Why professional sound systems suffer from ground loop humming
Good and bad balanced connections
Balanced interconnection systems used in professional audio world are naturally very good at rejecting noise problems. The shield grounded on both ends of the cable to equipment cases provides a very good shields agains RF interference and electric fields. The blaanced narure of the signals will very effectively eliminate the effect of small potential differences between the audio equipments. The twisted narure of the wires is good at reducing the effect of noise from the magnetic field.
Because those design choises, a balanced audio interconnection should be quite much free of any ground loop problems when the whole system is done using balanced inteconnections. And if there is need to connect anything unbalanced to such system, those should be connected using DI-boxes and isolation transformers. This should provide a hum free operation for equipments.
Unfortunately in real life the situation is not so simple. Many audio manufacturers, conciously or unconsciously, connect balanced cable shields (pin 1 in XLR connector, sleeve on 6.3 mm jack) to audio signal ground instead of the equipment case ground where is is suppored to be originally connected. The result of this design choise is that any currents induced int the shield modulte the ground where the shield is terminated. This also modultes the signal referenced to that ground and the result is typically annoying humming noise.
Unfortuantely the practice of connecting noisy cable shields to audio ground is very widely spread. This improper arrangement of shield grounds amazingly sometimes achieves somewhat acceptable performance. It performs well in many stadard qudio equipment bench test, but tge peformance on the field can be quite much varying. This means that getting good performance from such system need quite much testing and tweaking including ground lifts and isolation transformers.
The hum and buzz problems with too many balanced equipments with signal grounded shields habe given balanced equipments a bad reputation. This has created great confusion among system designers and users. Unfortunatley this is the situation where we have to live in.
The truth of a balanced interconnection is that a properly designed and built balanced interconnection is very much better than any unbalanced interconnection in rejecting humming noise. Properly dessigned balanced interconnection means that for fully balanced operation the signal wires should be well impedance balanced and the shield of the cable should connect to the chassis ground the the point of entry.
Balanced and unbalanced interconnections
Balanced and unbalanced systems are not designed to interface together directly. In nowadays system there is unfortuanately many times need to interface balanced and unbalanced equipments to each other. This kind of interfacing is always very possible source of causing unwanted humming entering to the system.
The most proper way to do balanced to umbalanced interconnection is to use an transformer isolation in the interconnections. This gives you the most reliable results. In some cases just a special cable or adaptor will do the job, but in this case you have to be very careful what you are doing to avoid himming to enter the system.
Unbalanced interconnections (common in home audio) should be avoided in professional sound systems for many reasons. An unbalanced system uses only two wires, one for signal and one for ground, which arrangement has an inherent problem called common impedance coupling. If two different circuits share the same conductor or wire, a current flowing in either circuit will produce a voltage drop across the wire and that voltage drop will become summed to the signal in the cable.
Since the cable shield is effectively connecting the grounds of the devices together, it carries a current derived from the power line as well as the audio signal current. A receiving cannot tell the difference between signal and hum, and will amplify both if they are present.
If you have a humming problem with an unbalanced connection, the choice is to somehow reduce the problem is to reduce the interchassis current. We could eliminate it by simply breaking the chassis to chassis shield connection. This is usually best done using an audio isolation transformer fitted to the line. Simply just cutting the audio cable ground wire will not stop the effects of voltage differences between equipments. On the other hand cutting the mains cable ground wire connection is potentially dangerous and against electrical safety code.
Tomi Engdahl <Tomi.Engdahl@iki.fi>