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Self on Audio This Page is Intentionally Left Blank Self on Audio Second Edition Douglas Self AMSTERDAM•BOSTON•HEIDELBERG•LONDON NEWYORK•OXFORD•PARIS•SANDIEGO SANFRANCISCO•SINGAPORE•SYDNEY•TOKYO NewnesisanimprintofElsevier NewnesisanimprintofElsevier LinacreHouse,JordanHill,OxfordOX28DP 30CorporateDrive,Burlington,MA01803 Firstpublished2006 Copyright©2006,DouglasSelf.Allrightsreserved TherightofDouglasSelftobeidentifiedastheauthorofthisworkhasbeen assertedinaccordancewiththeCopyright,DesignsandPatentsAct1988 Nopartofthispublicationmaybereproducedinanymaterialform(including photocopyingorstoringinanymediumbyelectronicmeansandwhetherornot transientlyorincidentallytosomeotheruseofthispublication)withoutthewritten permissionofthecopyrightholderexceptinaccordancewiththeprovisionsofthe Copyright,DesignsandPatentsAct1988orunderthetermsofalicenceissued bytheCopyrightLicensingAgencyLtd,90TottenhamCourtRoad,London, EnglandW1T4LP.Applicationsforthecopyrightholder’swrittenpermissionto reproduceanypartofthispublicationshouldbeaddressedtothepublisher PermissionsmaybesoughtdirectlyfromElsevier’sScienceandTechnologyRights DepartmentinOxford,UK:phone:(+44)(0)1865843830;fax:(+44)(0)1865853333; e-mail:permissions@elsevier.co.uk.Youmayalsocompleteyourrequeston-linevia theElsevierhomepage(http://www.elsevier.com),byselecting‘CustomerSupport’ andthen‘ObtainingPermissions’ BritishLibraryCataloguinginPublicationData AcataloguerecordforthisbookisavailablefromtheBritishLibrary LibraryofCongressControlNumber: 2006927676 ISBN–13:978-0-7506-8166-7 ISBN–10:0-7506-8166-7 For information on all Newnes publications visitourwebsiteatwww.newnespress.com PrintedandboundinGreatBritain 06 07 08 09 10 10 9 8 7 6 5 4 3 2 1 Working together to grow libraries in developing countries www.elsevier.com | www.bookaid.org | www.sabre.org Contents Introduction viii Preamplifiers and related matters 1 Advanced preamplifier design (November 1976) 1 2 High-performance preamplifier (February 1979) 13 3 Precision preamplifier (October 1983) 24 4 Design of moving-coil head amplifiers (December 1987) 37 5 Precision preamplifier ’96, Part I (July/August 1996) 49 6 Precision preamplifier ’96, Part II (September 1996) 67 7 Overload matters (February 1997) 86 8 A balanced view, Part I (April 1997) 90 9 A balanced view, Part II (May 1997) 103 10 High-quality compressor/limiter (December 1975) 117 11 Inside mixers (April 1991) 127 12 Electronic analogue switching, Part I: CMOS gates (January 2004) 142 13 Electronic analogue switching, Part II: discrete FETs (February 2002) 155 vi Contents Power Amplifiers 14 Sound mosfet design (September 1990) 170 15 FETs versus BJTs: the linearity competition (May 1995) 181 16 Distortioninpoweramplifiers,PartI:thesourcesofdistortion (August 1993) 187 17 Distortion in power amplifiers, Part II: the input stage (September 1993) 201 18 Distortion in power amplifiers, Part III: the voltage-amplifier stage (October 1993) 217 19 Distortion in power amplifiers, Part IV: the power amplifier stages (November 1993) 232 20 Distortion in power amplifiers, Part V: output stages (December 1993) 251 21 Distortion in power amplifiers, Part VI: the remaining distortions (January 1994) 265 22 Distortion in power amplifiers, Part VII: frequency compensation and real designs (February 1994) 279 23 Distortion in power amplifiers, Part VIII: Class A amplifiers (March 1994) 294 24 Power amplifier input currents and their troubles (May 2003) 310 25 Diagnosing distortions (January 1998) 318 26 Trimodal audio power, Part I (June 1995) 333 27 Trimodal audio power, Part II (July 1995) 347 28 Load-invariant audio power (January 1997) 363 Contents vii 29 Common-emitter power amplifiers: a different perception? (July 1994) 380 30 Few compliments for non-complements (September 1995) 391 31 Loudspeaker undercurrents (February 1998) 400 32 Class distinction (March 1999) 404 33 Muting relays (July 1999) 421 34 Cool audio power (August 1999) 434 35 Audio power analysis (December 1999) 448 Index 463 Introduction ThisbookisacollectionofthearticlesIwroteforthejournalWirelessWorld (nowElectronicsWorld)betweentheyears1979and1999.Thevastmajority of these deal with hi-fi preamplifiers and power amplifiers, and this book concentrates on this field. Inthelasttwenty-fiveyearsthescopeoftechnologyavailabletotheaudio designer has greatly widened. At the beginning of this period the only choice in preamp design was between discrete transistor stages and the relatively new op-amps. The latter had dubious characteristics as regards noise and distortion—particularly crossover distortion. This most unloved of audio defects was tolerated in power amplifiers because it had to be, but there was considerable resistance to incorporating it in preamplifiers. At this time no-one would have considered using valve circuitry in a new design. My association with this influential and much-loved magazine actually