an exacting energiser - ifi audio · may 2017 pro iesl tech notes in terms of performance, only...
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May 2017
Pro iESL
Tech Notes
AnExactingEnergiser
Partone.
AlittlebackgroundTheProseriesiESListhebestwayofdrivingelectrostaticheadphones.WeareconfidentthattheiESLProdrivenbyeithertheiCANProoryourexistingamplifiercangotoe-to-toewiththebestamplifiersoutthereforelectrostaticheadphones.Itsdefiningcharacteristicisatransparencyandanultra-widedynamicrangethatistotallynatural.
WhydoIneedanelectrostaticheadphoneamplifier?WehavequotedfromWikipedia:
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ELECTROSTATIC DRIVERS CONSIST OF A THIN, ELECTRICALLY CHARGED DIAPHRAGM, TYPICALLY A COATED PET FILM MEMBRANE, SUSPENDED BETWEEN TWO PERFORATED METAL PLATES (ELECTRODES). THE ELECTRICAL SOUND SIGNAL IS APPLIED TO THE ELECTRODES CREATING AN ELECTRICAL FIELD; DEPENDING ON THE POLARITY OF THIS FIELD, THE DIAPHRAGM IS DRAWN TOWARDS ONE OF THE PLATES. AIR IS FORCED THROUGH THE PERFORATIONS; COMBINED WITH A CONTINUOUSLY CHANGING ELECTRICAL SIGNAL DRIVING THE MEMBRANE, A SOUND WAVE IS GENERATED. ELECTROSTATIC HEADPHONES ARE USUALLY MORE EXPENSIVE THAN MOVING-COIL ONES, AND ARE COMPARATIVELY UNCOMMON. IN ADDITION, A SPECIAL AMPLIFIER IS REQUIRED TO AMPLIFY THE SIGNAL TO DEFLECT THE MEMBRANE, WHICH OFTEN REQUIRES ELECTRICAL POTENTIALS IN THE RANGE OF 100 TO 1000 VOLTS. DUE TO THE EXTREMELY THIN AND LIGHT DIAPHRAGM MEMBRANE, OFTEN ONLY A FEW MICROMETERS THICK, AND THE COMPLETE ABSENCE OF MOVING METALWORK, THE FREQUENCY RESPONSE OF ELECTROSTATIC HEADPHONES USUALLY EXTENDS WELL ABOVE THE AUDIBLE LIMIT OF APPROXIMATELY 20 KHZ. THE HIGH FREQUENCY RESPONSE MEANS THAT THE LOW MID-BAND DISTORTION LEVEL IS MAINTAINED TO THE TOP OF THE AUDIBLE FREQUENCY BAND, WHICH IS GENERALLY NOT THE CASE WITH MOVING COIL DRIVERS. ALSO, THE FREQUENCY RESPONSE ‘PEAK’ REGULARLY SEEN IN THE HIGH FREQUENCY REGION WITH MOVING COIL DRIVERS IS ABSENT. WELL-DESIGNED ELECTROSTATIC HEADPHONES CAN PRODUCE SIGNIFICANTLY BETTER SOUND QUALITY THAN OTHER TYPES. ELECTROSTATIC HEADPHONES REQUIRE A VOLTAGE SOURCE GENERATING 100 V TO OVER 1 KV, AND ARE ON THE USER'S HEAD. THEY DO NOT NEED TO DELIVER SIGNIFICANT ELECTRICAL CURRENT, WHICH LIMITS THE ELECTRICAL HAZARD TO THE WEARER IN CASE OF FAULT.
Source:https://en.wikipedia.org/wiki/Headphones#Electrostatic
May 2017
Pro iESL
Tech Notes
Toputthistheoryintoperspectivewithsomefigures,
SennheiserHD800dynamicheadphones=102dB/1VTypicalpairofelectrostaticheadphones=102dB/100V!!!
