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Nominal impedance

In electrical engineering, the nominal impendance of an input or ouput is the impedance of the output or input (respectively) that it is designed to accept.

This article is intended to be a layman's introduction, and focuses on audio frequencies at which cable impedance is not significant. See impedance for a more technical discussion. See also impedance matching, cable impedance.

Most equipment is designed to operate with the internal impedance of a signal source roughly equal to the impedance of the input to which it is connected. This provides the most efficient coupling, and is best in most but not all situations. The nominal impedance is the assumed impedance for the purpose of deciding whether two pieces of equipment are compatible for connection, and is normally but not always similar to the actual impedance.

If impedance is mismatched between inputs and outputs that are connected together:

Table of contents
1 Signal level impedance
2 Loudspeaker impedance

Signal level impedance

Three common nominal impedances for low-level signals are:

High impedance

Guitar amplifiers normally have a nominal input impedance of about 50K ohms, both to suit the original guitar pickups and because this was a convenient impedance for circuits based on valves such as the 12AX7. The actual impedance of the output of a guitar with magnetic pickups and no inbuilt preamplifier is typically about 250K ohms.

The first solid-state amplifiers had relatively low actual input impedance, which was one possible cause of the disappointment some guitarists expressed with their tone. Since the invention of the FET, solid state amplifiers have had actual input impedances up to about 500K ohms.

Higher impedance means a relatively small current flows but a relatively large voltage is present. It creates greater noise problems, and this factor and the eventual loss of high frequencies limit the length of the cables that can be used.

The most common high impedance input connector is the 6.5mm (quarter inch) jack plug, which is also used for many other purposes.

Low impedance

Low impedance is used for longer lines than are practical for high impedance, particularly in conjunction with balanced lines and for microphone inputs. Typical impedances are from 50 to 600 ohms. The most common low impedance input connector is a 3-pin XLR connector, with pin 1 earth, pin 2 normal input, pin 3 inverting input. Stereo 6.5mm jack plugs are also sometimes used for low impedance balanced lines, with the right channel (the ring or collar) used for the inverting input. See tip ring sleeve.

High impedance unbalanced signals can be converted to and from low impedance balanced signals by use of an audio balun.

Line impedance

Line impedance is 600 ohms, and line level is 100 mV for full output. Both of these, and the name "line", come from the standards of the earliest telephone networks.

This is a common standard for connections between pieces of electronic equipment, but 100 mV at 600 ohms 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 preamplifier, and going the other way may require a dummy load.

The most common line input or output connector is the RCA connector, but 6.5mm jack plugs for unbalanced lines and XLR connectors for balanced lines are also common.

Loudspeaker impedance

Matching of loudspeaker impedance to the output of an amplifier is far more critical than matching of signal level impedance.

In the days of valves, most loudspeakers had a nominal impedance of 16 ohms. 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 ohm loudspeakers in parallel will give an impedance of 8 ohms. It is possible to use several taps at once but the calculations can get quite involved, so as to balance the power between the loudspeakers, match the impedance of the output, and avoid overloading any part of the output transformer secondary winding.

Since the advent of solid-state transformerless outputs, these multiple-impedance outputs have become rare, and lower impedance loudspeakers more common. The most common impedance for a single loudspeaker is now 8 ohms. Most solid-state amplifiers are designed to work with loudspeaker combinations of anything from 4 ohms to 8 ohms, and many will go down to 2 ohms or up to 16 ohms.