Some Features of Car Amplifiers:
• 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.
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 underbiased 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.
See more info at: caraudioproductreviews.com
