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Preamps are an essential part of the recording chain. This post is a clear explanation and helpful overview of the topic from Mark Garrison's book Encyclopedia of Home Recording.
A preamplifier, or preamp, is a device for boosting the level of a signal. Specialized preamps exist for a number of purposes, such as a guitar preamp or a phono preamp, but the most common for recording purposes is the mic preamp.
The preamp can significantly affect the tone of a signal. Much like microphones, many studios have a variety of preamps on hand to allow them to create the right tone for the application. The quality of the preamp’s construction and components, as well as whether it works on transistors or vacuum tubes, are among the factors that will affect its tone. Tube preamps are often used in digital recording to help “warm up” the tone of the signal.
Often preamplifiers are packaged with an equalizer and a compressor in the same unit. This combination is referred to as a channel strip.
Preamplifiers often have only one control, gain, which controls the amount of signal boost provided. This should be set at the highest level possible without the signal distorting in order to provide a healthy signal to the rest of the signal chain.
Most preamps have the ability to provide phantom power. Phantom power is a current supplied through the microphone cable to power condenser mics, active DIs, and some other devices. If a preamp has phantom power capabilities it will have a switch, usually labeled “phantom” or “48V”.
Many preamps also have a pad feature. Turning on the pad will reduce the signal at the input by a specified amount (usually labelled on the pad switch). This is handy in situations where too much signal is coming from the microphone.
A low-cut is also common on preamps. This will place a high-pass filter on the signal, cutting all signal below a set frequency. The cut frequency varies from one preamp to the next, but the switch will almost always be labeled with it. A low-cut can be used when recording an instrument that does not generate any frequencies in the range being cut. This will reduce unwanted low-frequency rumble that can make its way into the recording through the mic stand.
Phono preamps are used to connect a turntable to a mixer or other device.
In the early days of record production, manufacturers ran into the problem that creating a loud, low sound required a deep, wide groove to be cut in the record. This often resulted in the record lathe cutting right through the disc, or breaking the wall to the next pass of the groove. To solve this problem, records are recorded with a slanted frequency response, meaning the lower the frequency, the quieter it is.
A phono preamp takes the signal from a turntable, grounds it to prevent hum, restores the original frequency response, and provides a line-level output that can be accepted by other devices.