UNDERSTANDING, FINDING, & ELIMINATING GROUND LOOPS IN AUDIO & VIDEO SYSTEMS – 2005 Generic Seminar Template is a very good collection of information on ground loop problems on audio and video systems. The writer Bill Whitlock from Jensen Transformers really knows what he is talking about.
Here are some fact points picked from document:
The very meaning of the term ground has become vague, ambiguous, and often quite fanciful. Some engineers have a strong urge to reduce these unwanted voltage differences by “shorting them out” with massive conductors (the results are most often disappointing) or finding a “better” or “quieter” ground. There are several common myths about grounding. Many indulge in wishful thinking that noise currents can somehow be skillfully directed to an earth ground.
An excellent broad definition is that a ground is simply a return path for current. We must remember that current always returns to its source through either an intentional or accidental path.
In all real equipment, there are parasitic capacitances between the power line and the equipment ground. They are the unavoidable. These capacitances allow leakage current to flow between power line and chassis/ground inside each piece of equipment. Any connection between two such devices or such a device and a grounded one will carry this leakage current. We must accept this fact as reality.
For grounded equipment, the effects of leakage current are usually insignificant compared to voltage differences between outlet grounds. Substantial voltages are magnetically induced in premises safety ground wiring by the imperfect cancellation of magnetic fields that surround the two load-current-carrying conductors. Significant ground voltage difference (1 volt is not unusual) will exist between the chassis or local “ground” of any two pieces of safety-grounded equipment. We must also accept this fact as reality.
When a system contains two or more pieces of grounded equipment, whether via power-cords or other ground connections, a “ground loop” may be formed.
Transformer isolators are very good devices in solving ground loop issues, but you need to remember to check performance data for isolators carefully. Beware of products that are not well-specified. They can sometimes solve noise problems, but at the expense of sound quality.
Noise rejection in a real-world balanced interface is often far less than that touted for the input. That’s because the performance of balanced inputs have traditionally been measured in ways that ignore the effects of driver and cable impedances. In real life the ground noise rejection of ordinary differential amplifiers is extremely sensitive to impedance imbalances in the driving source and real-world outputs are very rarely so precisely matched. The CMRR can easily frop from its advertised or “rated” 90 dB down to 65 dB.
The ground noise rejection of ordinary differential amplifiers is extremely sensitive to impedance imbalances in the driving source. With unbalanced sources, their entire output impedance becomes “imbalance” and the noise rejection of differential amplifiers is quite poor.
Electric fields can capacitively couple noise into signal conductors. Grounded shield solves the entire problem well. Braided shields with 85% to 95% coverage are usually adequate. Note that shield ground connections can affect CMRR. Cable capacitances between each signal conductor and shield are mismatched by 4% to 6% in typical cable. The imperfect symmetry and/or mis-matched capacitances will cause signal current in the shield. This current should be returned directly to the driver from which it came. For shielded balanced audio cables, the shield should ALWAYS be grounded at the driver — whether or not the receiving end is grounded. The most widespread industry practice is to ground the shield at both ends. It provides a good guard against RF interference but compromises CMRR to some degree.
Be sure all balanced line pairs are twisted. Twisting makes shielded or unshielded balanced pair lines nearly immune to magnetic fields and makes unshielded balanced lines nearly immune to electric fields. This is especially important in low level microphone circuits.
Effective magnetic shielding, especially at power frequencies, is very difficult to achieve. Imperfections in real cables result in unequal induced voltages that add noise to the differential signal (SCIN = shield-current-induced-noise). Generally, the best cables have braided or counter-wrapped spiral shielding and the worst have foil shields and drain wires.
Bundle signal cables. All signal cables between any two boxes should be bundled. For example, if the L and R cables of a stereo pair are separated, nearby ac magnetic fields will induce a current in the loop area inside the two shields — coupling hum into both signals. Bundling all ac power cords separately helps to average their magnetic and electrostatic fields, which reduces their net radiation. Of course, keep signal bundles and power bundles as far apart as possible.