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PREFACE noise to be added to the signal. There are several ways of preventing this, of which the best known is the balanced interconnection, which uses two wires to carry the signal instead of one. It is now pos- sible for the receiving equipment to respond to the difference between the two signal conductors and (to a first approximation) ignore the unwanted voltage drop down the ground conductor. Like so many technical matters, the principle is beautifully simple but the practicalities can get rather complicated; this technology is thoroughly dealt with in Chapter 7 by an author who is a master of his subject. The section on "Power Amplifiers" makes up the fourth part of this book, and I have contributed this myself, having spent a good deal of time working on the apparently simple but, once again, deeply complex and fascinating matter of turning small voltages and currents into large ones without distort- ing them in the process. The three chapters are taken from my Audio Power Amplifier Design Handbook , which is now available in a greatly extended fifth edition. The first chapter taken from the Handbook explains the basic architecture of power amplifiers, and shows how they are put into different classes such as Class-A, Class-B, and so on, depending on their mode of operation, up to Class-S. Some less well-known types such as error-correcting and non-switch- ing amplifiers are covered. The intelligent use of negative feedback is absolutely fundamental to power amplifier design, and it is explained here. The second chapter goes into more detail on power amplifier functioning, and shows how the appar- ently hopelessly entangled sources of amplifier distortion can in fact be reduced to a few straightforward mechanisms, most of which can be dealt with quite easily once they are properly identified. Topics such as measuring open-loop gain are also covered. In the third chapter, I explain the operation and design of Class-G amplifiers, which significantly economize on power consumption and heat dissipation by running from low-voltage rails when repro- ducing small amplitudes, and switching to high-voltage rails to accommodate the relatively infrequent signal peaks. This technology is of increasing importance, as it has found its niche in subwoofers where relatively large amplifier power outputs are required. When properly implemented--as described here--it can give much better quality than the Class-D amplifiers which are sometimes used in this application. The fifth section of this book "Loudspeakers" deals with the tricky business of turning electricity into sound. It consists of four chapters, all taken from the book Loudspeakers by Philip Newell and Keith Holland. The first chapter sets out the most basic fundamentals of loudspeaker operation, covering acoustic radiation and the load a loudspeaker presents to a power amplifier. In the second chapter, diversity in loudspeaker design is explored, and the construction of both moving-coil and electrostatic loudspeakers is gone into in more detail. The third chapter describes loudspeaker cabinets, which are not just enclosures to keep the dust off, but an integral part of the functioning of the loudspeaker. Finally , the fourth chapter in this section deals with the crossover units that direct the appropriate fre- quencies to the high-frequency and low-frequency units of a multi-way loudspeaker system. This alone is a large and complex field of study. The next section of this book, the sixth, addresses the vitally important field of "Digital Audio." I am particularly conscious that this is a huge and ever-growing field of knowledge, and one section of one book can only give a taste of its extent. Digital audio entered most people's lives with the introduc- tion of the CD in 1982, and since then digital methods have extended from relatively simple storage media to highly complex manipulations known as Digital Signal Processing or DSP. Very sophisticated specialized processors have been developed to efficiently handle the enormous amounts of calculation required. Chapter 15, by Craig Richardson, gives a good account of the essential principles of digital audio, cov- ering sampling theory, Z-transforms (which are not as scary as they sound), and the basics of choosing a DSP chip. Another aspect of digital technology is the use of the MIDI standard for controlling electronic musical instruments. Back in the days when I had my own eight-track recording studio (a setup which dated back to when eight tracks referred to tracks on magnetic tape, and which unfortunately succumbed to viii