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Sound is derived from vibrations which in turn are generated or represented by changing current levels. Sound can never be derived from a static voltage or current. Your statement that "DC should never produce audio" is - to all intents - true. A ground loop is the unexpected flow of current in a circuit caused by a potential difference between two points ...


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Thing is, that if two points in a circuit are actually "ground" (i.e. 0V potential) then by definition, no current can flow between these two points. When current does flow between two arbitrary points on a circuit (that we may consider to be 'ground') this actually means that one or another of these points is actually not ground. Only one of them is. ...


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