beganwithadesignforacompressor/limiterin1975,whichstartedlifeas my third-year project at university. This design is not reproduced here as it is unarguable that it has been overtaken by advances in technology. This debut was followed by what I called ‘An Advanced Preamplifier’ in1976.TheAdvancedPreamplifiercertainlygave(andgives—Istillhave the prototype) exemplary performance, obtained by making each stage a discrete-componentoperationalamplifier.Thismadenecessarytheuseof dual IC regulators to produce +/−15V, and at the time the cost of this power-supply scheme was significant. Asareactiontothiscomplexity,Idecidedtotrymyhandatwhatmight becalled‘traditional’discretecircuitryinapreamplifier,andthisbecame the ‘High Performance Preamplifier’ published in 1979, though actually designed nearly two years earlier. It was conceived in an era when op- amps were still regarded with considerable suspicion by designers seeking the best possible audio performance. In the search for simplicity a single supply rail was used, without regulation, but with a simple RC filter after the reservoir capacitor to reduce ripple to a manageable 50mV or so. Experiment had proved that this minimal-cost arrangement could give hum and noise results that were as good as those yielded by the dual-IC- regulator approach. In contrast, the Precision Preamplifier of 1983 was designed at a time whentheremarkable5534/5532op-ampshadbecomeavailableatreason- able prices. Since they delivered very low noise with almost unmeasurable Introduction ix distortion,itwasclearlytimetotrya‘thirdway’asregardspreampdesign. Havingexploreddiscreteop-amps,andconventionaldiscretecircuitry,an IC solution was an obvious next step. The return to op-amps meant a return to dual power supplies, but this was a small price to pay for the convenience of dual rails. This design later gained a moving-coil head amplifier. I had designed several of these stages before, at least two of which made their way into commercial production, but this was the first version that got both noise and distortion down to what I considered to be acceptably low levels. The salient features are the discrete transistor inputdeviceswhichthen,andindeednow,providethebestpossiblenoise figure.Atthetimemanyheadampswereoutboardunits,oftenrelyingon battery power, presumably to sidestep intractable ground-loop problems. However, no difficulties were found in grafting this design onto existing preamplifiers. Someyearslater,havingdevotedmuchtimeinbetweentopowerampli- fier design, I felt the call to take another look at preamplifiers. My last design was twelve years old, and it seemed likely that some significant improvements could be made. Much thought and a lot of calculation and simulation led to the ‘Precision Preamp 96’ articles, including in-depth mathematical modelling of the noise generated by the RIAA stage. This allowedeachnoisecontributiontobestudiedindependently,andpermit- ted comparison between the actual noise and the theoretical minimum. The latter is rather sensitive to the exact assumptions made. It was also possible to discover why op-amps that appeared to be quieter than the 5534 in theory, were actually slightly noisier in practice. The answer was that op-amp bias-cancellation networks maybe great for DC precision, but the extra common-mode noise they generate in audio circuitry is just an embarrassment.The96preampisinfactnotsomuchanupdatedversion as a thorough re-design, with only the moving-coil input amp remaining essentially unaltered. It demonstrated, amongst other things, that obtain- ing an interchannel separation of 100dB on a stereo PCB is perfectly possible with careful component and track layout. In 1990 I had again turned my attention to power amplifiers. For many years I had felt that the output stages of power amplifiers presented very great possibilities for creative design, and so I explored some of them. OneofthefirstdifficultiesImetwithwastheproblemofdetermininghow much of the overall distortion was produced in the small-signal sections, and how much was generated by the output stage. Traditionally the latter was regarded as the major source of distortion, but there was very little published research to back this up, and so I attacked the problem myself. When I began it was not clear if there were two, twenty, or two hundred significant distortion mechanisms, but after a good deal of study it sud- denly became clear that seven or eight were sufficient to explain all the observabledistortion.Thisisnottosaythattherearenototherdistortion

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Whether you are a dedicated audiophile who wants to gain a more complete understanding of the design issues behind a truly great amp, or a professional electronic designer seeking to learn more about the art of amplifier design, there can be no better place to start than with the 35 classic magazine
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