The term microphone preamp can either refer to an electronic circuit or separate device that the microphone is plugged into. In either case, the function of the mic preamp is to increase the signal that reaches the speakers from that which the sound is fed through. This can be done by increasing the gain of the input signal, which allows the volume level of the sound to be controlled.
There are many forms of microphone preamps in use these days. The most common ones are rackmount, wireless, and ground models. Racksmount is a term used to describe a housing with one or more rear panel input terminals. They tend to have a high amount of gain which is good for high signal levels and volume control.
Wireless mic preamps are very popular because they are easy to set up. The only drawback of them is that they tend to pick up some extra noise that is picked up by the audio signal input terminals. Some manufacturers have come up with solutions to eliminate this issue, but most users find that it is not worth the trouble.
Ground models are somewhat similar to rackmounts, however, there is a significant difference in the way the audio signal is processed. The mic preamp uses push-pots or transistors on each channel. When the signal gets to the front of the speaker, the signal is amplified and pushed back into the mixer which is then sent out as an audio signal to all speakers in the room. This is accomplished by means of buffers. Some modern buffers have buffering capability built right in. Others use what is called phase shifting circuitry to change the signal once it is passed to the speakers.
A common feature found in many microphone preamps is a THX certified ground wire which will eliminate signal interference from ground loops and other sources. Many microphones have line input terminals so you can connect the mic preamp to your sound board. Other types will have separate line-level input terminals.
There are also several different ways to control the volume of a mic preamp. Some use knobs or pads on the front of the unit, while others have a switch on the back that will turn the volume up and down. Many professional quality preamps provide a number of different options when it comes to controlling the signal levels for recording purposes. For most users, however, using one of these knobs or pads is usually all that is necessary for creating a quality recording.
The gain stage is perhaps the most important setting for most users of any mic preamp. This is the point at which the signal levels get amplified so that it can be processed through the amplifier. Most professional quality units will come with several different levels of gain, so this is usually not something you need to change unless you are using the unit with an expensive piece of audio equipment. If you are simply processing one source of audio, however, it may be useful to experiment with different levels of gain until you find a setting that works well.
One feature that is often overlooked when looking for a mic preamp is that of a THX certified sound card. This is a special type of chip that guarantees the sound quality of any device that uses the microphone preamp as well as any source audio. Unless you are going to be doing professional sound checks or monitoring of your own work, this feature may not be needed. However, if you are using the unit with a computer or another source device, you will want to make sure there is a THX certification so that you are sure the audio is clear and audible.