TraditionalheadphoneamplifiersdonotdelivereithertheEHTvoltageortherequiredaudiovoltage.Soaspecial‘Energiser’isrequiredtomatchtheelectrostaticheadphones.Typicallyamaximumoutputof300Visneeded.Thisisequivalentto300Wattintoa300ohmheadphoneload!Hence,electrostaticheadphonesrequire:
• VERYhighvoltages –need300Vinaninstant• BIASvoltage –adjustablefrom230Vto640V
Therearetwooptionstomakesuchaproductto‘energise’electrostaticheadphones: ProductA: adedicatedhigh-voltageamplifier.
ProductB: anEnergiser,asolutionwhichisalmostoppositetoatubeampasitconvertslow-voltagetohigh-voltage.
Intherealworld,fewamplifierscandelivertheselevelsofvoltageoutputwhichiswhyan‘Energiser’isrequiredto‘charge’electrostaticheadphones.Someexistingtubeamplifiers(liketheiFiRETROStereo50)couldbemodifiedtodriveelectrostaticheadphonesdirectlyandsomesolid-statehighvoltageamplifiersalsoexist,howevertheneedforsuchextraordinarilyhigh-voltagescompromisetheactive‘directhigh-voltage’ofmostamplifiers.Bycomparison,anEnergisercanuseastandardspeakeramplifierorasuitablypowerfulheadphoneamplifier(>10Vinto16ohmsor>20Vinto64ohmsareneeded)andusetransformerstoproducethehigh-voltagerequired.TheiESListhesecondtypeofaproduct:anEnergiser.YettheiESLisnoordinaryEnergiser.Eachoneofthesethree-corecomponentsweredevelopedusingthebestclassicandlatestapproacheswithscantregardforcost,complexityordifficulty:
• Thetransformer• Thebiasvoltagegenerator• Thecomponentquality
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Togive:
TheTransformer–the‘sound’oftheEnergiserLikeagoodchassisinacar,atransformermakesorbreakstheEnergiser.Thisisfundamental,simplybecauseitisalmostsolelyresponsibleforthe‘sound’ofanEnergiser.Ideallyatransformerwouldhaveno‘sound’signatureofitsown,butsimplyandtransparentlypassontheaudiosignal.Inrealitythisisachallengingmandate.Atransformeriscomprisedoftwoparts,bothofwhichintheiESL,havebeenexecutedverymuchwithmorethananodtothe‘moneyisnoobject’daysofclassicAltecandWesternElectricdesign,materialsandworkmanship.Sincethegoldenageofstereointhe1950sand1960s,theartofmakinggreataudiotransformerswasmostlylostwhentubesfelloutoffavour.AtAMR/iFi,weretainthisarcaneknowledgeandwishtocarryitonwellintothe21stCentury.FirstwiththeAMRtransformers,thenwewentontotheoutputtransformersintheRetroStereo50.NowwehavemovedontotheiESL‘PPC’transformers.Tomakethesetransformers,wehadtosourcetherequiredmaterialsandimplementaclassic,yesteryearmethodofhand-windingtransformersthatisalmostalostartbutabsolutelyworththeeffort.
May 2017
Pro iESL
Tech Notes
1.1TheCore–the‘tone’TheiESLtransformercoreusesamixofMu-Metal(akaPermalloy)laminationandGrainOrientedSiliconeSteel(akaGOSS).Mostaudiotransformersforsuchhigherlevelsandpowerrestrictthemselvestousingsteel.https://en.wikipedia.org/wiki/Electrical_steel#Grain_orientation
+ GOSShandleshighsignallevelswell,whichisneededifwewishtoproduce300Vorevenmore.
- GOSShasproblemswithhysteresisatlowlevels,causinglossoflow-leveldetail.Insomecasesthis‘simplification’ofthemusicsignalmaybeadesirableeffect,howeveritisaclearandaudibledistortion.
