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The term nominal impedance in electrical engineering or audio has a number of different meanings. It refers to the approximate designed impedance under certain conditions. The actual impedance may vary from this nominal figure. The term is used when the exact value of impedance is not important to the operation of the circuit or system.
Most signal-processing equipment is designed to operate with the internal impedance of a signal source much smaller than the load impedance of the input to which it is connected. This provides the most efficient voltage transfer, and is best in most, but not all, situations. The nominal output impedance is the assumed impedance for the purpose of deciding whether the equipment can drive another piece of equipment with a known input impedance.
If impedances are mismatched between inputs and outputs that are connected together:
The nominal line impedance is 600 Ω. Both of these, and the name "line", come from the standards of the earliest telephone networks. It is unlikely that the cable will have an exact 600 ohm characteristic impedance, but this is generally unimportant for the relatively short runs of cable compared to the highest audio frequencies (20 kHz) in use. The main reason for standardisation is for the source and loads to be matched to each other for maximum power transfer.
This is a common standard for connections between pieces of electronic equipment, but 100 mV at 600 Ω is considerably more level than is common for a low impedance input, and represents a greater signal strength than is common for high impedance connections as well, so connecting a high or low impedance signal into a line input normally requires a pre-amplifier, and going the other way may require a dummy load.
Nominal impedance here refers to the impedance of the speaker at a defined low frequency in free air. The impedance will change with loading and frequency. In the days of valves, most loudspeakers had a nominal impedance of 16 Ω. Valve outputs require an output transformer to match the very high output impedance and voltage of the output valves to this lower impedance. These transformers were commonly tapped to allow matching of the output to a multiple loudspeaker setup. For example, two 16 Ω loudspeakers in parallel will give an impedance of 8 Ω.
Since the advent of solid-state transformerless outputs, these multiple-impedance outputs have become rare, and lower impedance loudspeakers more common. The most common nominal impedance for a single loudspeaker is now 8 Ω. Most solid-state amplifiers are designed to work with loudspeaker combinations of anything from 4 Ω to 8 Ω.
Cables for transmission of high frequency signals usually come in one of two nominal impedances: Either 50 ohm or 75 ohm. This means that the characteristic impedance of the cable is close to one of these figures. The figure may vary slightly with frequency.
The effective impedance of input or output is the equivalent impedance of all of the output or input circuitry of a device lumped into one (imaginary) component. It should not be confused with nominal impedance. An impedance is a combination of resistance, capacitance, and inductance.
See impedance for a more technical discussion.
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