Other noise and hum sources than groundloops
There are also other sources of noise which can cause humming to audio systems than groundloops. If you have done all the countermeasures against groundloops and still have the problems or your problems seems not be caused by groundloops look at the possibility of other sources of noise as described in this document.
Audio cables can act as affective antennas for RF energy. Preamplifiers and other audio input devices vary widely in their tolerance of RF signals. In audio system where there has been done lots to avoind ground loop problems, cable shields "float" at one end. The downside is that this makes each cable act as an effective whip antenna. If the cable length and ambient RF frequencies are just right (Murphy 's law will anttend to this), the whip antenna becomes tuned and can produce very high RF voltges at it's floating or ungrounded end (and that RF will then capacitively couple to audio signal wires inside the cable shield).
This RF voltage can be reduced by terminating the floating end of the cable at RF frequencies through a resies network which consists of reries connected 51 ohm resistor and 10 nF ceramic capacitor. This network terminates the line at frequencues above 300 kHz, but looks "open" at audio frequencies (so avoids forming "ground loop").
Hum and noise can also enter the audio signal path magnetically. Basic physics tells that any conductor (wire) exposed to avrying magnetic field will have an AC voltage "induced" to it. To avoid magneticly picked noise in the cable it is a good idea to keep the audio cables at some distance away form devices like high current carrying cables, light dimming systems, transformers (any equipment which has big transformers) and large electrical motors. All of those can be a serious source of electromagneti noise which can enter to audio cable and cause humming noise.
Unbalanced audio connections will pick magneticly coupled noise easily. The normal cable shield which is very effective in shielding against capacitively coupled noise has almost no effect on magnetically coupled noise. Only effectively ways to fight against magnetic noise in unbalanced connections are keeping the cables away form magentic noise sources (magnetic fields drop very rapidly with the distance from the source, so even few centimeters movement of cable can help to reduce the problem) or placing lot of iron (or other magnetic material) between the noise source and the cable (like putthing the audio cable inside an iron tube).
The only cable mechanical constuction which can avoid magnetic noise pickup in audio cable effectively is making cable such that two signal conductors have exactly the same distance to the magnetic source. Thightly twisting of balanced signal conductors help to make average distance of each conductor to any doutside magnetic field source the same. Combination twisted pair shielded cable and balanced connection is very effective way to fight against magnetic noise. A cable type callsed "star quad" improves magnetic noise resistance of the cable by effectively averaging the gmanetic pickup of four conductors (the downside of this cable is higher price, harder connection to connectors and higher cable capacitance than normal shielded thisted pair cables).
Even when you are using twisted pair cables all parts of your system are not completerly shielded against magnetically picked noise. Every part of the signal chain where the cable condictors are not twisted can still pick up magnetic interference. A mated pair of XLR connectors leaves many centimeters (3-5 centimeters) of signal conductors untwisted and is very vulnerable to magnetic pickup. It woudl be bad idea to lay pair of such connectors near any strong magnetic field (like on top of power amplifier or audio equipment mains power supply).
Wires are not the only components in audio system which can pick up magnetic fields. Any audio transformer can also pick up magnetic noise (good ones less, porer ones more). If your audio system used audio isolation transformers for audio signal isolation it si a very good idea to keep those away from magnetic noise sources.
Tomi Engdahl <Tomi.Engdahl@iki.fi>