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After what's turned out to be a rather lengthy discussion in these pages about balanced circuitry I was prepared to move on today. Recent postings by others, as well as multiple emails from some of my readers suggest perhaps we spend a few days more finishing the subject. Then we'll move on. Part of the issue turns out to be vernacular. What's the definition of a balanced audio circuit? To some it means symmetrical amplification chains. To others, like me, it means differential amplification taking advantage of common mode rejection. Definitions, by their very nature, can have multiple meanings. I think this is one of those cases. In the interest of thoroughness I will touch on symmetry for a few days and then we'll move on. But first, one last word about balanced audio. I promised at the beginning of this series to explain how one can tell if a circuit is balanced or not balanced, in so far as the input is concerned. One of the defining characteristics of a balanced input is that it ignores that that is in common, such as noise and distortion, while amplifying the differences between its two inputs (balanced audio always has two signal wires; unbalanced RCA has only one). z_fig3 (1) Those inputs are made different by flipping the phase (polarity) of one vs. the other; thus as one signal wire goes positive the other goes negative. balanced-waveform What this means is rather simple, yet important to grasp. Balanced audio signals are twice as loud as those of single ended audio. Technically referred to as +6dB, balanced signals that are rendered properly in an audio circuit with a differential input, are louder by double that of single ended signals. Why is this? Look at the example of the balanced audio signal showing Pin 2 (hot) going positive while Pin 3 (cold) is going negative. Note the distance between the two is double, relative to a single signal wire and ground. Like two cars traveling away from each other at the same speed, covering twice the distance as one car, opposing waveforms have twice the 'difference' between them - thus produce twice the voltage in an amplifier that only amplifies differences. Tomorrow we'll start on complementary circuits.
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Paul McGowan

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