In an audio set up, an amplifier is an electronic device that is used to increase (or amplify) the current, voltage, or power of an analog electrical signal. This amplified current is then feed into your loudspeakers, which creates the sound or music that you hear. A typical loudspeaker has an impedance of between 4Ω and 8Ω, thus a power amplifier must be able to supply the high peak currents required to drive the low impedance speaker.
Amplifier classes are mainly lumped into two basic groups. The first are the classically controlled conduction angle amplifiers forming the more common amplifier classes of A, B, and AB, which are defined by the length of their conduction state over some portion of the output waveform, such that the output stage transistor operation lies somewhere between being “fully-ON” and “fully-OFF”.
The second set of amplifiers are the newer so-called “switching” amplifier classes of D, E, F, G, S, T etc, which use digital circuits and pulse width modulation (PWM) to constantly switch the signal between “fully-ON” and “fully-OFF” driving the output hard into the transistors saturation and cut-off regions.
Class A
A Class-A amplifier is one in which both output stages of the device are constantly on at full power.
Because of the positive attributes associated with Class A operation, it is considered the gold standard for audio quality in many audiophile circles. However, there is one important drawback to these designs: efficiency. The requirement of Class A designs to have all output devices conducting at all times results in significant amounts of wasted power, which is ultimately converted to heat. This is further exacerbated by the fact that Class A designs require relatively high levels of quiescent current, which is the amount of current flowing through the output devices when the amplifier is producing zero output. Real world efficiency rates can be on the order of 15-35%, with the potential to drop into the single digits using highly dynamic source material.
Class B
Class B amplifiers were invented as a solution to the efficiency and heating problems associated with the previous class A amplifier and uses a push-pull amplifier topology. The output of a Class B amp incorporates a positive and negative transistor. To replicate the input, each transistor only conducts during half (180°) of the signal waveform. This allows the amp to idle with zero current, thereby increasing efficiency compared to a Class A amp.
There is a trade-off that comes with a Class B amp: the increased efficiency degrades audio quality. This happens because there is a crossover point at which the two transistors transition from the on state to the off state. Class B audio amps are also known to have crossover distortion when handling low-level signals. Needless to say, such distortion in sufficient amounts is audible, and while some Class B designs were better than others in this respect, Class B didn’t receive much love from audiophiles.
Class A/B
A hybrid of the Class-A and Class-B designs, the Class-A/B approach allows for both of the output stages to be on at the same time, but with one output receiving the current flow for between one half and one full cycle, while the other output receives just enough current flow to remain on so it can respond instantly to the input signal. This approach leverages the strengths of both designs with greater efficiency than Class A, and without the non-linear crossover distortion of class B. The advantage of this small bias voltage, provided by series diodes or resistors, is that the crossover distortion created by the class B amplifier characteristics is overcome, without the inefficiencies of the class A amplifier design. So the class AB amplifier is a good compromise between class A and class B in terms of efficiency and linearity, with conversion efficiencies reaching about 50% to 60%.
Class D
A Class D audio amplifier is basically a non-linear switching amplifier or PWM amplifier. There is no period during a cycle were the voltage and current waveforms overlap as current is drawn only through the transistor that is on. These amps are highly efficient (often up to 90% or higher) because the output transistors are either fully turned on or fully turned off during operation. This approach completely eliminates the use of the linear region of the transistor that is responsible for the inefficiency of other amplifier types.
In today’s market, Class A/B dominates the scene, and for good reason: they perform very well, are relatively cheap, and their efficiency is perfectly adequate for low powered applications. Of course, as amplifier manufacturers try to push the envelope of power delivery, they turn to Class D designs to avoid having their amplifiers double as space heaters. Meanwhile, on the other end of the market are Class A aficionados who can forgive a lack of efficiency in the hopes of purer sound.
In the end, amplifier classes aren’t necessarily as important as some people might ascribe. Yes, there are important differences, particularly when it comes to cost, amplifier efficiency, and consequently weight. Certainly a 500W Class A amplifier is a bad idea, unless the idea of using your amplifier as an oven appeals to you. On the other hand, the differences among the classes don’t inherently define sound quality. In the end, that comes down to engineering and implementation.
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