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The easiest way to understand preamplifiers is to think of them as the "control centers" of a stereo system because they have all the controls necessary to enable the listener to choose a listening source (ie. cd player, tape deck, etc.) and alter the sound (ie. changing volume and tonal quality). But remember that preamplifiers must be connected to power amplifiers in order to make various listening sources loud enough to be audible.
To enhance the enjoyment of entertainment and facilitate the "home theater" experience, some preamplifiers include some form of surround sound circuitry. The term "surround sound" is often used very loosely. Identical sounds coming from the front and rear of a listener can be described as surround sound. The same thing with slightly time-delayed rear sound reproduction also qualifies. Such surround sound can be used with music or any other sound sources but the forms of home surround sound reproduction that are specifically designed for specially processed movie soundtracks include Dolby Surround, Dolby Pro Logic Surround, Lucasfilm THX Home Theater, and Dolby Digital (formerly known as Dolby AC-3). But more on these later under Surround Sound.
Among the features found in a preamplfier are tone controls, a loudness switch or control, a high filter, low filter, a source selection switch, tape monitors, tape dubbing switches, a moving coil input switch, mute switch, and a balance control. A rare feature that may be found on some preamplifiers is a pair of d-a converters.
Tone controls alter the frequency response (see frequency response) of the sound. There are usually two or three controls: one for bass, one for treble, and the other for midrange. The bass control only alters low frequencies. For example, turning down the bass control would de-emphasize the sound of bass guitars and bass pedals while turning it up would make it louder. The treble control only affects high frequencies. So turning up a treble control would increase the music's clarity. Cymbal crashes, for example, would sound louder and more vibrant. The midrange control makes sounds in the middle of the audible frequency range softer or louder. Most of the vocal range of sound lies within this range so this control could make singers sound more or less distant, depending on its setting.
Loudness switches or controls are similar to tone controls because they also alter certain sounds. Their purpose is slightly different, however. When we listen to music at low levels bass and treble sounds seem to lose their presence and impact. With a loudness switch or loudness control these sounds can be emphasized to restore the lost impact.
One of the flaws we sometimes hear with some recording is hiss; it's the continuous "sssssss" sound that comes from primitive or mediocre recordings and noisy radio broadcasts. With a high filter, this hiss noise is reduced to make you music sound more listenable. Switching in this filter, on the other hand, compromises the clarity of the sound, particularly the highs (high frequencies). As a general rule of thumb, remember that there's no such thing as a free lunch.
For those listeners who still have LP's, a low filter or subsonic filter might be useful. As a result of heat or physical mishandling, some records may become warped. When they're played the warped portions are reproduced as low tones that are so low they're inaudible. These bass tones can actually damage the speakers if they're loud enough. Therefore, it's a good idea to use a low filter to remove these frequencies.
A source selection switch allows the listener to choose the tuner, turntable ("phono"), cd player, tape deck, or other audio signal as a listening source. But the setting of this switch can be overridden by a tape monitor.
