May 2017

Pro iESL

Tech Notes

AnExactingEnergiser

Partone.

AlittlebackgroundTheProseriesiESListhebestwayofdrivingelectrostaticheadphones.WeareconfidentthattheiESLProdrivenbyeithertheiCANProoryourexistingamplifiercangotoe-to-toewiththebestamplifiersoutthereforelectrostaticheadphones.Itsdefiningcharacteristicisatransparencyandanultra-widedynamicrangethatistotallynatural.

WhydoIneedanelectrostaticheadphoneamplifier?WehavequotedfromWikipedia:

Tech Notes Pro iESL

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

Tech Notes Pro iESL

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.

Tech Notes Pro iESL

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.

Tech Notes Pro iESL

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!

Tech Notes Pro iESL

• 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

Tech Notes Pro iESL

(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.

Tech Notes Pro iESL

Forfurtherinformation,pleasecontact:VictoriaPicklespress@ifi-audio.com/+44(0)1704227204

AboutiFiiFiAudioisheadquarteredinSouthport,UK.ItisthesisterbrandofAbbingdonMusicResearch(AMR).Theyrespectivelydesignandmanufactureportableanddesktop‘ultra-fidelity’audioproductsandhigh-endaudio‘home-based’components.Thecombinedin-househardwareandsoftwaredevelopmentteamenablesiFiAudioandAMRtobringtomarketadvancedaudioproducts.