Mu-MetalisauniquemagneticalloywithverydifferentpropertiestoGOSS:https://en.wikipedia.org/wiki/Mu-metal
+ Mu-Metal/Permalloyisgreatforlowsignallevels(low-levelresolution)but
cannothandlehigh-levelsignalswell.Mu-Metalbasedtransformersarewellknownfortheircrystalclearsound,butgenerallycannothandlehigh-levels,restrictingthemmostlytouseinmicrophonepre-amplifiersorformovingcoilLP-pickups,wheresignallevelsaremeasuredinthousandsofaVolt,ratherthanhundredsofaVolt.
- Mu-Metal/Permalloyrequiresheattreatmentafterbeingworked(e.g.beingcut)torestoreitsmagneticproperties.ItishugelymoreexpensivethanGOSSandinlimitedandforeverdwindlingsupplyasexistingmakerscloseupshoporareacquiredanddroplessprofitable,lowervolumeproductssuchMu-Metal/Permalloyfromtheirlines.
Combiningthetwomaterialsinpinstripefashion,aprocesspioneeredbyPeerlessTransformersUSAinthe1950sgivesthebestofbothmaterials.
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PeerlessTransformersfeaturedinthelegendaryWesternElectricandAltec(“AllTechnicalServices”)recordingequipmentfromthegoldenageofstereo.
GettingidenticallaminationsinGOSSandMu-Metal/Permalloyischallenging,gettingultra-thinlaminationssuchasweprefertouse,moresoandthenmanuallyandindividuallyassemblingeachcorewithdifferentmaterialsinarepeatablepatternhastobedonebyhand.Arduous?Yes,butworthit.
1.2TheWinding–‘VoiceoftheTransformer’Whileagoodcoreisthekeytomakingatransformerthathasgooddynamicsandcorrecttone,thewindingmustbeequallyconsidered.Duetomanylimitingfactors,makingatransformerwithawidebandwidthandlowdistortionisverychallenging.Thewindingstructuredeterminesthebandwidthoftransformers.Typicallywindingaudiotransformersisacompromisebetweenlowfrequencycut-off,lowfrequencydistortionandhighfrequencycut-off.Thesethreeelementstendtoformoneoftheclassic"chooseanytwo"triads,especiallyforclassic‘layered’constructiontransformers.Oftentransformershaveahighfrequencycut-off(-3dB)barelyhigherthan20kHzandlowfrequencycut-offsbarelybelow20Hz.AnEnergiserusingsuchtransformersislikelytoexhibitamuffledtopendwhilebassiscompromisedbythelackofreallylowcut-offandexcessivephase-shiftintheaudiblerange.Asanabsoluteminimumspecification,wetargeted<10Hz–40kHz,-3dB,whichisrequiredbytheJAS‘High-Res’logo,whichwestrivetoearnwithallourproducts.
Toattainsuchperformance,somerarelyusedtechniqueswereemployedtosidestepthe‘chooseanytwo’limitationsfortheiESL.ThekeyistheuseofScheibenwicklung(disk-winding)...
May 2017
Pro iESL
Tech Notes
TheScheibenwicklungisalsoquiteold,GermanpatentDE861406CissuedtoSiemensGMBHJan5,1953withafilingdateofMay14,1943.(http://google.com/patents/DE861406C?cl=ko)coversthis.
Themuchmorecommonlayeredwindingisnotabletoofferaswidebandwidth.
