Class A Amplifiers
A class A amplifier is a linear amplifier. It uses transistors or output devices to conduct electricity throughout the entire cycle of the signal input. It usually runs hot because the transistor is always on and at full power. It produces a constant signal called bias signal. It produces accurate sounds, but it is very inefficient because it loses four to five watts of output power to heat. It is generally expensive due to its tight tolerance and the additional components needed for regulating the heating and cooling processes.
Class B Amplifiers
A class B amplifier boosts a signal whose positive and negative halves are handled by various circuit parts. It uses at least two transistors that continually turn on and off to amplify both halves of the waveform. It requires a large input signal to operate properly, functioning directly opposite of a class A amplifier.
Class B Amplifiers usually run cooler than Class A Amplifiers. However, they does not produce pure sounds because of many crossover distortions. Class B amps are less expensive than class A models, as they requires two cheap transistors instead of a high-end one.
Class AB Amplifiers
A class AB amplifier combines the best features of class A and class B amplifier. It provides the excellent sound quality of a class A amplifier and the efficiency of a class B amplifier. It usually operates on lower output levels. It sets an output bias to allow the flowing of current in a transistor for more than one-half of a signal cycle but not more than one entire cycle. It is ideal for general use at home, outdoors, or in the car.
Class C Amplifiers
A class C amplifier features anode conduction with an angle of less than 180 degrees. It provides poor linearity, but it does not usually affect the single frequency power it produces. It uses a tuned circuit that restores a signal to near sinusoidal shape. Class C Amplifiers are more efficient than a class A, B, or AB amplifier.
Class D Amplifiers
Many audio enthusiasts usually confuse a class D amplifier with a digital amplifier. Some models can use coherent binary data, allowing them to run in digital mode. However, this is not true for all the amplifiers in this class. It is better known as a switching amplifier as it uses a transistor that rapidly turns on and off two times in every cycle. The switching frequency of Class B Amplifiers allow the transistor to switch on and off millions of times every second. It offers an almost 100% efficiency. It is ideal for subwoofers and other non-high-fidelity applications.
Other amplifier classes
Some amplifiers belong to class G and class H. They look like Class AB Amplifiers, but both classes uses a dual power supply that turns on or off according to the imputed musical signal. They offer enhanced efficiency and more power.
Class G amplifiers are ideal for pro audio designs, while class H amplifiers are ideal for optimum output device operations. However, they amplify signals in a complex manner that significantly degrades their sound quality. | 
