Methods for breaking ground loop
Fort check that you have made the cabling between equipments properly. Proper audio cable connection methods are essential in getting humm free system. Rane technical note Sound System Interconnection is a very good source for information how the audio wiring between equipments should be connected. This is a well worth to read article.
Breaking of ground loop can be done in many ways, some which are better and some are less good.
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.
Is a GFCI a substitution for ground conductor ?
Ground fault interrupterscan be used as replacement for ground connections in three prong receptables according NEC. The NEC requires that three prong receptacles without ground that are protected by GFCI must be labelled as such.
The NEC, section 210-7(d), and CEC, section 26-700(9), are quite explicit that GFCIs are a legal substitute for a grounded outlet in an existing installation where there is no ground available in the outlet box. But your local codes may vary.
Because the NEC talks only about existing installations without ground it seems that you can't use GFCI as ground fault problem solver, because your system already have grounding (and GFCI is a substition for grounding in installation where there is no ground conductor).
Transformer isolation of whole studio system
You can isolate your system completely from the power company with a special transformer. But if you're going to go to the trouble of doing this, you might as well consider rewiring your studio with good power system which has proper start grounding. The use only professional equipments with balanced audio input and outputs and do the wiring correctly. If you stil face problems you can consider using balanced power system to power your equipments.
Solving the ground loop hum problem using the right ground loop isolator is the safe, effective way to eliminate unwanted hums at an affordable price. Typically ground loop problems can be solved with audio isolation transformers connected to the audio line. There are also commercial audio distribution amplifiers which provide this isolation.
Regardless of what type of ground loop isolation method is used, remember, never remove the ground prong on the power cable.This may work as a temporary solution, but personal safety should not be compromised.
If you can't find the ground loop and don't want to use isolation transformer, try tempting the circuit with an even better ground. Heavy-gauge wire has very little electrical resistance, so less current is likely to flow through the shield. Get some heavy gauge wire and run lengths of it from your computer's chassis to each other chassis. On audio equipment, there's usually a screw marked for this purpose. You should hear the hum go down as soon as you make the connection. If heavy-duty ground connections don't stop the hum, you'll have to use suitable isolator in that audio line.
Direct (DI) boxes as ground loop problem solvers
Direct boxes are devices that convert unbalanced instrument-level (or line-level) outputs into balanced, microphone-level inputs. They are used heavily in large, arena-size sound reinforcement systems where the keyboards, etc., are located far away from the mixer. Impedance differences between the keyboard and mixer prohibit one from running cables directly out of the bass amp of keyboard and to the mixer.
To use the Direct Box, one takes the guitar amp's output (it usually has one on the back), and, using a 1/4" patch cable, plug the other end into the direct box input. Make sure that the switch (if it has one) labeled "Speaker/Instrument" is on "Instrument." Plug a normal XLR cable into the output end and the other into your cabling which goes to the mixer.
If the system is grounded at bothe ends and ground potentials start flowing, you'll get a ground loop. Ground loops can be identified by a low hum (60Hz in the US and 50 Hz in Europe) through the sound system. First place to check: direct box. Direct box usually has a switch labeled "Ground Lift" for solving this type of problem. The ground lift switch will lift the ground (safely) and the hum should stop. If the switch isn't there, use a special ground lift cable on the XLR cable. Do not, by any means, disconnect the grounding pin of the AC cord.
How to use noise gate to reduce hum
Noise gate does not let any sound through the gate unless the sound has a signal level which is higher than the set threshold value. If the problematic hum source is some instrument and the hum is very faint a noise gate can make less annoying. When you set the noise gate threshold so that the gate does not pass any signal when there is no signal from the instrument then the hum is not added to the main mix. When the instrument is played a very weak humming is quite hard to notice.
Commercial products for audio line ground loop elimination
- ES034 Ground loop isolator - isolation transformer for car audio from Scosche Industries
- FGA-40 Ground Isolator is 1:1 audio isolation transformer with 10 kohm impedance from Monacor (Best.-Nr. 06.4370)
- Ground LP Isolator (270-0054) from Radio Shack, designed for car audio
Equipment to remove hummign already in signal
- N'Hummer 1a AC Line Noise Eliminator from Stro Lion Technologies
- SN-700 Noise/Hum Eliminator - a single-ended, realtime noise eliminator which tries to eliminate huming noise form audio lines, made by Roland.