GOODSOUND!GoodSound! "How To" Archives

Published July 15, 2001


An Amplifier "Drives" Your Audio System

An automobile requires an engine to provide power to turn the wheels. And every car -- from the powerful muscle cars to the smallest econobox -- has an engine. Similarly, every stereo system (or home-theater system) employs an "engine" -- the amplifier -- to drive the speakers. That's because the signal strength of source components on their own is insufficient to power the speakers directly.

In some systems, the amplifier is built into an all-in-one style component. If you have a receiver, it consists of a radio tuner, a preamplifier (see below) and a power amplifier, all combined into a single chassis, operating as one unit. If you have an integrated amplifier, it eschews the radio section completely and is made up of a connected preamplifier and power amplifier. You may also see systems in which each of these elements exist as separate components -- not surprisingly, they are called separates for short.

The job of a power amplifier is to take the relatively small (low-level) signals from the preamplifier and, using the electrical power from the AC outlet, convert those signals into several, successively larger copies of the input signal. That's a complex task in itself, but a power amplifier must also be able to control (drive) a wide range of loudspeakers as well.

The preamplifier is the control unit for a sound system. It has two basic functions: source selection and volume control. The various source components (CD/DVD, tape, tuner, VGR, TV) connect to the preamplifier and it controls switching from one component to another, routing signals to special outputs for tape recording and setting the loudness level for the entire system.

Everything that connects to the preamplifier produces the same sort of electrical signal except for turntables. Turntables, even modern ones, produce very weak electrical signals that must be magnified (amplified) considerably to reach the same levels as those coming from a CD player. These signals also require complex equalization to convert the signal from an LP to full-frequency sound. Therefore, LP playback requires a pre-preamplifier called a phono section. Since most listeners no longer own turntables, so-called line-stage preamps eliminate the internal phono stage. These are now the most common type of preamplifier. Connecting a turntable to a line-stage preamplifier requires a separate outboard phono stage.

We've noted that an integrated amplifier becomes a stereo receiver with the addition of a radio tuner. Similarly, a home-theater receiver adds a surround-sound processor to a stereo receiver. It also replaces the two-channel amplifier section with a multichannel section of five or more channels.

Certain integrated amplifiers and receivers offer a pre-out or pre-out/main-in feature. A pre-out is exactly what its name implies -- an output taken directly from the internal preamplifier section.

Pre-out/main-in jacks on integrated amplifiers or receivers separate the internal preamp and power amplifier. Metal straps or jumpers are typically used to connect the preamp output to the amplifier input. By moving the straps or jumpers you can use the pre-out and main-in jacks to connect signal processors or other devices between the two components. As already noted, you could use the pre-out to connect the unit to a better quality or higher powered amplifier. On the other hand, you could bypass the preamplifier, using a better preamp connected to the unit's internal amplifier. As an upgrade move, a pre-out could also be used for line-level (standard preamp-level signal) hook-up of a subwoofer. Whether you end up using it or not, a pre-out/main-in gives you flexibility if you ever need to upgrade your stereo system -- and flexibility is always a good thing to have.

Amplifiers (whether stand-alone, integrated or receiver-based) can use different amplification devices. Solid-state amplifiers use transistors as their amplification device. Tube amplifiers use vacuum tubes (or valves, as the British like to call them) instead.

Amplifiers are divided into several classes of operation, which are determined by two attributes: How inherently linear or low-distortion they are and how efficiently they convert wall voltage into power to drive loudspeakers. The nomenclature of the most common amplifier classes (Class A, Class A/B and Class D) does not pertain to sonic quality like a test score, where A is better than D. Good-sounding amplifiers are made using all modes of operation.

Class A is the least efficient category, but it inherently offers the lowest distortion. It dissipates up to 75% of the wall power it draws into heat. Class A amplifiers always run hot and, if they are solid-state designs, require massive heat sinks to conduct the heat away from the circuit. Class A/B is up to 50% efficient and is the most common method used for stand-alone amplifiers, integrateds and receivers. Class D, also called switch mode, is the most efficient method (up to 90%) and is the least linear, unless it employs sophisticated corrective signal processing. It is predominantly used in less critical applications, such as subwoofer amplifiers. However, a new breed of digital amplifiers uses Class D (or variations thereof) for high-end applications.

In summary, every audio system needs an amplifier. Its presence is most obvious with a separate box that states "power amplifier" on its front panel. It's less obvious with an integrated amplifier (and even less so with a receiver). Superb performance can be obtained with either separates or integrated components. In making your choice, you will want to consider price, space, and convenience. One-box solutions tend to be more affordable than equivalent separates and take up less space. However, if a future upgrade path is important to you, make sure that a receiver or integrated offers pre-out and/or pre-out/main-in jacks. Even if you never use them, knowing you have options can be a great comfort.

And now you know all about amplifiers, integrated amplifiers and receivers. Start your engines!

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