I found this on the web, good reading:

Dangerous method: break the mains ground connections

Logically you could think, you could eliminate ground loops by disconnecting the power-cord ground pins on all your gear. Some people might try to break the ground connection by cutting the grounding pin in the connector, using cheater plug, cutting the ground wire in equipment, taping over the grounding connector etc.

Do not do this. Removing the ground connection isn't right. It is against electrical safety regulations and potentially very dangerous. Removing ground connection can defeat the actions of your noise filter or spike protectors inside the equipments. If the ground connection is cut then a fault in the isulation inside equipment will cause dangerous voltages to the equipment case instead of burning a fuse. Removing the ground connection from the equipments which have it is dangerous, against electronic safety regulations and you risk damaging your equipment. Running without a power ground will not automatically electrocute you but will make this much more propable if something goes wrong in your system.

NEVER use a three wire to two wire adapter on ANY piece of audio gear where a human can POSSIBLY come into contact with it. You're asking for a nice 120v "signal" thru someone's body. Yes it MIGHT eliminate the hum, but there's a MUCH safer way to do the same thing.

EBTECH®
A Division of Sound Enhancement Products, Inc. www.ebtechaudio.com
How to Find Ground Loops and Prevent AC Hum
What is a Ground Loop?
When you hear hum in an audio system, it's almost always caused by a loop antenna effect between two or more pieces of gear, across
signal lines. A loop antenna is formed by having a loop of wire where the beginning and end of the loop are connected - the loop can
be any shape. The loop antenna(e) is basically a form of radio antenna and they tend to pick up the 60Hz AC signal being broadcast by
a building's electrical wiring. They also pick up 120Hz, 180Hz, and all the other harmonics of 60Hz and, usually to a lesser degree,
electrical noise being broadcast from all over such as radio/TV, hair dryers, etc. These loop antennae are closed circuits usually
through the ground wires and hence are commonly called ground loops.
Examples of Ground Loops:
1. Going up the AC power cord ground from the electrical system wiring to a keyboard, going across a signal line ground from the
keyboard to a mixer across the signal ground, down the mixer's power cord ground reconnecting to the electrical system wiring.
2. Going across the signal ground from a mixer to a reverb unit, going from the reverb unit across the signal ground back to the
mixer and reconnecting inside the mixer.
3. Going up the AC power cord to the mixer, across the signal ground to the amplifier, down the amplifier's power cord ground and
reconnecting to the electrical system wiring
4. Going up the AC power cord to a guitar amplifier, going across the input signal ground to an effects device left channel output,
from the effects device right channel output to another guitar amplifier, down the second guitar amplifier's power cord ground and
reconnecting to the electrical system wiring.
Which connection has the Ground Loop? (AKA Playing Audio Detective)
Identify the ground loop causing the trouble; not all ground loops cause noise or hum. For complex systems you may need to repeat
these steps starting with a different piece of equipment in various combinations to locate the problem:
1. Strip the system down to one piece, such as the mixer, by disconnecting all interconnects and AC cords except for the mixer.
2. Add one piece of equipment at a time; hook up AC and interconnects (making sure all grounds are connected and in good
condition) then listen for hum or noise.
3. Turn on and off the power each time you switch equipment to avoid pops and shorted outputs.
4. Proceed until you find the offending piece(s) causing the problem.
5. Plug the Hum Eliminator™ in all lines between the offending equipment and the rest of the system. For example ... insert the line
outs of the keyboard into the inputs of the Hum Eliminator™, then insert the line outs of the Hum Eliminator™ into the inputs of
the mixer.
It is often helpful to listen through a pair of headphones. Quite often you will only hear hum coming from a particular input channel
on a mixer and that is where the ground loop will be. Alternatively, if you hear hum coming out the speakers with all the mixer's
channels turned down, it's likely that the problem is between the mixer and amplifier or other equipment that comes after the mixer.
Another common path for ground loops is through a chassis into the rack and then into another chassis. Test this by removing the
chassis from the rack. The Hum Eliminator™ will help but you should also try isolating the chassis from the rack with electrical tape
and insulating the rack screws with nylon washers.
Note: Never use the Hum Eliminator™ between an amplifier and speaker or the equipment may become damaged. Only use
the Hum Eliminator™ on line level signals.
Page 2
- 2 -
185 Detroit St Cary, IL 60013 telephone 847-639-4646 ex 30 fax 847-639-4723
revision 09/02/05 www.ebtechaudio.com
What about clipping or lifting the AC ground or signal ground?
While these methods may or may not remove your hum, they have some real drawbacks!
Removing or disabling the AC ground:
• Can cause electrocution
• Can cause distortion due to floating signal references
• Can cause some pieces of equipment to oscillate or become damaged
• Can cause current and noise meant for the AC ground to be dumped down the interconnect (line level) to another piece of
equipment instead
Cutting the shield at one end of the interconnect cable:
• Can hinder the ability of the cable to serve as a signal return
• Can cause distortion and/or clipping of the signal since there is no voltage translation matching (shifting a signal to match
ground and power supply).
• Can alter the cable's frequency response.
• Can defeat the shielding effect.
Why using the Hum Eliminator is the safer and better solution:
The Hum Eliminator™ is completely transparent; its audiophile quality components don't change your sound. With a flat
frequency response from 20Hz to 70kHz (way beyond the range of human hearing) the Hum Eliminator™ is the answer.
The Hum Eliminator™ breaks the ground loop, keeping all AC grounds intact. It provides isolated signal returns and
performs automatic voltage translation matching.
The Hum Eliminator™ automatically converts from unbalanced to balanced without signal loss. With the Hum
Eliminator™ you can run a signal across a room from a pre-amp, effects unit or keyboard without picking up AC hum from
power cords and without the signal loss you get from a DI box. Balanced outputs from the Hum Eliminator™ benefit from
true common mode rejection (CMR), canceling out noise from AC power cords and other sources.
The Hum Eliminator™ will match any ground potential difference between two pieces of equipment. If the ground of your
keyboard is 6 volts higher than the ground of your mixer, the Hum Eliminator™ will shift the entire signal of the keyboard
down by 6 volts to compensate without affecting the keyboard at all.
The Hum Eliminator™ is equipped with “smart jacks” (TRS); so you can run all types of line level signals; balanced or
unbalanced.
No buzz, no hum. No signal degradation. No filtering. No noise gates. No dangerous ground lifts.
NO PROBLEM!

