The amplifier is the central component of a stereo system. It takes the signal from an output device such as a tape deck or CD player, amplifies the signal and transmits it to a set of speakers so the sound can be heard. Any system with speakers requires some type of amplifier.

Amplifiers are rated by the power they produce, expressed in watts per channel. One channel of amplification powers one speaker, and for stereo, two amplified channels power two speakers. You can purchase an amplifier producing as little as 15 watts per channel for home speakers to as much as 200 watts per channel. Most moderately priced amplifiers provide from 20 to 100 watts-per-channel which is plenty of power for an average room.

The preamplifier tells the amplifier which signal source, such as CD or tape deck, to amplify and output to the speakers.

The receiver obtains radio signals via antennas.

The tuner allows you to choose which radio frequency you wish to hear.

Amplifiers commonly come with preamps, radio receivers and tuners integrated into the component, though some audiophiles may choose to connect three or even four separate devices together. For ease of identification, these integrated packages are referred to as amplifiers, and very often, as receivers, too.

Almost all amplifiers are solid-state transistorized designs. However, reaching back to the early days of radio, a few audio companies have reintroduced vacuum tube amplifiers, eschewing the transistor in favor of "warmer" sounding tubes. Because of their tendency to be very expensive, the high-end amps are manufactured in low quantities.

Features

All amplifier combinations offer basic volume, balance and tone controls. These allow you to adjust the volume louder or softer, adjust the volume balance between the right and left channels, or increase or decrease the volume of bass and treble.

Many other features are available in amplifiers such as:

• Inputs for a 75- or 300-ohm FM antenna, and an AM wire loop antenna.
• Outputs for multiple sets of speakers that can be driven individually or simultaneously.
• Radio tuner with memory for AM and/or FM station settings, call letters or numbers.
• Input jacks for a phonograph, CD player, tape deck and other audio components.
• Recording (input) and playback (output) jacks for audio recording decks, such as a tape, or a recordable digital format such as CD-R or MiniDisc.
• A headphone jack with a separate volume control.
• Separate preamplifier outputs and main amplifier inputs with controls to join or separate them (this feature enables you to connect a separate power amplifier that may supply more power and that turns your receiver into a preamplifier and switcher).
• A universal remote control that allows you to operate every device hooked into the receiver.
• A display window that provides source, function and mode information.
• Multiple AC outlets, including at least one switched, allowing you to plug in a device that will power up when you turn the receiver on and power down when you turn the receiver off; and one unswitched, which supplies power to another device whether the receiver is turned on or off.

If you plan on connecting your TV or VCR into your integrated amplifier or receiver, consider an audio/video control center, also known as an A/V receiver. An A/V receiver contains inputs for video equipment (VCR, digital disc players, satellite TV & cable box) a surround sound decoder and additional amplifiers to power surround sound speakers. A surround-sound system can give the listener the feeling of being in a concert hall or jazz club, a football stadium or a movie theater. The combination audio/video surround sound amplifier is discussed under Home Theater.

Bridging

Most power amplifiers power multiple channels: a two-channel amplifier supplies power to a pair of speakers; a four- or six-channel amp can power two or three pairs of speakers.

Many multi-channel amplifiers can be bridged so that two single channels can be connected to create a single channel supplying twice the power. In other words, a four-channel, 60-watt-per-channel amplifier can be bridged to create a two-channel, 120-watt-per-channel amplifier. This kind of amplifier affords the highest level of flexibility should your taste in speakers change or should you decide to alter your system configuration.

Most companies include instructions for do-it-yourself bridging, but if you've never done it, it is best left to an authorized service technician.