If the absolute maximum power is to be transferred from a source to a load, the impedances must match exactly. The source impedance output, the cable impedance, and the load impedance input have to be equal. This is called a matched circuit. However, when a circuit is matched, the voltage swing is reduced 6dB, lowering the best possible S/N ratio by that amount. Because of low-noise preamplifier circuits, there is a better audio trade-off of power transfer for a larger voltage swing. Therefore, the idea that circuits should be impedance-matched is fallacious, and instead modern audio equipment should simply be connected via balanced input and output lines. Systems that use high impedance mics are limited to cable lengths of around 10’ or so, but there is no reason to worry about the actual impedance value; it is either high or low.
See bridging circuit.
