Aside from the built-in preamps in mixers, sound cards and computer audio interfaces, external preamps are available with tube and solid-state circuitry. Each has its own sonic advantage and extra features depending on type, model and use, so experimentation is key to their use. Preamps that do not alter the microphone sound are known as "transparent" preamps and are used primarily to boost the microphone signal. "Color" preamps add warmth, depth and tonal characteristics for applications requiring it.
Using a "color" preamp for vocal recording can make weak voices sound stronger, and can add warmth and edginess where warranted. Transparent preamps in "live" sounding recording rooms are sometimes used to take advantage of the room sound when additional coloration may not be desirable.
Color and transparent preamps are used extensively for musical instrument recording, particularly when using microphones with acoustic instruments. Preamps can add warmth and sparkle to pianos, brass, woodwinds, acoustic guitars and other stringed instruments, but can also be used to beef up tone of electric guitars, basses and keyboards.
Recording of drums and percussion is a perennial challenge for even the best-equipped studios. With multiple tones and frequencies requiring different mics and placement techniques, preamps help fine-tune the desired tone. Although transparent preamps are often used for drums, color preamps are frequently used to add sizzle, depth and sonic separation to cymbals, and other low-level, frequency-specific percussion instruments.
Experimentation and documentation of results is key to using preamps. With so many variables -- microphone type, mic placement, room sound, performance style, musician tone, instrument type and others -- there is no easy answer as to when, what type or how preamps should be used. Ongoing documentation of these variables and their results is part of the learning process. Eventually, studio engineers get to know what works and what doesn't for repeatable and predictable results in future recording sessions.