NEVER use a cheater plug or cut the ground plug off an ac power cord.
From the Owners Manual
Diagnosing / Troubleshooting
System Owner DT recently posted the following:
To troubleshoot ground loops problems, listen to the audio signal at the mixer with headphones. If the buzz exists at the mixer then do the following:
One by one, disconnect the inputs and outputs to and from the mixer and note if the buzz decreases. If the buzz slowly diminishes with each input or output that is disconnected including those connected directly to microphones and devices not powered from an AC wall plug, then the buzz may be caused by the snake running too close to a noisy AC power line. If the hum reduces dramatically only when inputs or outputs are disconnected which are connected to devices that are AC powered, then it is likely caused by a ground loop.
If the hum is not present in the mixer headphones but only audible from the L1™s, try adding direct boxes between the mixer and the L1™s.
Usually if all equipment powered from AC power is plugged into the same AC wall plug, there will be little or no problem with ground loops. Because the mixer is usually at the back but other equipment powered by AC is at the front, it is impractical to use the same AC wall plug (long extension cords do not qualify as the same wall plug). Try using a direct box with ground lift and connect between the mixer or other audio devices which are AC powered that seem to be responsible for the ground loop. AC devices with 3-prong plugs are most suspect but 2-prong AC powered devices can also cause ground loops. Do NOT modify or isolate the ground pin of devices that have 3-prong AC power cords.
I helped two churches with L1™s reduce hum to acceptable levels by using direct boxes at the audio output of the electric organ and at the audio input to the L1™s . We used a Radial PRODI for the electric organ. I recommend a balanced line-level to balanced line-level direct box for the L1™ inputs. The Radial Twin ISO is a two channel line to line direct box. If the two channels are used on two separate L1™s put the two channel direct box close to the mixer. Alternately, have your local pro audio store help you select single channel line to line or line to mic direct box to be used with each L1™. Be sure that the ground lift switch is in the "lift" position.
The direct boxes can be expensive, check to see if you can return them if they don't solve your problem or if you find that you don't all of them.
In some cases ground loops or buzz in the presence of certain light dimmers can be difficult to completely solve and some residual hum may be audible. There are some types of light dimmers that produce less electrical noise. One technology is known as "zero crossing" switching which generate less electrical interference.
Hopefully some of this info is relevant and helpful.
Dave also recommended reading Ground Loops at Radial Engineering.
Ground Loops (article excerpt)
Almost all cases of noise can be traced directly to ground loops, grounding or lack thereof. It is important to understand the mechanism that causes grounding noise in order to effectively eliminate it. Each component of a sound system produces its own ground internally. This ground is usually called the audio signal ground. Connecting devices together with the interconnecting cables can tie the signal grounds of the two units together in one place through the conductors in the cable. Ground loops occur when the grounds of the two units are also tied together in another place: via the third wire in the line cord, by tying the metal chassis together through the rack rails, etc. These situations create a circuit through which current may flow in a closed "loop" from one unit's ground out to a second unit and back to the first. It is not simply the presence of this current that creates the hum -- it is when this current flows through a unit's audio signal ground that creates the hum. In fact, even without a ground loop, a little noise current always flows through every interconnecting cable (i.e., it is impossible to eliminate these currents entirely). The mere presence of this ground loop current is no cause for alarm if your system uses properly implemented and completely balanced interconnects, which are excellent at rejecting ground loop and other noise currents. Balanced interconnect was developed to be immune to these noise currents, which can never be entirely eliminated. What makes a ground loop current annoying is when the audio signal is affected. Unfortunately, many manufacturers of balanced audio equipment design the internal grounding system improperly, thus creating balanced equipment that is not immune to the cabling's noise currents. This is one reason for the bad reputation sometimes given to balanced interconnect.
A second reason for balanced interconnect's bad reputation comes from those who think connecting unbalanced equipment into "superior" balanced equipment should improve things. Sorry. Balanced interconnect is not compatible with unbalanced. The small physical nature and short cable runs of completely unbalanced systems (home audio) also contain these ground loop noise currents. However, the currents in unbalanced systems never get large enough to affect the audio to the point where it is a nuisance. Mixing balanced and unbalanced equipment, however, is an entirely different story, since balanced and unbalanced interconnect are truly not compatible. The rest of this note shows several recommended implementations for all of these interconnection schemes.
Please see the full text: Sound System Interconnection
- Two Prong Cheater Plub for AC Power
- Grounded Plug
- Ground Loops at Radial Engineering.
- Laptop Playback Noises.
- Magnetic Pick-up Noise
Download: Sound System Interconnection.pdf