CAR AMPLIFIERS INFORMATION:

An amplifier takes a signal from a source and makes it larger. This makes the sound louder. The hard part is amplifying the signal without adding any distortion. Amplifiers must have a power supply strong enough to keep them going and must store enough current to deliver all of the transients in the music.

How they work
A cassette radio or CD player produces a maximum of about nine real watts into four speakers given the power it gets from the car's electrical system. So what if you want more than nine real watts to power each of the speakers? You obviously need an amplifier. Amplifiers give more power allowing you to play your music more loudly.
Amplifiers also improve the sound quality of music at all volume levels.

Features of Car Amplifiers
Here are some important facts you should be aware of if you want to choose the right amplifier...

• Bridging amplifiers are very flexible because they can be used in different system configurations. One amplifier can be used to power one, two, three or four speakers.

• Speaker-level inputs are the only way your car amps can get signals if your radio doesn't have RCA-type output jacks.

• Preamp outputs are very useful when creating a multi-amplifier system because they allow an un-amplified signal to pass out of one amplifier to another.

• Remote bass control is available in some amplifiers that have a port to add an optional control knob. This varies the amount of power going to the subwoofer driven by the amplifier.

• High-pass filters, low-pass filters and crossovers give you the flexibility to remove the frequencies you don't want to send to the speakers powered by the amplifier.

• Bass boost is available in some car amplifiers. This lets you boost the bass significantly up to 18dB through a circuit.

An amplifier can have one channel of output or as many as eight channels. The most common amplifiers are two and four channel models but mono amplifiers are also becoming popular.

Main classes of amplifiers:
Class A amplifiers gives the least distortion, but are terribly inefficient. They are rarely used in commercial amplifiers because they produce too much heat and give too little output power for the effort. In class A the output transistor(s) are always conducting, even at idle with no signal. The power draw of a class A amplifier is roughly constant and they are most efficient at full-output. If the load ends up drawing too much current the amplifier can wind up leaving class A (one transistor switches off in a push-pull, for example) during a cycle.

Class A/B is what most commercial amplifiers are. They are much more efficient, but do produce more distortion. A class A/B car amplifier is formed when a class B style output stage is biased so that around the crossover point both transistors are conducting. This yields more distortion than either a proper class A or class B amplifier, however the bias point is much less critical.

Class B amplifiers have two transistors, one per supply rail. In properly-biased class B, only one conducts at a given time, but there is always one conducting. Much of the bad name class B has is due to amplifiers actually being under biased into class C where there's a portion of the cycle around the crossover point where neither is conducting.
Furthermore, Class B proper biasing may be made very difficult to achieve due to thermal stability issues (especially proper thermal coupling and tracking between the biasing circuit and the output power devices). Class C amplifiers are useless for audio. They are used in RF applications where the harmonics can be filtered out.
Nonetheless, the so called "Class G" is just the plain combining of a normal Class AB car amplifier output stage with a Class C "booster" enabled to operate only if high power peak are required by the load. If properly designed they performances are equivalent to that obtained by normal class AB amplifiers.

Class D amplifiers are a rather new phenomenon in the hifi world. They are extremely efficient (80%) and can give a very good result. They use pulse width modulation to amplify the signal; this lets them use the output transistors in switch mode where they're most efficient and dissipate the least power. Originally only for subwoofers, newer designs have since surfaced making this technology capable of sounding very good as a full-range amplifier.

A class D amplifier is one in which the output transistors are operated as switches. When a transistor is off, the current through it is zero. When it is on, the voltage across it is small, ideally zero. In each case, the power dissipation is very low. This increases the efficiency, thus requiring less power from the power supply and smaller heat sinks for the amplifier. These are important advantages in portable battery-powered equipment. The “D” in class-D is sometimes said to stand for “digital.” This is not correct because the operation of the class-D amplifier is based on analog principles. There is no digital coding of the signal. Before the advent of the class-D amplifier, the standard classes were class-A, class-AB, class-B, and class-C. The “D” is simply the next letter in the alphabet after “C.” Indeed, the earliest work on class-D amplifiers involved vacuum tubes and can be traced to the early 1950s.

The high switching frequency used in class-D amplifiers is a potential source of rf interference with other electronic equipment. The amplifiers must be properly shielded and grounded to prevent radiation of the switching harmonics. In addition, low-pass filters must be used on all input and output leads, including the power supply leads.