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Explanation of class A, class B, class AB and class D amps

Amplifiers: what are classes A/B/AB/D

The basic function of an amplifier is to increase a low intensity signal capture. It can be a direct source such as a microphone, a musical instrument, or a signal corrected by an intermediary preamplifier (which processes the signal source through equalizers, filters, etc.)...

Amps are often presented in classes (A, B, AB, D). These classes correspond not to the active components of our amps (lamps, bipolar transistors or MOS), but to the input signal ratio they treat:

  • Class A: Uses the entire input signal (100%) and thus has a conduction angle of 360°.
  • Class B: Uses half of the signal (50%) with a conduction angle of 180°.
  • Class AB: Uses more than half of the signal (between 50% and 100%) thus between 180° and 360°.


We call these linear amps, since they use either part or the totality of a signal for processing. Their operation is based on transistor- or lamp- type active components, and depending on their class, the assembly and polarization (powering-on) of these components operate differently.

Class A

The output stage (just before the HP output) has only one active element (a transistor or lamp), which is always conducting. Because it amplifies the entire input signal, it is the most accurate (most linear) as it limits distortions of the output signal. Its power however, is limited, since this type of amplifier shows weak efficiency (relationship between the energy supplied and consumed in the process), showing a significant, constant consumption rate. These amps generally do not exceed 20 watts and are used more often for guitar than bass. They also require high amplitude in order to maximize their efficiency.

Class B

The active components of these amps provide half as much. A class B system amplifies only half the signal and will therefore drive harmonic distortion. For this reason, we use the "Push-Pull" principle that consists of mounting two active components on the output stage (e.g. two transistors). They will share the work whereby one will amplify the negative part of the signal and the other the positive part. The entire signal is amplified and maintains high efficiency without suffering from high distortion levels. These amps offer the possibility of very powerful capabilities, but also contain a fault: between the amplification of the positive and negative signal (with the signal usually oscillating between positive and negative values) is a non-linear region commonly known as "crossover distortion” which becomes audible in low amplitude signals. That's why we created a new class of amplifiers, Class AB.

Class AB

Amps of this class represent a blend of the two preceding classes. As long as you keep it on low power (up to 20 watts), the polarization remains of type A.

And as soon as you surpass this limit, the amplifier can access the B system and its output stage uses both poles. This system is most common in units and combos, even today. The first advantage of the latter is to avoid crossover distortion and remain linear below 25 watts; the second is to offer the power of class B. 

Class D, E and F

Then there are the new amplifier classes: "switching" amplifiers (Class D, Class E and Class F). They don’t decline in the same way since by nature they have a conduction angle of zero (they do not amplify the original signal directly). They are therefore organized according to the technologies they employ. For us, class D is the most familiar.

Contrary to what one might think, Class D has nothing to do with the word ‘digital.’ We call it D simply because it follows the C class (which I won’t talk about, because it mainly concerns radio transmitters).

The principle of such an amplifier is literally to chop the input signal: the first signal is modulated (pulse width modulation). In a curve, it becomes a square wave signal with two states, having constant amplitude and frequency, but also a variable pulse width (the average varies depending on the amplitude of the input signal).

As their amplitude is constant, the amplifying components can act in commutation (as switches). They are then either blocked or saturated to transmit the amplified signal in an unbeatable performance. Then the output signal is filtered with a low-pass filter that can isolate slices of unnecessary harmonics generated by modulation. We thus find an output signal close to the input signal, but which is amplified. The quality of the filter ensures the linearity of the output signal.

But why convert a perfectly good input signal, which never asked anything of anyone? As I have already said, for the efficiency. But the benefits also affect convenience: there is very little energy loss and therefore little heat generated during the process, so the Class D system is not nearly as much of an energy guzzler (since its components commute). We can therefore use venting components (heat sinks) and smaller power supplies.

So you benefit from the format, and in the case where the power supply is commutated, you end up with a downright light system. Remember, the power transformer is what weighs down a cumbersome system. But I'll stop there, if I go further I’ll lose you completely and I would not want to start writing things that exceed the extent of my knowledge!