Looking at a mixer you notice there are several strips known as input channels with identical buttons on each strip. A microphone preamplifier is basically a stand alone input strip and when you buy a recording console you are pretty much committed to the sound quality of each strip because they are all the same. That is where a microphone preamp comes in, a microphone preamps main duty is to boost a microphones low signal to a line level that is more usable for any device you are recording too. There are plenty of variety, design, features and sound quality that allow you to mix and match different preamps for your recordings.
The common features you will find on a microphone preamp is gain control, which allows you to adjust the strength of the signal being picked up by the microphone, be careful because too much gain can cause distortion and not enough gain can bring noise into the recording . Some microphone preamplifier's come equipped with EQ (equalizer), which can cut or boost certain frequencies. A helpful side note when mixing, try to cut frequencies that you do not need first before boosting frequencies.
A compressor is another feature found on some microphone preamplifier's. Compression squishes the dynamic rage of a signal, then boosts that signal so that the quiet parts of a recording is louder, for example, during a loud part of a song you can still hear the quiet acoustic guitar that you compressed. Be careful because once you use compression on your records you are committed to that compression because It is very hard to remove compression once you are editing.
Microphone preamps are also a great choice for systems based around a digital audio workstation which does not require a big console but can use a few quality preamps, which can be connected directly into an audio interface or sound card. When thinking about purchasing a microphone preamp remember that it is purely a personal choice on what you choose just make sure they come with the features you require. Microphone preamps are a very cost effective way of experimenting with your recordings, so have fun.
Sometimes you will find yourself in a critical situation and a microphone preamp can offer low distortion specs, a special sound, or use of tube and integrated circuit technology, which is why microphone preamps are used. When it comes to the selection of a microphone preamp it is purely a personal choice. The days where there was only a few ways to get a great recording is gone, now you can choose from an abundance of equipment, so it is important to try and test microphone preamps to find the one that will do the job you want.