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CHAPTER 7: Grounding and Interfacing > GROUNDING - Pg. 228

PART III Preamplifiers, Mixers, and Interconnects Typical copper braid or aluminum foil cable shielding has little effect on magnetic fields at audio fre- quencies. If a shield is grounded at both ends, it behaves somewhat like a shorted turn to shield the inner conductors from magnetic fields (Ott, 1988, pp. 47­49). Depending on the external impedance between the grounded ends of a cable shield, it may begin to become effective against magnetic fields somewhere in the 10 kHz to 100 kHz range. Box shields of aluminum or copper are widely used to enclose RF circuits because they impede magnetic fields through this eddy current action and are excellent shielding for electric fields as well. See Morrison and Lewis, 1990, pp. 144­145 for an excel- lent explanation of this high-frequency shielding. However, copper or aluminum shielding is rarely an effective way to prevent noise coupling from audio-frequency magnetic fields. GROUNDING Historically , grounding became necessary for protection from lightning strokes and industrially-gener- ated static electricity--i.e., belts in a flour mill. As utility power systems developed, grounding became standard practice to protect people and equipment. As electronics developed, the common return paths of various circuits were referred to as ground, regardless of whether or not they were eventu- ally connected to earth. Thus, the very term ground has become vague, ambiguous, and often fanciful. Broadly, the purpose of grounding is to electrically interconnect conductive objects, such as equipment, in order to minimize voltage differences between them. An excellent general definition is that a ground is simply a return path for current , which will always return to its source. The path may be intentional or accidental--electrons don't care and don't read schematics! (Gerke and Kimmel, 1998) Grounding -related noise can be the most serious problem in any audio system. Common symptoms include hum, buzz, pops, clicks, and other noises. Because equipment manufacturers so often try