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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