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.
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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