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Audience: Professional audio designers and electronic engineers; advanced amateur audiophiles and electronic designers; audio engineers involved in using and maintaining high-end domestic and professional audio systems.
The Economic Importance of Power Amplifiers
Audio power amplifiers are of considerable economic importance. They are built in their hundreds of thousands every year, and have a history extending back to the 1920s. It is therefore surprising there have been so few books dealing in any depth with solid-state power amplifier design.
The first aim of this text is to fill that need, by providing a detailed guide to the many design decisions that must be taken when a power amplifier is designed.
The second aim is to disseminate the results of the original work done on amplifier design in the last few years. The unexpected result of these investigations was to show that power amplifiers of extraordinarily low distortion could be designed as a matter of routine, without any unwelcome side-effects, so long as a relatively simple design methodology was followed. This methodology will be explained in detail.
To keep its length reasonable, a book such as this must assume a basic knowledge of audio electronics. I do not propose to plough through the definitions of frequency response, total harmonic distortion (THD) and signal-to-noise ratio; these can be found anywhere. Commonplace facts have been ruthlessly omitted where their absence makes room for something new or unusual, so this is not the place to start learning electronics from scratch. Mathematics has been confined to a few simple equations determining vital parameters such as open-loop gain; anything more complex is best left to a circuit simulator you trust. Your assumptions, and hence the output, may be wrong, but at least the calculations in between will be correct ...
The principles of negative feedback as applied to power amplifiers are explained in detail, as there is still widespread confusion as to exactly how it works.
Origins and Aims
The core of this book is based on a series of eight articles originally published in Electronics World as 'Distortion in Power Amplifiers'. This series was primarily concerned with distortion as the most variable feature of power amplifier performance. You may have two units placed side by side, one giving 2% THD and the other 0.0005% at full power, and both claiming to provide the ultimate audio experience. The ratio between the two figures is a staggering 4000:1, and this is clearly a remarkable state of affairs. One might be forgiven for concluding that distortion was not a very important parameter. What is even more surprising to those who have not followed the evolution of audio over the last two decades is that the more distortive amplifier will almost certainly be the more expensive. I shall deal in detail with the reasons for this astonishing range of variation.
The original series was inspired by the desire to invent a new output stage that would be as linear as Class-A, without the daunting heat problems. In the course of this work it emerged that output stage distortion was completely obscured by nonlinearities in the small-signal stages, and it was clear that these distortions would need to be eliminated before any progress could be made. The small-signal stages were therefore studied in isolation, using model amplifiers with low-power and very linear Class-A output stages, until the various overlapping distortion mechanisms had been separated out. It has to be said this was not an easy process. In each case there proved to be a simple, and sometimes well-known, cure and perhaps the most novel part of my approach is that all these mechanisms are dealt with, rather than one or two, and the final result is an amplifier with unusually low distortion, using only modest and safe amounts of global negative feedback.
Much of this book concentrates on the distortion performance of amplifiers. One reason is that this varies more than any other parameter – by up to a factor of 1000. Amplifier distortion was until recently an enigmatic field – it was clear that there were several overlapping distortion mechanisms in the typical amplifier, but it is the work reported here that shows how to disentangle them, so they may be separately studied and then, with the knowledge thus gained, minimized.
I assume here that distortion is a bad thing, and should be minimized; I make no apology for putting it as plainly as that. Alternative philosophies hold that as some forms of nonlinearity are considered harmless or even euphonic, they should be encouraged, or at any rate not positively discouraged. I state plainly that I have no sympathy with the latter view; to my mind the goal is to make the audio path as transparent as possible. If some sort of distortion is considered desirable, then surely the logical way to introduce it is by an outboard processor, working at line level. This is not only more cost-effective than generating distortion with directly heated triodes, but has the important attribute that it can be switched off. Those who have brought into being our current signal-delivery chain, i.e. mixing consoles, multitrack recorders, CDs, etc., have done us proud in the matter of low distortion, and to willfully throw away this achievement at the very last stage strikes me as curious at best.
In this book I hope to provide information that is useful to all those interested in power amplifiers. Britain has a long tradition of small and very small audio companies, whose technical and production resources may not differ very greatly from those available to the committed amateur. I hope this volume will be of service to both.
I have endeavored to address both the quest for technical perfection – which is certainly not over, as far as I am concerned – and also the commercial necessity of achieving good specifications at minimum cost.
The field of audio is full of statements that appear plausible but in fact have never been tested and often turn out to be quite untrue. For this reason, I have confined myself as closely as possible to facts that I have verified myself. This volume may therefore appear somewhat idiosyncratic in places. For example, field-effect transistor (FET) output stages receive much less coverage than bipolar ones because the conclusion appears to be inescapable that FETs are both more expensive and less linear; I have therefore not pursued the FET route very far. Similarly, most of my practical design experience has been on amplifiers of less than 300 W power output, and so heavy-duty designs for large-scale public address (PA) work are also under-represented. I think this is preferable to setting down untested speculation.
