独立电唱机前置放大器英语资料Word文档格式.docx
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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
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