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I'll try not to extend this series on balanced audio too much longer. I don't want to wear out my welcome. But I do think its an important subject to understand and not as many people as I would hope seem to grasp its importance—especially the British. As best I can tell British hifi companies and their customers seem "opposed" to balanced audio, while us Yanks take it in stride. Who knew there was such a divide? Whether you're for or against the benefits of balanced audio, it would seem useful to at least understand what it is. We're not talking politics or religion here! Well…it is high end audio… Yesterday we were introduced to the diff pair—two amplifying devices connected together in such a way that common input signals are cancelled, while difference signals are amplified. Here's what it looked like. Diff Q1 and Q2 are the amplifying devices, the arrows of their emitters tied together. This whole circuit consumes five parts, two of which are transistors (though they could be tubes) the other three are resistors. This very topology, with a handful of extra parts and a power supply, is essentially what a few expensive preamplifiers have, and not much else. Here's the takeaway for today. Power supply noise rejection. Believe it or not, this simple little handful of parts not only lowers input noise from a balanced cable, it also performs the same magic with the power supply feeding it. Ripple, hum, noise, common to the power supply, are reduced handily by the same means that lowered input noise, common mode rejection—the propensity of a circuit to ignore that which is in common, and amplify that which is not. The positive and negative power supplies connect equally to both sides of the circuit. Tomorrow we'll cover balanced outputs, then pull all three elements together in the last few posts, before moving on to…hell, I don't know.
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Paul McGowan

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