Microphones generate a very low level signal. Mic preamps amplify that signal to "line level".
A preamp is an individual electronic unit that amplifies one mic signal. However, you can put a mic preamp (or multiple preamps) into a stand-alone box or into a mixing console. For example, if you put four of these preamps into a box you now call it a four-channel preamp because you can now amplify four separate mic signals.
The signal chain goes mic > mixer (or stand alone) preamp > recording device. For example, I record my acoustic guitar in stereo by plugging two separate mics into two of my mixer's mic preamps, then go from my mixer's main-outs into two of my computer soundcard's line-ins. It's that simple.
If you have a stand-alone preamp you don't need a mixer unless you are recording multiple tracks at once (and have a recording device with multiple inputs, as well).
Ya,,, I recently bought a Tube Pre the Presonus TubePre
it has a tube in it, which gives u a better tone ... and i really dig it ... i record vocals and acoustic with it ... also if i need one mic for drums ... congas or something like that
as WX3 said, your mixer has preamps in it, generally cheap ones.
A mixer's mission is to take multiple tracks and make them into 1 or 2. A preamp's mission is to raise the gain of a signal input. You can plug as many mics into the pre as it has channels, usually 1 or 2, but could be as many as 8, as in an octopre. Most people use the best preamp they can lay their hands on for 1 or 2 mics, and a mixer for multiple sources, especially multi-mic'ing drum kits.-Richie