Different Types of Amplifier Classes Explained

Selecting an audio amplifier in the audio market today is not as a simple task as it looks. In order to fully enjoy a proper music listening experience, getting the right kind of audio amplifier is essential. But not all people, especially the basic-level consumer know what proper audio amplifier to get for their listening needs nor the proper class type to get.

But what exactly are the classes of audio amplifiers that one should get for themselves? Is there a certain criterion or requirement for getting a specific one? Read on to find out.

From Questyle

What Are The Different Types of Audio Amplifier Classes?

Picture this scenario: you are buying a new home theater audio setup for your new home. The store staff then asks you what is the class of the amplifier you would like to have with your audio setup. That is where the confusion of what exactly is the audio amplifier class that you want, or in other cases, need for your present audio setup.

audio amplifiers explained
Diagram of some classes of audio amplifiers (From Circuit Digest)

Currently, there are four classes of audio amplifiers available in the market for purchase by consumers and all of them are classified by letters (either in single or double format). The commonly sold classes of audio amplifiers today are the following: A, A/B, C, D, G, and H.

Those class letters can be usually seen on the specification sheet/user manual of the audio amplifier and the specification panel located on the back of the device. Aside from giving the device a label for its specification, it is also used for determining what kind of power output it has and its topology (i.e. function level).

Do take note that the primary function of an audio amplifier is to simply amplify the waveform sent to it via an audio source by the usage of a preamplifier inside the unit. It does so without distortion (or at least with as much little distortion as possible). Below is a simple diagram explaining how it works, being called a sine wave diagram, which shows a full wavelength wherein it represents 360 degrees.

Sine wave diagram, wherein it shows the degrees a waveform travels (blue line) (From Audioholics)

Class A Audio Amplifiers

The simplest in its class of audio amplifiers, the waveform that is being amplified by this class will be conducted through the whole 360 degrees cycle. Aside from its simple waveform conduction, it is also known as single-ended amplifiers or push/pull amplifiers due to its simplistic nature of handling waveforms of a specific audio source.

The amplitude (the loudness) of an audio source has two charges, positive and negative charges as it is an electric current. In a Class A audio amplifier, the waveform of an audio source will be conducted a full 360 degrees in both positive and negative charges.

For a single-ended amplifier, it is usually a single unit doing the whole process (sine wave diagram) over and over again during usage. However, a push/pull amplifier works in pairs; one handles the positive charge flow (+V) and the other handles the negative charge flow (-V). And the process is repeated multiple times during usage.

Class A audio amplifier diagram (From Electronics Tutorial)

A Class A audio amplifier uses a single output switching transistor (it could be a Bipolar, FET, IGBT, or something else) and is placed at the Q-Point (the one with the blue circle in the diagram). The Q-Point is also known as the operating point of a device wherein it is the steady-state DC voltage or current of the device.

Class A: The Golden Standard

Thanks to the simple design and ease of use of the Class A audio amplifier, it has been set as a gold standard by the music industry and in the consumer market when it comes to amplifiers. It has been even agreed upon in most audiophile communities that a Class A audio amplifier is better than the rest of the classes.

The only known downside for this class is its efficiency as it consumes a lot of power. Remember that the design of Class A audio amplifiers is for it to have all output devices conducting at all times during usage. That means it uses a lot of power during that time but unfortunately, not all power consumed is efficiently used and therefore is wasted.

That wasted power also turns into unwanted heat (which makes its output devices hot to the touch over a long period of usage).

Another thing about Class A audio amplifiers is that they need a high amount of quiescent current (amount of current flowing through output devices) even if the amplifier is not producing any output. It causes the efficiency rate of this class to drop to only 15% to 35% and could even drop further into single digits for amplifiers using highly dynamic source materials.

Class B Audio Amplifiers

This class instead focuses on the previous class’ downside and improves on it even further: efficiency. Instead of conducting all of the time at 100%, Class B audio amplifiers instead use a similar push/pull arrangement to Class A minus the continuous 100% conduction. Instead, it uses only half of the output devices for conducting when in use.

To simply explain, one half of the output device would handle the +V -180 degree section and the other would handle the -V +180 section, forming a complete 360-degree waveform in a sine wave. Thanks to that design, the efficiency rate of Class B audio amplifiers has been rated at an average of 78.5% for most devices.

Class B audio amplifier diagram, wherein it has two Q-Points (blue circle) (From Electronics Tutorial)

Even though it boasts a higher efficiency rate than Class A, the reality is that there are few Class B audio amplifiers present in the market today, as not many manufacturers use this kind of class due to one significant issue: crossover distortion.

Class B: Crossover Distortion Issue

The crossover distortion issue with Class B audio amplifiers is the result of the two output devices handling the positive and negative portions of the waveform, wherein a delay in the handoff between both devices occurs. Due to that delay, a small amount of distortion can occur, in which many consumers (especially audiophiles) will notice a dip in output quality, making it an imperfect choice over Class A units.

