独立电唱机前置放大器英语资料Word文档格式.docx

独立电唱机前置放大器英语资料Word文档格式.docx

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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"

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