Expert Car Audio

Amplifiers classes:

Car Amplifiers are clasified by 5 main classes:

Class A Amplifiers:
Although class A amps provide clear sound, they are considered to be inefficient because they get heated very fast. Class A amplifiers also don’t provide enough power for the effort they consume. Class A amplifiers are ideally used for car tweeters.

Class A/B
Unlike class A amps, class A/B amps are considered to be more efficient but 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 amps consist of two transistors, one for each 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.

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.