Love listening to your favourite music through speakers? Looking for the right equipment to amplify your music? There are a number of essential equipment that every audio engineer needs to know, when setting up a sound system. Whether be it home entertainment systems or professional ones, you will still need to ask yourself – What is an Audio Power Amplifier?
Don’t worry too much, as you have come to the right place to learn more about this vital audio equipment. In this article, we will go through the basic concepts surrounding the audio power amplifier. Understand the fundamental principles of its design and function. Also learn more about its application in the audio world. Excited yet? Then let’s begin!
Introduction – Why Use It?
Simply referred to as a power amp, the audio power amplifier is essentially an electronic amplifier which amplifies electrical audio signals that has low power, (signals with frequencies of 20Hz – 20kHz, the human audible range) to a level that is strong enough to drive loudspeakers and ultimately resulting in the signal being audible to the audience or listeners.
In a typical sound system chain, it is known as the final electronic stage, before the signal is transmitted to the loudspeakers. The stages before this, consists of low power audio amplifiers which are involved in pre-amplification (usually applied to microphone or instrument signals), equalization, tone controls, mixing various audio signals or adding effects (such as reverberation).
The input signals can come from various audio sources such as CD players, digital music players and electronic musical instruments. In order to adhere to line-levels, these low-level signals are needed by most audio amplifiers.
A power amp’s input signal usually measure around a few hundred microwatts, but its output can range from a few watts for small electronics devices (like portable radios), to hundreds of watts for a home theatre system.
A nightclub’s sound system may consume several thousand watts and the sound reinforcement systems used in large concerts may require tens of thousands of watts. Standalone power amp units, are usually catered to the hi-fi audiophile and professional sound reinforcement market.
As for most consumer level audio products, such as TVs and car stereos, the power amplifiers are integrated within the chassis of the main component.
Let us now look at the various topics that we will be discussing:
- Design parameters
- Filters and pre-amplifiers
- Further developments
History – Tube Amplification
Invented by Lee De Forest in 1909, the triode vacuum tube (or valve) was the core component in the early audio amplifiers. Essentially a three terminal device with a control grid, the triode is designed to modulate the flow of electrons from the filament to the plate. The first AM radio was engineered to include the triode vacuum amplifier.
All audio power amplifiers used to be based on vacuum tubes and some of them are designed to be of notably high quality (such as the Williamson amplifier in 1947-9). Today, the majority of modern audio amplifiers are build using solid state devices (transistors such as BJTs, FETs and MOSFETs), however tube-based amplifiers are still in demand by people who prefer the “valve sound”.
Transistor-based audio power amplifiers became practical, when a variety of cost-effective transistors were widely available in the late 1960s.
Audio power amplifiers have a couple of major design parameters which includes frequency response, gain, noise, and distortion. The functions of these parameters are intertwined. For example, an increase in gain may result in an unwanted increase in noise and distortion. While “negative feedback” causes gain reduction, it also reduces distortion. Most audio amplifiers are linear amplifiers operating in class AB.
Filters and Pre-amplifiers
Most modern digital devices such as CD and DVD players, radio receivers and mp3 players already provide a “flat” signal at line level. Hence, a preamp is only needed to serve the function of a volume control and source selector. An alternative to a separate stand-alone preamp is the integrated amplifier, which integrates passive volume and switching controls, into a power amplifier.
The opinions surrounding solid state power amplifiers (some years after its introduction) were very poor, as they were believed to be unable to produce the pristine audio quality of the best valve amplifiers. This shaped the opinion among audiophiles that “tube sound” or valve sound had a unique value due to the vacuum tube technology itself.
In 1970, a new form of distortion was observed in solid-state amplifiers, which is called transient intermodulation distortion (TIM). TIM happens when there are rapid increases in amplifier output voltage. Problems with TIM stem from reduced open loop frequency response of solid state amplifiers. This issue was finally solved by decreasing the preamp frequency bandwidth, and inserting a lag compensation circuit in the amplifier’s input stage.
The Baxandall Theorem was the next step in audio amplifier design. Created by Peter Baxandall in England, this theorem explains the comparison (in ratio) between the input distortion and the output distortion of an audio amplifier. This new concept allowed audio engineers to more accurately observe the distortion processes in an amplifier.
Audio power amps are used in PA systems, live sound reinforcement systems, and home systems such as hi-fi or home-theatre system. They also drive instrument amplifiers like an electric guitar amplifier. Some instrument amplifier designs require the power amplifier to be integrated into a single “head” unit, which consists of a preamplifier, tone controls, and electronic effects.
Standalone power amplifier units are typically used by audio engineers for the setting up of PA systems and sound reinforcement systems. Power amps in stereo systems have two channels, in order to drive left and right speakers and a single channel power amplifier for the subwoofer. The size of the venue and audience determines the number of power amplifiers needed in a sound reinforcement system.
A small pub may only require a single power amp unit driving two loudspeakers. A nightclub may need several amp units for the FOH speakers, one or more units for the monitor speakers (facing the performers) and also another unit for the subwoofer.
Larger venues such as a stadium may utilize a large number of power amp units mounted on racks. Consumer electronics such as TVs, home theatre surround sound systems, “combo” guitar amps and car stereos, are usually designed to have a built-in power amplifier within the chassis of the main component.
That’s it friends, we have come to the end of this article. Hopefully, you now know the importance of an audio power amplifier in any sound system!
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