The Study of Amplifier Design
Although solid-state amplifiers have been around for some 40 years, it would be a great mistake to assume that everything possible is known about them. In the course of my investigations, I discovered several matters which, not appearing in the technical literature, appear to be novel, at least in their combined application:
The need to precisely balance the input pair to prevent second-harmonic generation.
The demonstration of how a beta-enhancement transistor increases the linearity and reduces the collector impedance of the voltage-amplifier stage (VAS).
An explanation of why BJT output stages always distort more into 4 Ω than 8 Ω.
In a conventional BJT output stage, quiescent current as such is of little importance. What is crucial is the voltage between the transistor emitters.
Power FETs, though for many years touted as superior in linearity, are actually far less linear than bipolar output devices.
In most amplifiers, the major source of distortion is not inherent in the amplifying stages, but results from avoidable problems such as induction of supply-rail currents and poor power-supply rejection.
Any number of oscillograms of square waves with ringing have been published that claim to be the transient response of an amplifier into a capacitive load. In actual fact this ringing is due to the output inductor resonating with the load, and tells you precisely nothing about amplifier stability.
The above list is by no means complete.
As in any developing field, this book cannot claim to be the last word on the subject; rather it hopes to be a snapshot of the state of understanding at this time. Similarly, I certainly do not claim that this book is fully comprehensive; a work that covered every possible aspect of every conceivable power amplifier would run to thousands of pages. On many occasions I have found myself about to write: 'It would take a whole book to deal properly with....' Within a limited compass I have tried to be innovative as well as comprehensive, but in many cases the best I can do is to give a good selection of references that will enable the interested to pursue matters further. The appearance of a reference means that I consider it worth reading, and not that I think it to be correct in every respect.
Sometimes it is said that discrete power amplifier design is rather unenterprising, given the enormous outpouring of ingenuity in the design of analog integrated circuits. Advances in op-amp design would appear to be particularly relevant. I have therefore spent some considerable time studying this massive body of material and I have had to regretfully conclude that it is actually a very sparse source of inspiration for new audio power amplifier techniques; there are several reasons for this, and it may spare the time of others if I quickly enumerate them here:
A large part of the existing data refers only to small-signal MOSFETs, such as those used in (CMOS) op-amps, and is dominated by the ways in which they differ from BJTs, for example in their low transconductance. CMOS devices can have their characteristics customized to a certain extent by manipulating the width/length ratio of the channel.
In general, only the earlier material refers to bipolar junction transistor (BJT) circuitry, and then it is often mainly concerned with the difficulties of making complementary circuitry when the only PNP transistors available are the slow lateral kind with limited beta and poor frequency response.
Many of the CMOS op-amps studied are transconductance amplifiers, i.e. voltage difference in, current out. Compensation is usually based on putting a specified load capacitance across the high-impedance output. This does not appear to be a promising approach to making audio power amplifiers.
Much of the op-amp material is concerned with the common-mode performance of the input stage. This is pretty much irrelevant to power amplifier design.
Many circuit techniques rely heavily on the matching of device characteristics possible in IC fabrication, and there is also an emphasis on minimizing chip area to reduce cost.
A good many IC techniques are only necessary because it is (or was) difficult to make precise and linear IC resistors. Circuit design is also influenced by the need to keep compensation capacitors as small as possible, as they take up a disproportionately large amount of chip area for their function.
The material here is aimed at all audio power amplifiers that are still primarily built from discrete components, which can include anything from 10 W mid-fisystems to the most rarefied reaches of what is sometimes called the 'high end', though the 'expensive end' might be a more accurate term. There are of course a large number of IC and hybrid amplifiers, but since their design details are fixed and inaccessible they are not dealt with here. Their use is (or at any rate should be) simply a matter of following the relevant application note. The quality and reliability of IC power amps has improved noticeably over the last decade, but low distortion and high power still remain the province of discrete circuitry, and this situation seems likely to persist for the foreseeable future.
Excerpted from Audio Power Amplifier Design Handbook by Douglas Self Copyright © 2009 by Douglas Self. Excerpted by permission of Focal Press. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.
|1||Introduction and general survey||1|
|2||History, architecture and negative feedback||31|
|3||The general principles of power amplifiers||62|
|4||The small signal stages||75|
|5||The output stage I||109|
|6||The output stage II||166|
|7||Compensation, slew-rate, and stability||186|
|8||Power supplies and PSRR||239|
|9||Class-A power amplifiers||259|
|10||Class-G power amplifiers||294|
|12||FET output stages||328|
|13||Thermal compensation and thermal dynamics||339|
|14||The design of DC servos||385|
|15||Amplifier and loudspeaker protection||397|
|16||Grounding and practical matters||432|
|17||Testing and safety||454|
Posted August 15, 2005
I had a simple project to build a 25W x 2 RMS and this book was not helpful at all. The author goes on and on about how he figured out this or that with his buddy but provides no instructions on how to build an amp. I was looking for something a little more how to. If you want to build an amp for your project and move on - get another book. If you're an EE and would love reading a novel about this guys low THD amps that he never shows how to build then this is the book.
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