Help with a weird ground loop problem?

Any major flourescent lighting near your amp? (long shot..but is an old prob sometimes) ....also...try running all power thru same outlet source if haven't yet. curious what model amp....

Another culprit is circuits with variable voltage switches (incandescent light switches) which may not be isolated from the receptacles properly.
 
I interesting method for finding EMI is to use a single coil guitar (only one coil online) and run it around the area your gear is in. If you have that available, can leave your recording setup powered, but use a single coil (either just through the amp, or all the way to the audio interface). Just be careful of gains when doing it.

thanks for the help you guys...
 
mettelus, unfortunately no luck with the single coil experiment.
 
Jim, I assume you mean from a single duplex more so than an single outlet?  sorry if that seems like a stupid question...  the DI box I purchased has already been sent back but it did not have a ground lift circuit... but after a bit of reading and shopping I see that some of them do.  I'm guessing this would be a good next piece of equipment to try.
 
thanks everyone. the guy at sweetwater gave up on my problem and no one at gearslutz even commented.  so I truly appreciate you all helping me!  not sure where else I'd turn...

it might be a ground loop, it might be a pin-1 problem, it might be any number of things, and it is going to be difficult to diagnose on a forum, but let's give it a shot eh?
 
First - is everything properly grounded. If all the gear has a three prong power cord is the ground conductor on the outlet connected to ground? Don't laugh, I've run into that many times, especially with low cost outlet strips and power "conditioners". If you don't know how to test this, or don't have a friend that knows how to do this then go to a hardware store and pick up  one of those outlet testers (whatayaknow - even Amazon sells them - LINK) It won't identify every possible problem, but it will let you know if all three conductors are connected correctly.
 
Second - start taking things apart. It sounds like you've already started down the path. If you disconnect the multi-effects rack from the amplifier does the hum go away? In theory (and I love theories<G>) the output from the multi-effects rack ought to be isolated already. You didn't mention what this rack is, can you provide more detail?
 
Third - should have been first I suppose - you also didn't mention details about this isolator you inserted . What is it? Many such products are pure snake oil, the only way to get good sound and proper galvanic isolation is a transformer, and a good transformer is expensive.
 
Fourth - powering everything from a single outlet can work, but not if the outlet strips are poorly constructed. I've measured 10s of Ohms of impedance at 60 Hz on some cheap rack mount power strips. UGH!
 
If you can provide some more details we can make the hum go away. I've yet to see a power line noise problem that could not be solved. Some take longer than others to figure out, that's all!

Man, I got a lot of good stuff from this thread. Thanks everyone.

Do those ferrite deals you clip on to your cables help? You see those built into many wallwarts and other devices with cables.

Another option to consider with this is (if tracking) is a program with noise reduction by noise print capability. If a second or two of "silence" is captured at the ends of each take, it makes for simple destructive edits. There was a thread posted last week mentioning Adobe Audition 3 (there is also a link to the 3.0.1 patch lower in that thread). That has this capability, and can also be added to the Utilities menu for quicker editing if desired.
 
Unfortunately, for performance, this route won't cut it.
 
I took my amp to max power settings for the first time in a long time last weekend and ended up just doing the above (but was just tracking with it). My amp actually recommends not doing this, since it has some excessive signal boosting built in and compounds any noise issues (even minor ones).

hmmm.....

Beepster
 
Well sheeeyote. I never venture down into this sub forum (for some stupid reason) but for some reason poked my head in today and saw this thread. I too have persistent hum issues AND have been looking for a decent, but cheap, DI/HiZ thingiemadoodle. I was going to try out the MOTU Z-Box but darned if this doesn't look much more suited to my noisy arse setup. Less expensive too. Thanks.
 
http://www.sweetwater.com/store/detail/Zdirect

Cool, I like Art, I have a 20 year old preamp that's still kickin after some seriously heavy use throughout the years :)