For the purposes of this article, an amplifier and its various parts will refer to a music amplifier such as a guitar amplifier. An amplifier can generally be split into two parts: the preamp and the power amp. Each part does a separate job to produce a powerful sound that is both ample in volume and clear in tone. The preamp is responsible for changing the tone and clarity of the signal, while the power amp is responsible for powering the speakers and the volume of the tone.
While these components are two separate entities, they are often contained in one unit, called a head. In other cases, such as rack-mounted systems, the preamp and the power amp are two separate units that are connected by speaker cable to produce the final tone. The components are separated to reduce noise from the power amp from affecting the tone of the preamp. Though the two separate units produce the clearest tone, they can be cost prohibitive. They take up more space than an amp head as well, and an amp head contains both amps in a single unit, which makes transporting and storing the amps much easier.
The preamp often includes tone controls such as low-, mid-, and high-frequency dials. These dials shape the tone of the signal being processed, allowing the user to boost or lower bass frequencies, treble frequencies, or mid tones. The preamp is a versatile tool in shaping the sound of an amplified instrument or microphone. Modern preamp units also contain other tone controls as well as effects.
For example, it is not uncommon to find controls for gain and presence on the preamp. The gain adjustment can boost the tone signal, causing a loud, distorted tone. A presence adjustment will modify the highest frequencies of your instrument's tone to affect the clarity and crispness of the output sound. In addition to these common controls, a preamp may also include an effects processor that allows the user to add chorus, flange, tremolo, distortion, and various other effects that create a unique tone.
Once the preamp shapes the tone of your instrument's signal, the signal is sent to the power amp. The power amp is responsible for powering the speakers that will produce the final sound. A power amp comes in a variety of sizes measured in watts; the higher the wattage, the more sound the amp is able to produce. While the preamp does almost all of the tone modeling of your signal, keep in mind that the volume of your signal will also affect your tone somewhat.