An amplifier's purpose is to take a quiet signal and make it louder. A preamp is merely an amplifier that precedes the main amplifier. The typical function of a power amplifier is to output a signal to a set of speakers, and frequently is designed to handle very specific signal levels –- typically, a line level signal. Therefore, any device that outputs a lower-level signal must be brought up to line level for the amplifier to operate properly.
If you have ever selected a component on your system and noticed a drastic drop in volume compared to the other devices, you have a device connected which may require a preamplifier. Nearly all turntables, for instance, have phono outputs that must be amplified prior to the speaker amplification stage. Additionally, most microphones and electric instrument signals require a preamp, but rarely will such devices be used in a serious home theater system.
Dedicated phono preamps are common, ranging from very inexpensive utility boxes all the way to extremely expensive audiophile units. The latter are typically designed with a specific model or range of turntables in mind, a “matched set” which should bring out the best qualities of both. A high-quality preamp will improve the sound of even lesser-quality turntables, but only to a certain extent; don't expect to get the same results from a 30-year-old Sears record player as you would from a Music Hall turntable. Similarly, running that audiophile turntable through an inexpensive utility preamp will work, but it will probably fail to do justice to the subtleties of vinyl.
More commonly used are combination source-selector/ preamps, which allow the listener to integrate all home theater components and specify the amount of gain (signal boost) applied to each source. These form the backbone of any serious home theater setup and tend to provide many features in addition to pre-amplification. This is usually where the actual processing occurs for stereo and surround systems, and may also contain the DACs (digital/ analog converters) that will define the quality of your high-definition listening experience. One may consider this component as taking the “raw” sound from each source and turning it into the final, polished mix that is sent to the power amplifier and speakers. Even if the attached components are line-level already (and thus do not require signal boosting), the preamp will almost certainly add character, clarity, and space to the sound.
Occasionally, you may find a component which purports to output a line level signal and yet has a noticeably different volume range than the other components in your system. Professional or audiophile audio devices have rigorous standards to combat this very problem, but consumer audio and home theater equipment standards can vary much more drastically. A telltale sign of questionable signal levels is the hum or noise apparent in the signal when boosted to a volume comparable to the other equipment in your setup. Sometimes the culprit Is an amplifier stage within the component itself; a common example is a television or portable device (mp3 player, etc.) connected to the system. A preamplifier may assist with troublesome signals from such devices, especially if the preamp uses some sort of noise filtering and cancellation, but often you may find that replacing the questionable component with a higher-quality alternative is a better permanent solution.
Preamps come in both the solid-state and tube variety. For many serious listeners, a preamplifier using vacuum tubes (or “valves”, depending mainly on your side of the Atlantic) is absolutely necessary for the ways in which the tubes can enrich the sound, especially to mitigate an overly “stale” or “brittle” digital sound. While there is still healthy debate over the real benefits of tubes, many discerning listeners testify to the added dynamics and warmth. Detractors argue that tubes add an inherent distortion and “color” to the sound, and tend to be more expensive and fragile, while solid-state preamps should provide more accurate sound reproduction and an increase in general durability. Regardless, there is no lack of high-quality, well-regarded preamplifiers of either type. As always, let your own ears (and needs) be your main guide
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