Class A/B Audio Amplifiers

In order to solve the low-efficiency rate of Class A units while avoiding the crossover distortion issue that Class B units produce, manufacturers have designed and created the combined Class A/B audio amplifiers as a result.

Class A/B audio amplifier diagram. It uses a small bias voltage from series diodes or resistors to overcome the crossover distortion effect (From Electronics Tutorial)

Even though Class A has been mentioned as the golden standard when it comes to audio amplifiers, Class A/B units majorly dominate most (if not all) consumer markets present when it comes to audio amplifiers globally.

The Class A/B uses a similar push/pull arrangement as a Class B unit, but instead of both outputs only covering 180 degrees on each side of the sine wave (both positive and negative sides), it instead covers 181 to 200 degrees. That results in covering the artificial “gap” that is the byproduct of the Class B setup and the main cause for a crossover distortion.

As for efficiency rates, it is not far off from the rates of Class B as it has an average rate of 50% to 70% for most devices present in the market today. However, do take note that the average rate will also depend on other factors in its setup (i.e. having no issues with your audio setup, no internal damages, etc.).

Class C Audio Amplifiers

An uncommon class of audio amplifiers, Class C units boast a very high-efficiency rate (80% average) at the cost of poor linearity overall. The previous classes mentioned above are considered linear amplifiers, which means that their output is proportional to its input without any issues. That is not the case with Class C units as it suffers from disproportionate outputs that result in heavy distortion during usage.

Class C audio amplifier diagram (From Electronics Tutorial)

The reason for such heavy distortion in this class is due to its conduction angle in a sine wave being less than 180 degrees; it instead sits around 90 degrees. That means that its output current during usage is zero for more than half of the input signal process that it receives, which results in the high-efficiency rate and heavy distortion mentioned earlier.

Due to the heavy distortion produced by Class C units, they are rarely used as audio amplifiers by many. Even in many audiophile communities, Class C units are a rarity and are almost a forgotten relic at this point. However, Class C units are commonly used for high-frequency sine wave oscillators and specific types of radio frequency amplifiers.

Class D Audio Amplifiers

When it comes to the class of audio amplifiers with the highest efficiency rate (90% or higher), Class D units win in that category without any issues whatsoever. Commonly being referred to erroneously as “digital amplification” due to a misconception of the letter “D” (wherein many thought of it as “digital”).

The 90%+ efficiency rate of this class is possible thanks to the rapid on and off switching of its output devices, even if the amplifier itself is considered to be effectively idle.

Class D: Analog and Digital

Class D audio amplifiers come in two kinds: analog and digital. As the name suggests, Class D analog amplifiers use both analog input signals and control units in their entire operation cycle, accompanied by a feedback error correction.

Its counterpart, the Class D digital amplifiers use instead controls that are digitally generated without an error control present during the process of switching power stages. However, there are some digital versions with their own error control present in the form of DAC in the front of the unit in a similar fashion to analog versions.

Do take note that when it comes to overall performance, it has been proven that Class D analog amplifiers are better than their digital counterparts thanks to lower impedance output and having an improved distortion profile.

Other Classes of Audio Amplifiers

Aside from the four classes mentioned above, there are still other classes of audio amplifiers that are present in the market today. Some are standalone classes (i.e. Class F and Class I), while others are further improvements or upgrades of other classes (i.e. Class T is an upgrade of Class D).

Below are some of those other classes that are used by the public and other entities such as corporations and businesses.

Class F Audio Amplifiers

A class of audio amplifiers that focuses on boosting both efficiency and overall output by the usage of harmonic resonators in its network. Those harmonic resonators result in the shape of the output waveform into a square-shaped form. This class can reach an efficiency rate of 90% and higher only if an infinite harmonic tuning is used during its usage.

Class G Audio Amplifiers

This class further improves the good output and performance of the Class A/B amplifiers by enhancements through the usage of multiple power supply rails in its network. During usage, it automatically switches between those supply rails depending on the input signal as it changes. Thanks to that process, power consumption is significantly reduced overall, and less power loss due to wasted heat.

Class S Audio Amplifiers

A non-linear class of amplifier that uses a switching mode method similar to Class D. FOr its input, it converts analog signals to digital square waves with the usage of a delta-sigma modulator, amplifies it, and finally is demodulated by a bandpass filter.

The switching filter present in this class is purely digital and is either “on” or “off” at a given instance during usage, therefore allowing this class to hit the 100% efficiency rate. This makes this class perfect for those seeking an amplifier with a near-perfect efficiency rate during its overall usage.


This article has covered the various type of audio amplifier classes which differ by virtue of efficiency, power consumption and output. Hence, when choosing to purchase that next component in your audio set-up, checking the spec sheet is often a good idea to see which amplifier suits your application needs.

An important point to note, however, is that the class of amplifier does not inherently define sound quality – there can be stellar amps, for example, found in each case.

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