What is a Power Amplifier?

Investing in a power amplifier can be daunting if you’re unsure what you’re buying. With so many things to consider, such as power and impedance and whether the amplifier will be compatible with your speakers, without experts on hand to ask for advice, there’s always a concern that you might make an incorrect choice.

With Prebeat, there’s no need to worry! In this guide, we’re breaking down everything you need to know about power amplifiers, from the basics, such as what one is and what it does, to the more complicated stuff, like how a power amplifier works, to give you the knowledge you need to make an informed decision when you shop.

Table of contents:

What is a power amplifier?

How does a power amplifier amplify a signal? 

What is power handling?

What are the main types of power amplifiers?

What is a power amplifier? 

A power amplifier sits between a sound source and a passive loudspeaker and is used to take a line-level input signal and increase its amplitude or power to fill a venue or room with sound.

Usually the final block in an amplifier chain, power amplifiers are designed to drive loads directly to output devices, such as passive speakers. They do this by pushing an amplified signal output to the speakers, which is converted into audio sound. For this to work effectively, the power amplifier and speakers must have a compatible impedance to transfer the power efficiently. 

How does a power amplifier amplify a signal? 

A power amplifier will only operate efficiently if its input signal is over a certain threshold, so rather than passing a raw audio signal directly to a power amplifier, it is first preamplified with a current or voltage amplifier (known as a preamplifier). It is then sent as an input to the power amplifier after necessary modifications have been made. 

What is power handling?

To ensure you’re choosing the correct power amplifier for your PA setup, you’ll need to know about power handling, watts and impedance. 

Power handling, put simply, is how much power your speakers can handle. When choosing a power amp for a passive PA speaker setup, you should opt for one that can deliver a power output to twice your speakers’ continuous rating. 

For example, a speaker with a normal impedance (or electrical resistance) of 8 ohms and continuous power of 350 watts will require an amplifier of 700 watts into an 8-ohm load. You’ll need an amplifier with 700 watts into an 8-ohm load per channel for stereo speakers. 

To achieve optimum performance from your PA setup, the total ohm load of your passive speakers should match your amplifier. If the total impedance of your loudspeakers is too high, it will reduce the power delivered to them. In contrast, if it’s too low, the power delivered to the speakers may be too high, damaging the amplifier and overloading the speakers. 

What are the main types of power amplifiers?

Power amplifiers are divided into three types depending on the output device connected to them. These are the following: 

Audio power amplifiers 

Audio power amplifiers increase the power of a weak audio signal and are used in the speaker-driving circuits of mobile phones, headphones and televisions. 

The output of an audio power amplifier can range from a few milliwatts, in the case of headphone amplifiers, to the thousands of watts needed for home theatre systems. 

RF Power Amplifiers

With wireless transmissions, modulated waves need to be sent through the air over long distances. Antennas transmit these signals, and the range of transmission available depends on the power of the signals given to the antenna. 

When it comes to FM broadcasting, for example, the antennas require large input signals that are at thousands of kilowatts of power. Here, RF, or Radio Frequency power amplifiers, are used to increase the power of modulated waves to a high enough level that they can reach the required transmission distance effectively. 

DC power amplifiers

In electronic control systems, DC power amplifiers amplify the power of pulse width modulated signals (PWM). These systems require high-power signals to drive actuators or motors. 

DC power amplifiers take input signals from microcontroller systems and increase the signal’s power before feeding the amplified version to DC motors or actuators. 

The specific power amplifier needed usually depends on the application’s requirements, such as desired power level, frequency range and efficiency. 

Power Amplifier classifications

As there are multiple ways of designing power amplifier circuits, how they operate and the characteristics of each circuit configuration differ. Because of this, letters are assigned to each power amplifier to signify its operation methods and the characteristics of its circuit. 

Generally, these are divided into two categories: those designed to amplify analogue signals (Categories A, B, AB and C) and those designed to amplify digital PWM signals (categories D, E and F). 

The power amplifiers that are most commonly used are those in audio amplifier circuits and are the ones that you’re most likely to come across when investing in a power amplifier for your PA setup. They are classified under categories A, B, AB and C, so these circuits are the ones we’ll look at in more detail.

Class A Amplifiers 

Class A Amplifiers use a single transistor to amplify both the positive and negative halves of the analogue waveform, and the entire input waveform gets used in the amplification process. Class A Amplifiers have a simple design and are the most commonly used type of power amplifier. 

As the transistor, the electric component or active element used for amplifying, is in use all of the time, even when there is no input signal, this generates a lot of heat, meaning that Class A amplifiers aren’t as efficient as other power amplifier options. However, as they use 360 degrees of the waveform, the input signal is less likely to distort, making for a much more accurate, high-frequency performance. 

Class B Amplifiers 

Class B power amplifiers solve the efficiency problems of having a single transistor in the amplification circuit. Instead of using one transistor, the circuit uses two, one that amplifies the positive half of the waveform and the other which amplifies the negative. 

Because of this setup, each transistor amplifies 180 degrees of the signal, meaning the full 360 degrees is amplified more efficiently. However, this can lead to some distortion at the crossover point. 

Class AB Amplifiers 

Due to the potential distortion that can be experienced at the crossover point of Class B amplifiers and the lack of efficiency of Class A amplifiers, class AB amplifiers were designed. 

Class AB amplifiers combine Class A and B features to achieve a compromise between signal linearity and efficiency. 

Class C Amplifiers 

Unlike A, B and AB power amplifiers that mainly focus on the clarity and quality of the amplified signal, Class C amplifiers are much more efficient but sacrifice signal quality as a result. The conduction angle of a Class C power amplifier is less than 90 degrees, which equates to greater distortion, making them unsuitable for audio amplification but perfect for the amplification of Radio Frequency signals and high-frequency oscillators. 

Choose a power amplifier for your rig or PA system by browsing the range at Prebeat

In this guide, we’ve summarised what a power amplifier is and how it works, and we’ve broken down some of the differences between different types of power amplifiers and their classifications to improve your knowledge when you come to shop. If you’re still unsure about which you’ll need, browse our extensive power amplifiers collection at Prebeat, then contact us for more information on the best choice for your audio setup. 

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