High Performance Audio Power Amplifiers
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High Performance Audio Power Amplifiers for music performance and reproduction Newnes An imprint of Butterworth-Heinemann Ltd Linacre House, Jordan Hill, Oxford OX2 8DP A division of Reed Educational and Professional Publishing Ltd OXFORD BOSTON JOHANNESBURG MELBOURNE NEW DELHI SINGAPORE First published 1996 Reprinted with revisions 1997 © Ben Duncan 1996 © B. D. 1997 All rights reserved. No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1P 9HE. Applications for the copyright holder's written permission to reproduce any part of this publication should be addressed to the publishers TRADEMARKS/REGISTERED TRADEMARKS Computer hardware and software brand names mentioned in this book are protected by their respective trademarks and are acknowledged. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN 0 7506 2629 1 Typeset by P.K.McBride, Southampton Printed and bound in Great Britain High Performance Audio Power Amplifiers for music performance and reproduction Ben Duncan, A.M.I.O.A., A.M.A.E.S., M.C.C.S international consultant in live show, recording & domestic audio electronics and electro-acoustics. Foreword Ben Duncan is one of those rare individuals whose love and enthusiasm for a subject transcends all the usual limits on perception and progress. In fact, without the few people of true independent spirit, progress in the world would be swamped by the xylocaine of vested interest, narrow attitude and corporate monoculture. Amongst my early experiences of Ben Duncan’s thinking, many years ago, were his contention that electronic components have qualitative audio properties and his recom- mendation that we listen to the sound of capacitors of various dielectrics. The outcome was the exclusive use of polypropylene capacitors in all Turbosound’s passive hi-pass networks. This is not only illustrative of the depth to which the man goes, but also his extensive seen and unseen influence on the whole audio community. He is an holistic thinker and I believe there are very few things in the Universe that he has not, at one time or another, considered having an effect on audio quality. Does he keep his flights of fancy and strokes of brilliance to himself? Not one bit of it. He communicates compul- sively and in large quantities as anyone who has followed the general audio press for the last dozen or so years will tell you. A memorable early experience of power amplifiers was with the then relatively new transistor variety powering a P.A. I had built for the Pink Fairies, that was at the original Glastonbury in 1971. After the sixth failure of an HH TPA100, for no apparent reason, I was running out of working stock. On sitting down to consider the hopeless situation it became worse when I found the live soldering iron. My next immediate thoughts were about a change of career. Anyhow, the point of this sad little tale is that in those days power amplifiers were absolutely horrible things because despite the fact that they had somewhat puny voltage swings they were, nevertheless, always blowing up at the slightest opportunity and particularly in the hour before show time. These days things have pro- gressed a long way and sound system operators bask in the luxury of equipment that is almost indestructible and capable of audio quality usually associated with esoteric hi-fi as well as delivering arc welding levels of power. I am extremely grateful to Ben that he has undertaken the Herculean task of collating all the relevant facts on, and to do with, power amplifiers ranging from the in depth assess- ment of household mains to determinations as to whether it actually sounds any good. The breadth of the book enables an average human to purchase or design power ampli- fiers knowing that all relevant information is at their disposal and as such this book should be considered a positive contribution to the sum total of mankind. I hope it has a similar effect on his bank balance. Tony Andrews, Hoyle, Surrey March 1996 Contents Preface ....................................................................................................................... xi Acknowledgments ................................................................................................... xiii System of presentation ............................................................................................. xv 1 Introduction and fundamentals 1 1.0. What are audio power amplifiers for ?............................................... 1 1.1 What is the problem ? ......................................................................... 1 1.2 What is audio ? ................................................................................... 2 1.3 What’s special about audio ? .............................................................. 2 1.4 The ramifications of quality on audio ................................................ 3 1.5 Some different aims of sound reproduction ....................................... 3 1.6 About people and their hearing .......................................................... 4 1.7 Limits of a ‘objectivity’. Why listen ? ............................................... 5 1.8 Why are power amplifiers needed for audio ? ................................... 6 1.9 Music fundamentals ........................................................................... 8 1.10 Adjectives that describe sound ........................................................... 9 1.10.1 Tonal qualities .............................................................................................. 10 1.10.2 Broader tonal descriptors ..............................................................................11 1.10.3 General sonic adjectives ...............................................................................11 1.10.4 Dynamics...................................................................................................... 12 1.10.5 Space ............................................................................................................ 13 1.10.6 Botheration or Abomination ........................................................................ 14 1.11 Nature and range of music (alias programme) ................................. 14 1.12 Bass and subsonic content ................................................................ 15 1.13 HF dynamics and ultrasonic content ................................................ 16 References and Further reading ................................................................... 18 2 Overview of Global Requirements 19 2.1 Common formats for power amps ................................................... 19 2.2 Loudspeakers .................................................................................... 21 2.2.1 Loudspeaker drive-unit basics ..................................................................... 21 2.2.2 Loudspeaker sensitivity vs. efficiency ......................................................... 25 2.2.3 Loudspeaker enclosure types and efficiencies ............................................. 26 2.2.4 Loudspeaker configurations: a résumé ........................................................ 27 2.3 The interrelation of components ...................................................... 32 2.3.1 What loudspeakers look like to the amplifier .............................................. 32 2.3.2 What speakers are looking for ..................................................................... 35 2.3.3 What passive crossovers look like to amplifiers ......................................... 38 2.4 Behaviour of power amps as voltage sources .................................. 40 2.4.1 Drive-unit power ratings after EIA/AES ..................................................... 40 2.4.2 Output power capability requirements......................................................... 41 2.4.3 Loudspeaker vulnerabilities ......................................................................... 43 2.4.4 High power, the professional rationale ........................................................ 44 2.4.5 Active systems, power delivery requirements ............................................. 46 2.5 Current delivery requirements .......................................................... 46 2.5.1 The low impedance route ............................................................................. 47 References and Further reading ................................................................... 48 3 The input port – Interfacing and processing 49 3.1 The Input........................................................................................... 49 3.1.1 Input sensitivity and gain requirements ....................................................... 49 3.1.2 Input impedance (Zin) ................................................................................... 52 3.2 RF filtration ...................................................................................... 58 3.3 The balanced input ........................................................................... 59 3.3.1 Balancing