Some applications, RF for example, require terminators with a 50 or a 75 ohm impedance. An improperly terminated connection could result in "reflections" and signal distortion. Severe cases could result in equipment damage. Impedance MATCHING between source and load is required in these situations. The interconnect cable has to have a characteristic impedance (of 50 or 75 ohms) for maximum energy (not voltage, not current) transfer from source to load.

Audio frequencies are a different story. Audio frequency signal sources, at the "line" level (signals on the order of a volt or two), such as a pre-amp, cd player(analog out), dvd player (analog audio out), AM-FM tuner, etc, have output circuits that are voltage sources. Ideal voltage sources have 0 ohm output impedance. Real-world voltage sources are different. It is generally considered "good" if the above mentioned pieces of equipment have an output impedance in the low hundreds of ohms. Lower is better.

The line level inputs on equipment of the above mentioned type (and power amps also) have impedances that are very high compared to the equipment that is supplying the signal. These input impedances are usually tens of kilo-ohms. This allows near-maximum VOLTAGE (not current, not energy) to be transfered from the source to the load (examples: from the CD player to the preamp, from the preamp to the power amp and in the majority of applications, from the power amp to the speaker).

When an input (say, of a preamp) with nothing connected to it is selected, noise generated in the input section of the input stage is amplified by the input stage (and also the following stages of gain). With this (preamp) input connected to a source (with a low impedance output stage) such as a CD player, much of the noise generated by the input stage (of this preamp) is "shorted" out by the low impedance of the source device's low output impedance. Of course, any noise in the source component output will be passed onto the next piece of equipment.

Without breaking the noise into its component parts of current noise and voltage noise, etc., connecting an unused input (on a device like a preamp) to a shorting plug (a true short or 50 ohms or 75 ohms) simulates the low impedance provided by the connection to a signal source. The 50 or 75 ohm version of these shorting plugs can be used in situations that require a 50 or 75 ohm impedance. Compared to the tens of kilo-ohms input impedance of a pre-amp, the 50ohm or 75 ohm or near 0 ohm "shorting plug" placed at the input acts as a short for the majority of uses in this situation, yielding the same result: somewhat reduced noise.

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the 1derful1


p.s. Edited to make spel and gramr mistakes less noticeable and to increase happiness of reading.

[This message has been edited by Paul J. Stiles (edited June 01, 2002).]