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TransformerIllustrationfromhere:http://elektronik-kurs.net/elektrotechnik/transformatoren/ TransformersintheiEnergiserareuniqueincombiningScheibenwicklungandlayeredwindinginacomplexinterleavedpatterntoachieveresultsthatconsiderablyexceedeithertechnique.Thedesignofsuchtransformersismostlyanextinctart,withvirtuallynotextremaining(nevermindinprint)andfewpeoplewhostillhavetheknow-how.Andmanufacturingofsuchdesignsisordersofmagnitudemorechallengingthangenerictransformers.OurtransformersmatchandexceedthestandardssetbySiemens(Germany)andPeerless(USA).Withabandwidthof<5Hzto>60kHz(-3dB),transformersintheiESLfarexceedtherequirementsofeventheJASHigh-ResLogo.Torecap,inorderproducehigh-voltagesrequiredbyelectrostaticheadphones,theProiESLemployscustom-madePinstripePermalloyCoreTransformer(PPCT)ofthehighestquality:
• Ultra-widebandwidthsignal:attains<10Hzallthewayupto60kHz.• Perfectlylinear:fromlowthroughtohighsignallevels.• TheextremelycriticalcoreofthetransformersisaGOSS/Mu-Metal
hybrid.Thisreducesdistortiondramatically,whencomparedtotraditionalcores,betheyGOSS,amorphousironorsimilarmaterials.
• Inordertocombineahighstep-upratiowithgoodbehaviourwithoutexcessiveresonancesorbandwidthlimitationsourcustomtransformersuseacomplicatedmulti-sectionwindingwithbothverticalandhorizontalsectioning.Extremelythinwiremustbewoundpreciselyandtightlytocreatetheperformancerequired.
CombiningthiscomplexwindingwithanexceptionalcoreproducesatransformerthathandlesALLlevelswithoutdistortiondrawbacksaswellasofferingacompletelyflatfrequencyresponsewithoutcolorationswellbeyondtheaudioband.
May 2017
Pro iESL
Tech Notes
Intermsofperformance,onlysuchanexceptionaltransformercanapproachandevenexceedthebesttransformer-lessamplifiers.
Part2:BiasVoltageInadditiontotheactualmusicsignal,apairofelectrostaticheadphonesrequirestheso-called“biasvoltage.”Whereasso-calleddynamicheadphoneshaveastaticmagneticfieldwhichprovidesthe‘bias,’inthecaseofelectrostaticheadphones,astaticelectricityfieldisneeded.Itisimportantthatthisvoltageisbothextremelycleanandprecise.
Exceedingthebiasvoltagesignificantly(evenonlyduetohighmainslinevoltages)candamagetheelectrostaticheadphone,noiseonthebiasvoltagewillmodulatewiththemusicsignal.Thebiascircuititselfwiththeheadphoneformsalow-passcircuitthatiseffectiveinsuppressingsupersonicnoise(byatleast100times),howeverlow-frequencynoisespasswithoutanyreduction.Thismakesclearthatthebiascircuitryforelectrostaticheadphonesis‘quite’critical,yetinmostEnergisersanddedicatedESheadphoneamplifiers,onemightbeforgivenforconsideringthebiascircuitrywasamereafterthought.Therearetwocommonapproachestoofferingbiasvoltages:
• HighVoltageTransformer&Rectification:Customhighvoltage(appx410VforStaxProBias)transformersprovide50/60HzACwhichisrectified.Thesearecustomtransformerswithveryhighvoltagewindingswhichareunavailableofftheshelf.Theyareveryrareandareseldomseen,evenonelectrostaticspeakers!
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• Cascaderectifier:achieveshighvoltagesbutisverynoisy.Thenoisecanbe
filteredbutthisisnotpreferredassomeresidualnoisealwaysendsupinthemusicpathwhichissub-optimal.
BackgroundtoBiasingVoltagesTheCascaderectifiercircuitisperhapsthemostcommonyetisalsothemostcompromised.Themostcommonapproachtothebiasvoltageiscreatedusing50/60Hzmainspowerandaso-calledVillardorGreinachercascaderectifier(whichisalsosometimescalledaCockcroft-Waltonvoltagemultiplier).
