A preamplifier is a unit that connects several sound sources (a CD, a DAC, a turntable, etc.) and the amplifier (a power amplifier for speakers, active speakers or a headphones amplifier). Should a preamplifier be embedded into a power amplifier, that is called integrated amplifier.
The most common controls on a preamplifier are a source selector (activating one source at a time), a volume control and (rarely) the tone adjustment.
A separate unit is a phono preamplifier which is specifically designed to amplify a signal from a turntable (this will be covered in another article).
Preamplifiers may be divided into active and passive ones.
This type of preamplifier can do both: atenuate (when the output is weaker than the input) and amplify (when the output is stronger than the input) the signal coming from the source.
By electronic approach they are divided by their architecture of active elements: transistors, integrated circuits and valves.
Every type of active elements (including different models and brands inside those elements) give the sound a specific character that may or may not appeal to the listener. So, whenever we use an active preamplifier, we have to take into account that we operate with an electronic device with its strenghts (amplification, control, character) and weaknesses (distortion, noise, …).
A passive preamplifer has no active electronics. The signal at the output can be equal or lower to the one at the input. Usually this type of a preamplifier holds only the volume control (signal atenuator) and the source selector. Most of them don't even use electric power, except if the switcher is based upon relays.
Since many sound sources have a sufficiently strong output signal, it is just fine if we plug them directly into the power amplifier.
The advantages of a passive preamplifier are that it doesn't alterate the signal and it injects its own „will“.
You can get various sizes of these preamplifiers: small ones for desktop use, or larger ones for high end audio.
Since passive preamplifiers have no active electronics, we might think of them as a cheaper solution. Well, it's not. They are mostly constructed with higher quality elements and, in plenty of cases when wiring is involved, also hand crafted. The price of both types is not that much different, it is only up to you what you expect from a preamplifier to do for you.