外文英文翻译独立电唱机前置放大器.docx
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外文英文翻译独立电唱机前置放大器.docx
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外文英文翻译独立电唱机前置放大器
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.
Figure2
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