Last Updated: 20060803, mei
Sunday, October 8, 4:30 pm — 6:30 pm
T25 - GROUNDING AND SHIELDING
Bill Whitlock, Jensen Transformers, Inc. - Chatsworth, CA, USA
Many designers and installers of audio/video systems think of grounding and interfacing as a “black art.” Do signal cables really “pick up” noise, presumably from the air like a radio receiver? Equipment manufacturers, installers, and users rarely understand the real sources of system noise and ground loop problems, routinely overlooking or ignoring basic laws of physics. Although myth and misinformation are epidemic, this tutorial brings insight and knowledge to the subject. Signals accumulate noise and interference as they flow through system equipment and cables. Both balanced and unbalanced interfaces transport signals but are also vulnerable to coupling of interference from the power line and other sources. The realities of ac power distribution and safety are such that some widely used noise reduction strategies are both illegal and dangerous. Properly wired, fully code-compliant systems always exhibit small but significant residual voltages between pieces of equipment as well as tiny leakage currents that flow in signal cables. The unbalanced interface has an intrinsic problem, common-impedance coupling, making it very vulnerable to noise problems. The balanced interface, because of a property called common-mode rejection, can theoretically nullify noise problems. Balanced interfaces are widely misunderstood and their common-mode rejections suffer severe degradation in most real-world systems. Many pieces of equipment, because of an innocent design error, have a built-in noise coupling mechanism dubbed the “pin 1 problem” by Neil Muncy. A simple troubleshooting method that uses no test equipment will be described. It can pinpoint the exact location and cause of system noise. Most often, devices known as ground isolators are the best way to eliminate noise coupling. Signal quality and other practical issues are discussed as well as how to properly connect unbalanced and balanced interfaces to each other. While immunity to RF interference is a part of good equipment design, it must often be provided externally. Finally, power line treatments such as technical power, balanced power, power isolation transformers, and surge suppression are discussed.