独立电唱机前置放大器英语资料.docx

独立电唱机前置放大器英语资料.docx

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独立电唱机前置放大器英语资料

StandalonePhonoPre-amp

AHIGH-PRECISIONPHONOPREAMPLIFIER

byFredNachbaur,Dogstar1997,

1:

INTRODUCTION

Therehasbeenmuchpresslatelyaboutthemerits（anddrawbacks）ofthevenerablevacuumtube.Howmuchofthisretromovementisbasedindemonstrableprinciples,andhowmuchisrootedinnostalgiaorsubjectivityisadebatethatcouldfillvolumes.

Whatisclearisthatthereisconsiderablerenewedinterestinvacuumtubes,atechnologythateventwodecadesagowasconsideredasobsoleteasspatsandtophats.Nowthetrendisreversing,andanumberofmanufacturersareagainsupplyingtubegearforaudiophiles,musiciansandhobbyists.Inmanycases,thesearesimplyvintagecircuitsinnewpackaging.Otherproductsoffernewapproachestovacuumtubetechnology,addingwhatwe'velearnedinthemeantimetocomeupwithsometrulynoteworthydesigns.Theprojectdescribedherefitsintothelattercategory.

Whyusetubesanyway?

Thecentralargumentforthepro-tubemovementisthatspecscanbealmostmeaningless,andthatwhatcountsishowitsoundstotheindividuallistener.Highlysubjectivedescriptionsarethereforeused,insteadofthetechno-babblewe'vemore-or-lessgottenusedtoinrecenttimes.Theoppositecampclaimsthatnumbersdon'tlie,andthatyoucan'timprovesomethingbyaddingdistortionofanykind.

Bothcampshavevalidpoints.Mostofushaveheardexpensivegearwithspectacularspecifications,butwereleftcoldbythe"toogoodtobetrue,"almostclinicalsoundofsuchequipment.Similarly,mostpeoplewillagreethatjustbecausesomethinghastubesinitdoesn'tmakeitworthlisteningto.Turnononeofthoseold"All-AmericanFive"table-topAMradiosifyouwantastrikingdemonstrationofjusthowbadvacuumtubeequipmentcansound.

Perhapsawayofreconcilingthetwoviewpointsistoconsiderthedistinctionbetweenmusicalequipment,andreproductionequipment.Formusicalgear,theindividualfrequency,waveformandphasedistortionsarepartofwhatdefinesthesoundof,say,aFendertubeamp.Justasno-onewouldtrytodefineafine

Stradivariusviolinwithspecsanddistortionfigures,soalsoitwouldbespecioustoarguethatacertaintubeguitaramphasover12%THD（totalharmonicdistortion）at35watts.

Reproductionequipment,ontheotherhand,hasalwaysbeenexpectedtogiveaperfectrenditionofthesignalappliedtoit.Soundsgoodinprinciple;ifsuchathingexisted,weshouldbeabletoexactlyandperfectlyreproduceeverythingfromagrundgebandtotheNewYorkPhilharmonic,makingthereproductionsindistinguishablefromtheoriginalperformances.

Butthereinliestherub.Theoverallsoundofastereosystemdependssoheavilyontheroomit'sin,thespeakers,volumelevel,personalpreference,andahostofotherfuzzyvariablesthataperfectreproductionsystemcannotbesaidtoexisteveninthisdayandage,andprobablyneverwill.Addthefactthatmostifnotallrecordingsareelectronicallysweetenedtosomedegreetomakethem"soundgood"（asopposedtobeinganexactcopyoftheperformance）,andargumentsfortheclinicalreproductionapproachlosecredibility.

Carriedtoextremes,an"ideal"reproductionsystemwouldn'tevenhaveanycontrols,exceptperhapsforselectinginputs.Ifyouthinkaboutit,personalpreferenceistheonlyreasonwhystereoshavevolumecontrols,equalizers,andotheradjustmentstoletuscustomizethesoundtosuitourveryindividualearsandbrains.

Itmightbebesttoviewthereproductiongearasacontinuationofthesameprocessthatstartedwiththeconstructionoftheinstrumentsusedintheperformance.Whatweultimatelyhearisthesumtotaleffectofeverythingfromthatoriginalinstrumentdesign,tothewayitisplayedbytheartist,throughtheentirerecording,mixinganddistributionprocess,tothegearweusetoplayit,andhowwe'vesetitscontrols.

Thereis,therefore,aconsiderabledifferenceindesignapproachbetweenthe

"instrument"and"reproduction"categories.Anearlierdesign（"TheRealMcTube"）documentedavacuumtubepre-amplifierforuseasanadjuncttoelectricinstruments.Thedesignapproachwaslargelyempirical,andtheemphasiswasonhighlightingtheuniquedistortioncharacteristicsofthevacuumtube.Thisarticleexploresthereproductionaspect;thedesignapproachwasquitemathematicalandprecise,andtheemphasisisoncontrollingthecharacteristicsofthevacuumtube.

Vinylvs.CD

Similarargumentsareongoingregardingvinylrecordsvs.compactdiscs.TheCDcamppointsattheCD'saccuracy,definition,andclarity,whilevinylloversbemoantheCD'slackofwarmthandclaimthatconventionalrecordssoundmore

"natural."

