Jensen AN-002 1 "Balanced" System as Wheatstone Bridge Jensen AN-002 ANSWERS TO COMMON QUESTIONS ABOUT AUDIO TRANSFORMERS by Bill Whitlock WHAT DOES "BALANCED" REALLY MEAN ? The purpose of a balanced line is to transfer a "signal" from one place to another while rejecting "ground noise", which is not white noise or hiss, but power line related hum and buzz. To accomplish this noise rejection, two signal lines are used and the IMPEDANCE of the two lines to ground must be equal or "balanced". Since anything connected to the line affects its impedance, always consider the "system" consisting of the driver, the line itself, and the receiver. It is a popular belief that the signals must have opposing polarity and equal amplitudes, or symmetry. Signal symmetry has NOTHING to do with noise rejection . The system must reject noise even when there is no signal and, in fact, this is usually how system noise testing is done. Illustrated below is a generalized balanced system schematic, rearranged to show that it takes the familiar form of a Wheatstone bridge. A Wheatstone bridge has a "generator", four impedance "arms", and a "detector". In our schematic, the "generator" is the ground noise, the "arms" are the common-mode impedances of the driver outputs Z C1 and Z C2 and receiver inputs Z C3 and Z C4 , while the "detector" is the receiver itself, which is sensitive only to the voltage between the two lines. If the ratio of Z C1 to Z C3 is precisely the same as the ratio of Z C2 to Z C4 , the bridge is said to be "nulled". In this ideal condition, none of the ground noise will appear at the receiver no matter how large the noise is. In the real world, however, all kinds of mass-produced equipment will be interconnected and exact matching of these impedances at inputs and outputs cannot be reasonably expected. Equipment designers can minimize the effects of these mismatches or "unbalances" through judicious choices of input and output stage circuit topologies and common-mode impedances. The Wheatstone bridge is most sensitive to small fractional impedance changes in one of its arms when all the arms are the same impedance. It is least sensitive when one pair of arms is very low impedance and the other pair is very high (we change the impedance of pairs because the ratios of the two sides must match in order to null the bridge). Therefore, we can minimize the
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