A preamplifier picks up sound from a microphone or a pickup and processes it before amplifying it. It lets musicians blend multiple signals into one, and all combinations systems require a preamplifier.
Defining the Preampliflier
The exact definition of a preamplifier often eludes consumers because those in the music industry use it in many different ways. Preamplifiers can be a pedal, built into an instrument, a rack-mountable unit, a mixer or a sound card. Many preamplifiers have overlapping features with a DI -- "direct injection" -- box. The three main factors to consider when selecting a preamplifier are whether its output is at the level and impedance you want, whether it has the correct output connector plug for what you want to attach it to, and whether it can provide the tonal qualities you are searching for. A secondary quality you can evaluate is whether the amount the signal is boosted -- the dB gain -- is sufficient for your needs. A pedal commonly has 20 or 30 dB of gain, which may be insufficient for a bass or guitar, which often needs 50 or 60 dB to drive a typical power amp.
Putting a Preamplifier to Work
Musicians use preamplifiers to control their sound before it is projected through the amplifier. Preampliers can increase a gain, boost a low signal, change the tone, adjust the volume or equalization, blend multiple signals into one, convert sound from unbalanced to balance or lower the output impedance. Preamplifiers can also help solve problems such as pickup output that is too low, sound that is thin, feedback or a signal that is noisy or scratchy.