What Is a Microphone Preamplifier and Why It Is Important?

Microphone preamp or preamplifier is an audio device for preliminary sound processing and its purpose is to amplify the microphone signal before it gets to the recording device or other equipment such as a Public Address system, amplifier or a mixing console. Microphone level or instrument level sound is generally faint and unusable. The preamp is needed to intensify the sound and fit it into line level sound without causing any additional and unwelcome noise and distortion. Playing a crucial part in the recording process and the creation of quality sound makes the preamplifier essential for producing a good final audio recording. Of course, it is only another link in the sound recording and producing chain, but choosing a good preamplifier will contribute significantly to achieving your goals. Not only does it add volume, but also it adds its own tonal character and dimension to the original tune and improves it.

Types of preamps

Sorted by form – built-in/ integrated/ internal and External

• The internal mic preamps are integrated into another tool such as a microphone, mixer or stereo system, etc. They are usually designed to be transparent and clean.
• The external preamps are separate dedicated devices containing preamplifier circuits. They are often considered to sound better and they provide additional features that go beyond volume boosts, such as tonal modifications and they also might have built-in equalizers, or analog to digital converters and compressors to control the dynamics of the signals.

Sorted by circuit design – tube, solid state or hybrid

• Preamps with vacuum tubes are used again to amplify but also mainly distort the sound and achieve harmonic tonal saturation and alterations.
• Solid-state circuit preamps use electronics such as transistors, resistors, and diodes to amplify the signal. They provide a clean and crisp sound with little to no distortion.
• Hybrid – include both tube and solid state circuit with the opportunity to select each alternative or a combination of the two.

Sorted by output sound – transparent or coloration

• Transparent or clean preamplifiers amplify the sound to line level omitting the unwanted noise from the environment. Increasing the signal means, increasing the volume of all the background sounds as well, so the real purpose of the transparent preamps is to boost the input signal while getting rid of the extra fuss to make the sound loud and clear, and as a plus to reproduce the original sound in its desired form without changes and tonal coloration. What they do is inaudible for the ears. It is worth saying that the modern electronic processing devices have advanced to a point where the models with transparent and clean produced sound are common and cost-effective and the undesirable distortion are practically inaudible. They allow the full potential and character of the microphone to be revealed.
• Coloration devices refer to the ones that allow for character to be added to the original signal. For quite some time, having in mind early mic preamp design, the sound distortion or additional noise which remain after processing a signal have been unacceptable, however, more and more often with the design development, some of those alterations and modifications have become appropriate and even desirable. Adding those changes is now generally referred to as contributing to the quality and enhancing the sound, described with the words of adding harmony, color, dimension, character, warmth, smoothness, silkiness and so on. This is considered to make the sound unique. One can safely say that just as microphones have their “own sound”, so do the microphone preamplifiers.

Basic Features

• Inputs and outputs
  – XLR mic input (at least one, more functionality is desirable thus more inputs are a plus)
  – hi-Z Direct Input (DI) – Instrument input for connecting an electric guitar or bass guitar
  – Some external preamps also feature Line inputs – balanced and unbalanced.
  – XLR line output
• Gain Control or Volume control (AGC or AVC, A stands for Automatic) – its purpose is to maintain or control the output range, despite the variation of signal at the input and is used to adjust the gain (increase of the power) and the average volume at the output in order to balance the differences of strength of the received signal.
• Switch for Phantom power – It is Phantom power because there is no external visible power supply or dedicated cable to charge condenser microphones. In fact, the mic is powered by the same input wire which is delivering the audio signal.
• Phase (Polarity) reverse – used in case of multiple mics to reverse or swap the signal polarity so that both signals are in phase and they do not cancel out each other.
• Low cut/ High-pass filter – filter which allows only a concrete range of high frequency to pass through it and reduces and removes the signals with frequencies lower than the certain boundary of low frequencies (called cutoff frequency).
• Trim control – refers to altering the range of a signal to match a particular device, fine-tuning input gain or output level.

In conclusion, it should be noted that there are a variety of microphone preamplifiers with a wide range of configurations and specifications at hand to match almost every need and situation. And no matter if it is to be used for studio or for live sound, the mic preamp is always an essential tool in the reinforcement of sound.