The Complete Handbook of Microphone Preamps

Chapter 1: Understanding Microphone Preamps

A microphone preamp is the first active electronic stage a microphone signal encounters after leaving the microphone itself. Its job is to take the tiny electrical voltage generated by the microphone and amplify it to a level that can be processed by the next piece of equipment, usually a mixing console, an audio interface, or a digital recorder. This is a simple concept, but the execution of this task determines a huge part of the quality of your final recording. The preamp is the foundation of your entire signal chain, and a weak foundation will degrade everything built on top of it, no matter how good your microphone or your converters are.

Let's ground this in a concrete example. Consider a typical dynamic microphone, like a Shure SM57, when it's placed a few inches from a guitar amplifier. The mic produces a signal of around 1.5 millivolts. This is a very weak signal. If you were to connect this microphone directly to the line input of a mixer or interface, you would hear almost nothing. You would have to turn the volume up so high that the noise floor of the next stage would become a loud hiss, drowning out the guitar entirely. The preamp's job is to take that 1.5 millivolts and boost it to about 1.5 volts, a gain of roughly 60 decibels (dB). This makes the signal as strong as a standard line-level audio signal, which can be processed cleanly without adding