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Why wires are noisy If your tracks are plagued by noise and low-frequency hum, it's probably because of a wiring problem. But the solution is simple, relatively inexpensive, and has been around for a hundred years. Balanced audio wiring was first used by the phone company, to send calls over hundreds of miles of low-quality wire without picking up too much noise.
In the early days of high fidelity music systems, it was crucial to pay attention to the impedance matching of devices since loudspeakers were driven by output transformers and the input power of microphones to preamps was something that had to be optimized. The integrated solid state circuits of modern amplifiers have largely removed that problem, so this section just seeks to establish some perspective about when impedance matching is a valid concern. As a general rule, the maximum power transfer from an active device like an amplifier or antenna driver to an external device occurs when the impedance of the external device matches that of the source. That optimum power is 50% of the total power when the impedance of the amplifier is matched to that of the speaker. Improper impedance matching can lead to excessive power use, distortion, and noise problems.
Sweetwater Tech Tip Check out the complete archive Click here » Matching impedance as far as microphones and preamplifiers goes, is a widely misunderstood term. Most microphones can be used very satisfactorily with most preamplifiers and as we’ll see later, true “matching” is actually undesirable.