Hi Bob,
a preamp is an amp that amplifies the incoming signal at an optimized output value that can be used for further working.

In most cases you'll need them to reach clean and loud (noise reducted) signals. But in some cases you may wanna create heavier tunes so you'd like to overdrive (distort) it a little bit.

Simply all front end instruments does it use for output leveling. Even the back end system - the power amp - does use a preamp for controlling the input levels balanced to the power amp input (this gives you the ability to overdrive your power amp!).

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Greetings from Frankfurt (Germany),
Sheriff ;-)

[This message has been edited by Sheriff (edited 04-04-2005).]