Without a preamp, most microphones provide an extremely low-level audio signal. Preamps are used to apply gain, or amplification, to this signal. This, in turn, makes the signal from the microphone similar to the levels of other instruments. Since it is louder, it is also easier to edit. Preamps come in a variety of shapes, sizes and price ranges. They are essential to the transmission of sound from a microphone. Because they boost the sound of a microphone so drastically, preamps tend to color that sound in characteristic ways; the overall sound quality of a recording is directly related to the quality of the preamps used to make it.
An audio interface connects musical instruments to your computer. Audio interfaces typically have a number of inputs for instruments and microphones. Some audio interfaces have built-in preamps. Once sound is captured from the external sources, the signal must be converted from analog to digital before it is sent to the computer. All audio interfaces contain analog-to-digital conversion hardware.
USB microphones contain the preamp and audio interface mechanisms within the microphone itself. Sound is sent into the microphone, an internal preamp boosts the signal, and the signal is converted into digital data. Then, it is sent to the computer via USB. If USB microphones did not have preamps, very low-level signals would be sent to the computer. These signals would then have to be amplified in a Digital Audio Workstation. In order to give the user some of the flexibility that preamps provide, some USB microphones feature preamp gain control directly on the microphone.
USB microphones simplify the recording process. They eliminate the need for external preamps and audio interfaces. Hence, USB microphones offer an easy way to capture vocal sounds in a simple and direct manner. However, USB microphones bypass a lot of the customization and features of using external preamps and interfaces. Because the overall sound varies depending on the preamp, a built-in preamp limits you to one sonic configuration. Furthermore, many audio interfaces many inputs and a variety of controls. By using a USB microphone, you are limited to only one input.