Thiscircuitcandeliververyhighvoltagesusingrathergenericandinexpensivecomponents,butitsoperationisnoisy.GiventhelowfrequenciesoftheACused,largevaluecapacitorstendtobeneeded,usuallyrequiringrathernon-linearelectrolyticcapacitors,whichhavehighleakagecurrentsandsothecascaderectifiermustoperateconstantlytokeeptheBiasVoltagefromdropping.Tous,bothapproacheswereunacceptable.Wedecidedtodiscardallexistingsolutions.Wewentwithathirdoption.WhiletheBiasVoltageishigh,thereisalmostnocurrentdraw.
May 2017
Pro iESL
Tech Notes
Option3:TheCapacitiveBatteryPowerSupplyListeningtoavintagebattery-poweredEnergiserfromourcollectionofeclecticandsuigenerisHiFicollection,wenoticedthatoncethepowerwasturnedoff,theelectrostaticheadphoneskeptplayingformanyminutesjustontheretainedchargeinasmallcapacitor.Moreapparentwasthatsoundqualitywasnoticeablybetterastheprimitiveswitchedmodestep-upcircuitwasoff.Thiswasareal"eureka!"moment.
The3rdsolutionWedecidedtousealargebatteryofcapacitors,usinghigh-gradeWimaGermanymadefilmcapacitorsratedat1,000VDC.Thentheaimwastochargethemuponcetotherequiredvoltageandtothenturnthechargercircuitoffentirely,leavingthecapacitorbankto‘float’attherequiredbiasvoltage.Thisgivesanessentiallyperfectbiasvoltagesource,purestaticelectricity,noelectronicnoisewhatsoeveratanyfrequency.Talkaboutkillingbirdswithstones!TheDesignInordertoprovidethisonetimecharge,weemployaveryhigh-frequencyswitchingsystem(@750kHz).Itusesatiny,fully-shieldedtransformerandexoticultra-fasthighvoltagerectifiers.Morecrucially,thissystemshutsdowncompletelyassoonascorrectbiasvoltagehasbeenestablishedinthecapacitorbank.Thisalsomeansthatthebiasvoltageisprecisionregulatedtoasmallfractionof1Vtotheofficialbiasvoltage,nochangesinmainsvoltageorotherfactorswillcausethebiasvoltagetoshift.Assomeminordischargeoccurseventhroughair,thechargingsystemchecksthecapacitorbankevery30secondsattemptingtorechargethecapacitorbank.Ifthebankisstillfully-charged,theprocessusuallytakesafewmillionthofasecond
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(microseconds)asusuallyoneortwoswitchingcyclessufficestoreplenishthelostcharge.Thentheswitchingsystemisagainoffcompletely.Anynoisethatthissystemcreatesisconfinedtomediumfrequencyradiobandsfortheinfrequentshortdurationsduringwhichitisactive.Inpractice,over99.999%ofthetimethechargingcircuitiscompletelyoff.Thepracticaloutcomeisineffectaperfecthigh-voltagebatterytoprovidethebiasforelectrostaticheadphones.Twocompletelyseparateandindependentbiascircuitsareemployed,oneforthe230V‘normal’biasandanotheradjustable,tocoverawiderangeofmodernelectrostaticheadphones.Thisisalldeployedinatotallyseparatecircuitwhichisstandaloneandisnotinthemusicpathatall.
Usingthisuniquesolution,theCapactiveBatteryPowerSupplyintheProiESLisabletoprovidebiasvoltagesforallknownelectrostaticheadphonesacrossanincrediblywiderange:Biasvoltageusage:
230V(Normal) “normalBias”StaxESHP
500V(Custom) SennheiserOrpheusHE-90
540V(Custom)SennheiserHE-60 KingSoundKS-H2/3/4
580V(Pro) “ProBias”StaxESHP
600V(Custom) KOSSESP/950,Jade
620V(Custom)Optional
640V(Custom)Optional
May 2017
Pro iESL
Tech Notes
Whilethisisanuncommonlycomprehensivewiderangeofadjustability,comparedtootherEnergisers,theiESLapproachtobiasvoltageisunique.
