installed broadcast studios in buildings with large AM transmitters in the next room... and they were always electrically quiet.
 

bluzdogIf a ground loop is the suspect the most effective way to troubleshoot is to remove a grounding path.

 
On this we agree, but do NOT remove the SAFETY ground - remove the ground in the audio path, there are several inexpensive transformer boxes that let you do just that. They sound terrible, and you wouldn't want to record through one, but they provide a SAFE and effective troubleshooting tool.
 

bluzdogI respect your opinion and I appreciate all of the information that you provide Bill but I disagree here.

And I disagree with you - and we'll probably have to settle for that. I've been at this for a very long time (so long I can no longer bring myself to say the number<G>), and I've been fortunate to work with and learn from some of the best of the bunch, including folks who quite literally wrote the book. There is just never a good reason to defeat the safety ground for troubleshooting, let alone to resolve a problem.
 
And to everyone else I repeat: never defeat the SAFETY ground - it is there for your safety.

Okay, I could continue but I'm not here to change anyone's point of view. I'll agree to disagree.
 
Rocky

done.