Updated: Jul 17, 2018
What's a mic pre and why do I need one?
The proper term is microphone pre-amp. It's an actual amplifier that takes the very low wimpy signal that comes out of professional microphones, and boosts the signal to make it usable in modern electronic gear such as mixers, tape machines, or other audio equipment. Without a mic pre your mic signal would be so low you could barely hear it. No bueno.
Often you have a mic pre and might not know it's there. If a mixer or other sound equipment has a jack labeled "mic" the very next thing that happens to the signal is the mic pre, controlled by a knob labeled "gain." You adjust how much boost you give the microphone signal by turning the gain knob up or down. Simple. Too much gain and the signal will distort, not enough and we're back to the wimpy problem.
Mic pre's can be simple and small like those found in mixers where the only adjustment possible is the level of the boost applied to the signal, or they can be much more complex, expensive, and large. The picture above shows a well known mic pre often seen in home and professional recording studios. It's features include impedance matching (allows different kinds of mics to interface at their full potential),
low cut filter (less mud), and phase switching. (Phase switching helps when two mics on the same source are canceling each other out.)
One can easily be awash in choices when shopping for a pre amp. They come in all shapes, sizes, and colors. The tone quality they provide is their calling card. Some will be referred to as "gain on a wire." This means they do nothing to color the sound and simply make the signal louder. Others will talk about "big iron" which means a transformer has been used in the design. These designs seek to flatter the sound with a heftier meatier tone.
The most often abused cliche term you'll hear in reference to pre amps is the simple word "warm." Tube mic pres are often said to be warm. Softer and less analytical tones are described this way. It's tough to define warm in any meaningful way though, because there are so many variables and reasons things could sound "warm."
A good approach to begin looking at pre amps is to ask yourself if you like the sound to be clean and clear or full and (oh brother I'm going to say it---) warm. Then define your price range. Many excellent choices are available from $200-$300 on up to the moon.
So to re-cap: Mic-pre's have two purposes. The first and most basic is simply to make the signal stronger and usable. The second is improve or flatter the tone quality. A good mic pre will greatly enhance the recording quality of your mic. It's almost like getting a new mic.