Part3:Audiophileparts,unparalleledsignalperformanceInordertoselectinputsignalsandtoswitchspeakerconnections,high-gradesignalrelaysareused.WIMAcapacitors
These1,000VoltratedcapacitorsarefromGermany.Thekeyistheveryhighinsulationresistanceandminimalcurrentleakage,asanysuchleakagecausesself-discharge.Thewholecapacitorpackhasaround35,000,000kOhminsulationresistance,whichmeansthatself-dischargetime(to30%)isaround100,000secondsor27hours.Hence,theyareperfectlysuitedforthecapacitivebatterypowersupplyoftheiESL.VishayMELFresistors
MELFsoffersuperiorSMDresistorperformanceintermsofaccuracy,stability,reliability,andpulseloadcapability.ThecylindricalconstructionofMELFdevicesprovidesanoptimalpowerratingandpulseloadcapability.MELFaremorecomplextomakeandsocostmore.WhenassemblingPCBsMELFneedspecialmachinesandtreatment.Hencetheyareseldomused.
Tech Notes Pro iESL
Gold-platedsilvercontactrelays
Theentiresignalswitchingfortheinputselectionutilisesgold-platedsilvercontactminiaturerelays,filledwithaninertgas.Thismakessurethecontactswillbeasnewformanyyearstocome.Sealed-silveralloycontactrelays
Thespeakerconnectionsareswitchedusingsealedsilveralloycontactrelaysforminimumimpactonthesoundqualityofthespeakerpath.WehopethisoutlinehasofferedasmallinsightintothemakingsoftheProiESL.Simpleintheorybutcomplexispractice.Thereyouhaveit,theiESL.ItisoneveryspecialEnergiserunitforSennheiserOrpheus,Staxelectrostatic‘EarSpeakers’anddynamicheadphones.
WethankyouforreadingthisandyourinterestinthehistoryanddevelopmentbehindtheProseriesiESL.
May 2017
Pro iESL
Tech Notes
AddendumnotesonProiESLusewithProiCAN
MostESHPareveryinefficient.DuetothelimitedVoltagehandlingandthelowsensitivityESHPgenerallydonotplayveryloud,especiallycomparedtotraditionaldynamicheadphones.Sodonotexpectthemtogoasloudasdynamicheadphones.
DependingontheimpedancecontrolsettingtheProiCANandProiESLcombocandeliverbetween320VRMS(910VPP)usingthe64/96Ohmsettingandupto640VRMS(1820VPP)usingthe16/24Ohmsetting.
TheseareveryhighvoltagelevelsandmayexceedtheratedlimitoftheESHP.PleaseconfirmwiththemanufacturerofyourESHPwhatlevelsareallowedandifanydoubtonlyusethehighimpedancesettings.
WhenusingtheiESLwithProiCANremembertokeepgain,X-Bass&3-DSoundsettingsmoderate.Maximumgainisrecommendedat9dB,3DonlyatthelowersettingsandthesameforX-Bass,otherwisethestressonbothamplifierandheadphonemaybeexcessive.
Undertheaboveconditions,thevolumeontheProiCANshouldbeabletobeadvancedtomaximum(usinganiFiDAC)withoutriskofprotectioncircuitryengagingordamagingANYESHPaslongastheProiESLimpedancesettingis64Ωor96Ω.
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Forfurtherinformation,pleasecontact:[email protected]/+44(0)1704227204
AboutiFiiFiAudioisheadquarteredinSouthport,UK.ItisthesisterbrandofAbbingdonMusicResearch(AMR).Theyrespectivelydesignandmanufactureportableanddesktop‘ultra-fidelity’audioproductsandhigh-endaudio‘home-based’components.Thecombinedin-househardwareandsoftwaredevelopmentteamenablesiFiAudioandAMRtobringtomarketadvancedaudioproducts.