A preamplifier, or control amplifier, functions as a stage between an amplifier and a source component in an audio system, raising the level of the signal. In many systems, a preamp is integrated into a receiver, and functions as a source switcher and volume control. For many audiophiles, preamplifiers are considered one of the most important parts of the audio chain. Preamplifiers are also used for microphones in professional audio applications.
Preamps
Preamplifiers were originally created in a separate chassis in the early days of audio to get them away from the heat and hum of tube amps. While many transistor-based amplifiers have lower heat output, there are still practical reasons for having a separate preamp in a stereo or home theater system. While many modern sources can drive an amplifier without a preamp, having an impedance buffer keeps the output impedance of the source low while allowing for variable gain (volume), resulting in better sound.
Phono Preamplifiers
Turntables and record players need an additional preamplifier between the main preamplifier and the player, because the voltage output from the cartridge/needle is very low. Also, the creation of a vinyl record applies a Recording Industry Association of America equalization to the sound, and this needs to be removed for the record to sound right. Phono preamplifiers can be made using either tubes or transistors. Phono preamplifiers are further divided into moving magnet and moving coil designs. Moving coil cartridges are considered superior, but have a lower output than even moving magnet, and require greater gain. Most moving coil preamplifiers can provide up to 62 dB of gain, while moving magnet designs provide up to 40 dB of gain. The best phono preamplifiers allow the user to select a moving coil output or a moving magnet output.
Microphone preamplifiers
Microphone preamplifiers have XLR style inputs on them, and can be either tube or transistor-based designs. Microphone preamplifiers also have to be able to provide 48 volts of phantom power that is needed for many microphones to work. Some microphone preamplifiers also have a USB connection to easily integrate into a digital audio workstation (DAW). Some also have other professional features, such as compressors or limiters, and analog-to-digital converters for integration into a DAW via a soundcard.
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