[This message was edited by Ken Fox on 30 March 2006 at 06:16 AM.]

All grounds are supposed to return to Mecca, but frequently they don't.

The safety ground (i.e. bare wire) ain't supposed to carry current, but, if it's incorrectly terminated or incorrectly bonded to the neutral in the panel box, you can get this ugly mess. Also, switch mode power supplies (like what's in PC's) do weird things to building power systems.

Electromagnetic Interference (the antennae effect) is technically a separate animal from the ground loop problem. However, they frequently occur at the same time. The shield is supposed to protect from EMI but when the shield is cauzing the EMI (because of ground loop current) you're stuck until the ground loop current is stopped.

[This message was edited by Ray Minich on 07 April 2006 at 10:41 AM.]

Make sure when you test the outlets that no equipment is pluged in - no cables running from unit-to-unit, etc. If you had an open ground on one outlet, you could be checking "OK" because you are reading back through cables to another outlet that is grounded.

If you are using some of those multi-outlet adapters make sure you check the outlet you are plugging into AND then check your adapter! I've found some adapters wired backwards (neutral and hot) from the factory. And check ALL outlets on the adapter. You could have a problem with one or more of the outlets on the adapter - check them all.

In my prior life I worked in the computer field -- did a lot of trouble shooting at customer sites that had problems with their terminals, etc. Missing/loose/faulty grounds was the number one problem followed by neutral/hot being crossed and folks using 2 wire extension cords.

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Don

West Coast Double 8 Nashville 112