Microphone amplifiers, often refered to as a mic "preamp", is an especially sensitive amplifier that has an input especially designed to amplify the weak signal coming from a microphone. Because of the extreme amplification needed, they are relatively expensive to make, unless you somehow cut corners in the design or choice of components.
Must have features
- Adjustable input gain (well duh!)
- A highpass filter that can be switched on or off to eliminate unwanted rumble. Not all microphone have this built in. Some preamps even offer cutting off the subsonics at several different frequencies, which is also useful.
- Phantom power for condensor mics.
Nice to have features
- Phase inverse switch (for working with more than one mic.)
- A pad button for working with very loud sounds.
- Limiter for avoiding nasty clipping if something unexpected happens.
- Insert jacks for adding compressors, EQs and such to the signal chain.
- Built-in analog to digital converter. This is useful if the input of your soundcard isn't that good.