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All types of audio information recorded for playback began as very low level electrical signals. For example, the signal levels at the outputs of microphones, musical instrument pickups, or audio tape heads will typically average out to only a few millivolts. Such signal-level voltages must be increased in amplitude (amplified) to become usable. Voltage amplifiers designed to accomplish this task are referred to as preamplifiers. For normal audio applications, signal-level voltages will be preamplified to about 1 or 2 volts RMS. This level is commonly referred to as the line level, and it is the common output level produced by almost all consumer audio equipment (i.e., tape decks, CD players, FM receivers, etc.). On some musical instrument amplifiers, the line-level output is referred to as the preamp output.
In addition to amplifying signal-level voltages, preamplifiers normally contain all signal conditioning circuits. Signal conditioning circuits modify, or customize, the original audio signal to accommodate the listener's preference, room acoustics, or system idiosyncrasies. Examples of signal conditioning circuits are tone controls, equalizers, bass boost controls, reverb units, and various types of filter and phase circuits. Preamplifiers used for professional recording applications and commercial public address (PA) systems may also incorporate pan controls, delay lines, and harmonic modification capabilities. Input devices (i.e., tape deck, CD player, FM receiver, microphone, etc.) may be switched within the preamplifier circuitry.
All preamplifiers are voltageamplifiers. Their function is to condition and amplify a signal voltage to a line-level audio signal, in preparation for sending it to an audio power amplifier. In contrast, an audio power amplifier is designed to amplify a line-level voltage to a corresponding high-level voltage and current, capable of driving a low-impedance speaker (typically 4 or 8 ohms).
A high-quality audio power amplifier is said to be "transparent." This means the output voltage is an (almost) exact replication of the input voltage, without any modifications having been made to phase relationships, harmonic integrity, transient response, or frequency response. If a "perfect" audio power amplifier could be constructed, the only difference between the line-level input signal and the output signal would be the increased amplitude at the output (both current and voltage). Any other difference between the input signal and output signal of an audio power amplifier is considered distortion.
This book provides the reader with the tools necessary to construct high-power, high-quality (almost distortionless) audio power amplifiers. Throughout this book, amplifier quality is equated directly with the lowest-distortion specifications, and I make no apologies for this fact. Throughout the last few decades, there has been an unfortunate trend among a minority group of audio esoterics to advocate the introduction of certain types of harmonic distortion mechanisms within audio power amplifiers. I am quite certain that I have the unanimous support of every legitimate sound engineer (at least the ones who can pay their bills) when I unconditionally state that all signal conditioning should occur at the line level (i.e., within the preamp stage or external signal processors). All audio power amplifiers should be as distortion free as possible because sonic accuracy is desirable for a host of practical and performance factors. (This subject will be discussed in more detail in Chapter 2.)
The Social and Economic Importance of Audio Amplification
It is very difficult to imagine what our world would be like without electronic audio amplification. Every radio, television, telephone, and stereo system contains one or more audio amplifiers. Imagine, if you can, the condition of our society without these devices!
Without audio amplifiers, Elvis Presley and the Beatles would have been limited to audiences within the confines of their audible vocal range. Rock 'n roll could not exist. All musical instruments would have to be acoustic, and there would be no point to recording them (you would have to have an audio amplifier to play back the recording!). Preachers could not be heard in large church buildings, dance bands would have to revert to Glen Miller's type of orchestration, and the city of Nashville would have to come up with an entirely new tourist attraction. In essence, our culture, as we know it today, would die.
The economic considerations are equally massive. Out of all possible consumer electronic building blocks, the humble audio amplifier is most common. As we have become more sonically sophisticated in this electronic age, the listening perception of the average person has increased logarithmically. When the first gramophones became the newest consumer gadgets, they were publicly acclaimed to provide the finest audio reproduction possible. Try getting someone to compliment the sound of a gramophone today! The point here is simple: The general public of today is more educated in the subtleties of good sound, and they are more than willing to pay for it.
At present, high-quality, high-power audio amplifiers capture a very substantial portion of the consumer electronics market, with the latest manufacturing targets being in the home theater and multimedia arenas. Dedicated audiophiles have increased in number since the 1960s, and the exorbitant price tags on audiophile-quality equipment prove this market to be in good health. Although not as dramatic as the consumer electronics field, the entertainment and professional requirements for high-quality amplifiers continue to prosper. Indeed, there are few homes today that do not contain at, least one high-power audio amplifier system.
Definition of High-Power Audio Amplifiers
Throughout this book, the technical definition of high-power is from 50 watts RMS and up. This is my personal definition, and I use it for several reasons. First, in the 1950s, the highest-power amplifiers available were capable of producing only about 80 watts RMS into an 8-ohm load. These were commercial vacuum tube amplifiers, most commonly used in indoor movie theaters. With the right kind of speaker system (usually the exponential horn "voice of the theater" type design), 80 watts was more than adequate to enliven a large movie theater with 35-foot ceilings. The same is true today. With an efficient high-performance speaker system for the amplifier to work through, a 50-watt RMS audio amplifier can produce sound pressure levels that are on the "destructive" borderline.
Second, sheer power is not all it's cracked up to be. Those who strive for the most powerful amplifiers are often those desiring the fastest car, the biggest gun, and the meanest dog. I know some musicians who would happily lug around a 30-kW amplifier with a crane if they could afford it! Professionals in the audio fields know that power is only one of many specifications that must be considered en masse to determine the true capabilities of any audio power amplifier. The classic audio power amplifiers are the ones that set exceptional standards in design, performance, and reliability. For the most part, maximum power output ratings have little to do with their popularity.
This book provides complete cookbook information and construction details for audio power amplifiers up to 550 watts RMS. In addition, the functional and design information contained herein provides the know-how to construct power amplifiers of even higher power ranges for those who are involved in large professional audio systems...