Thisprojectgivesyoutheopportunitytoexplorethesesubjectiveandcontroversialdebates,lettingyourownearsbethejudge.Youmayfindthattherearesomerecordingsthatsoundsbetterthroughthetubepre-amp,andothersthatbenefitfromtheimproveddefinition（whateverthatis,technicallyspeaking）thatgoodsolid-stategearcanoffer.

2:

DESIGNPHILOSOPHY

Thedesignphilosophywastoimproveon"vintage"tubedesignsbyincorporatingrefinementsnormallyonlyassociatedwithsolid-stategear.Primeamongtheseistheuseofdifferentialinputstages,inwhichtheinvertinginputisused

strictlyforfeedback.Anotheristheuseofdirectcouplingbetweenstages,atechniqueoncequitecommoninoscilloscopes,butrareinaudiogear.Theresultingcircuitsarethuscloselyrelatedtotheoperationalamplifier,andusedinasimilarfashion.

Inadditiontothequasi-opampidea,someotherdesignconceptsusedinthe

projectare:

Useofreadilyavailablepartswhereverpossible.Forthisreason,tubetypes12AX7Aand12AT7Awereemployed.

Directcouplingwasusedincrucialpartsofthecircuitry.Moreaboutthislater.

Flexibilityinusewasaprimeconsideration.Whilethisarticlesdetailstheuseofthiscircuitdesignasamagnetic（RIAA）andceramicphonopre-amplifier,thereisvirtuallynolimittothenumberofpossibleapplicationswithsuitablemodificationofthefeedbacknetworks.

AWordAboutNegativeFeedback

Itisperhapsanunfortunateaccidentofhistorythattheterm"negativefeedback"wasusedtodescribethelinearizationtechniqueofapplyingaportionoftheoutputsignalbacktotheinput,inoppositephasetotheappliedsignal.Itgivestheimpressiontoaudiophileswithenoughknowledgetobedangerousthat"negative"feedbackmustsomehowbea"bad"thing.I'veseendiatribesagainsttheevilsofnegativefeedback,yetinthesamebreathsuchcriticsextolthevirtuesof"ultra-linearoutputtransformers."Thejokeisthatsuchtransformersachievethislinearizationbyapplyinglocalnegativefeedbacktothescreengrids!

Perhapsabettermonikerfor"negativefeedback"wouldbe"dynamiccompensation"orsomethingonthatorder.Considerthefollowingcharacteristicsof

well-designednegativefeedbacksystems:

A:

Precisecontroloverclosed-loopgainandminimizationofgaindriftwith

componentaging.

B:

Considerablereductionindistortionandnoiseintroducedwithinagiven

gainblock.

C:

Reductionofeffectiveseriesoutputresistance.

D:

Precisecontrolandstabilizationoffrequencycharacteristics.Extension

ofhigh-frequencyresponsewhereneeded.

E:

Reducesoreliminatestheneedforcarefulcomponentmatching

3:

HOWITWORKS

Thesectionsthatfollowstepthroughtheprinciplesofoperationofthisdesign.Sinceitisquitedifferentfromtraditionaldesigns,Isuggestthatyoufollowthroughthisatleastonce.Ifthe"finepoints"areoveryourhead,don'tworryaboutit;hopefullyenoughoftheinformationwillsticktogiveyouatleastaworkingunderstandingofthecircuit.

Therearein-lineillustrationsoftheschematic,repeatedasnecessary.（Ifyouhavecachingturnedoninyourbrowser,you'llonlyhavetoactuallydownloadtheimagesonce.）These"Figures"（schematics）areshownhereinreducedsize.Clickontheimagetoenlargeit,preferablybyright-clickingandopeningtheimageinanewwindow.Full-size,printableversionsarealsoavailableintheResourcessection.Itishighlyrecommendedthatyouprintoutthesefilesandhavethediagramsathandasyoustudythisdocument,especiallyifyouplanonactuallybuildingyourselfoneoftheseunits.

Notethatintheschematicdiagrams,onlyonechannelisshown;theotherchannelisofcourseidentical.Pleasenotethatpartnumbersrefertothevarioussectionsasfollows:

（We'llbeassumingtheleftchannelinthisdiscussion.）

1-99Commonparts（powersupply,etc.）Alsotubedesignations.

100-199Leftphonopreamp

200-299Rightphonopreamp

3-A）HIGH-PRECISIONPHONOPREAMPLIFIER:

OVERVIEW

Considerableeffortwastakentodesignapreampthatisascleanaspossibleto

satisfydiscerningaudiophiles,whilemaintainingarelativelylowpartscounttosatisfylimitedbudgets.Itshouldbenotedatthispointthat,whilethediscussionisinthespecificcontextofanRIAAphonopre-amplifier,thisdesignlendsitselfwelltoawiderangeofotheruses.

This"universalpreampmodule"sportsahighgain（about60dB）anddifferentialinputstagesthatallowfordifferentialinput（asforbalancedmicpreamps）aswellaseitherinvertingornon-invertingsingle-endedapplications.Frequency

responseshapingandgainsettingareaccomplishedusingtheappropriatefeedbacknetworks.

Theschematicdiagramofthephono/micpreampisshowninFigure2.IfbuiltonaPCB,thephonopreampequalisationisaccomplishedusingasmall"daughter"

carddirectlyonthemainboard;ifyouusethepoint-to-pointwiringapproach,

thesenetworkscanbewiredonseparateterminalstripsforeasymodificationif

necessary.

Figure2P