what is the best way to describe what a pre-amp is/does

[jimbob]

Im trying to compare different  'preamps and was wondering what the best way to describe what one does / supposed to do--the ideal things to look for in considering what pre-amp effects to buy or build.

[brett]

Maybe it's an old practice now, but in the old days a pre-amp took a low level input (guitar, mic) and cranked it up to line level (1V or thereabouts) WITHOUT distortion.

Not a very difficult task in these days of op-amps, etc.

[Tim Escobedo]

A preamp does nothing more than make a signal louder. It offers voltage amplification. That's the least it's supposed to do.

Guitar related preamps often incorporate overdrive sections, tone controls, etc in addition.

[niftydog]

Depends what you're after... but there are some basic things that you might consider common to all good pre-amps.  (generally speaking that is.  Being a bass player I don't use boosters and pre-amps in a trad sence, so I can't comment on things in specific guitar pre-amps!)

A basic pre-amp "buffers" the signal.  Basically, it "separates" the input side from the output side.  ie; if your guitar was having trouble driving an old fuzz-face due to it's low input impedance, inserting a pre-amp before the fuzz would help.

A more fancy pre-amp has some "shaping" or equalisation and level adjustment.


High input impedance (input buffering); prevents pre-amp "loading" previous stage. (ie; pickups or other effect device)

Low output impedance; ability to drive the following stages without loss (ie; guitar amp input or other effect device)

Level adjustment;  a good preamp will have some adjustment in level so that you can fine tune the output.  Normally you'd adjust it so that the effected signal is the same volume as the clean signal.  This is especially important if the pre-amp has equalisation controls.

Equalisation; depending on your needs.  More eq the better some would say.  Parametric eqs are more flexible.

Again, depends on your needs... but some pre-amps will have an input level and an output level, allowing you to drive them into clipping, thus getting an overdrive into the bargain.