McIntosh MAC 1900 User manual

THEMAC1900SOLIDSTATEAMFM/FMSTEREORECEIVER
Price$1.25

YourMAC1900AM-FM/FMstereo
receiverwillgiveyoumanyyears
ofpleasantandsatisfactoryperformance.
Ifyouhaveanyquestionsconcerning
theoperationormaintenanceof
thisreceiverpleasecontact:
CUSTOMERSERVICE
McIntoshAudioDivision
2ChambersStreet
Binghamton,NewYork13903
Ourtelephonenumberis607-723-3512
TakeAdvantageof2 years
ofFREEFactoryService. . .
FillintheApplicationNOW.
CONTENTS
Guarantee. . .
Installation. . .
HowtoConnect. .
TapeRecorders. . .
RecordPlayers. . .
Antenna, . ,
MaximumPerformanceIndicator. . .
Loudspeakers. . .
MclntoshLoudspeakers. . .
WhattheControlsDoandHowtoUseThem. . .
UsingtheTuningDial. . .
UsingthePushbuttons. . .
BalancingYourStereo. . .
ListeningtoYourStereo. . .
PerformanceLimits. . .
TypicalPerformanceCharts. . .
TechnicalDescription. . .
BlockDiagram. . .
1
2
3
3
4,
4,
4,
6
7
8
9
9,
11
11,
12,
14,
17
21
5
5
5
10,11
12
13
15,16
GUARANTEE
MclntoshLaboratoryIncorporatedguaranteesthis
instrumenttobecapableofperformanceasadver-
tised.Wealsoguaranteethemechanicalandelec-
tricalworkmanshipandcomponentstobefreeof
defectsfora periodof90daysfromdateofpur-
chase.Ifsuchdefectsoccur.MclntoshLaboratory
oroneofItsauthorizedagencieswillrepairthe
defectatnocosttothepurchaser.Thisguarantee
doesnotextendtocomponentsdamagedbyim-
properusenordoesitextendtotransportationto
andfromthefactoryorserviceagency.
TWOYEARFACTORYSERVICECONTRACT
Anapplicationfora FREETWOYEARFACTORY
SERVICECONTRACTisincludedwiththismanual.
Thetermsofthecontractare:
ForTwoYearsfromdateofpurchase—
1.Mclntoshwillprovideallparts,materialsand
laborneededtoreturnthemeasuredperformance
oftheinstrumenttotheoriginalperformance
limitsfreeofanycharge.TheSERVICECON-
TRACTdoesnotcoveranyshippingcoststoand
fromtheauthorizedserviceagencyorthefactory.
2.AnyMclntoshauthorizedserviceagencywillre-
pairallMclntoshinstrumentsatnormalservice
rates.Toreceivethefreeserviceundertheterms
oftheSERVICECONTRACT,theSERVICECON-
TRACTCERTIFICATEmustaccompanytheinstru-
mentwhentakentotheserviceagency.
3.Alwayshaveservicedonebya Mclntoshauthor-
izedserviceagency.Iftheinstrumentismodified
ordamaged,asa resultofunauthorizedrepairthe
SERVICECONTRACTwillbecancelled.Damage
byimproperuseormishandlingisnotcoveredby
theSERVICECONTRACT.
4.TheSERVICECONTRACTisissuedtoyouasthe
originalpurchaser.Toprotectyoufrommisrepre-
sentationthiscontractcannotbetransferredtoa
secondowner.
5.TheSERVICECONTRACTisgiventopurchasers
wholiveinthe50UnitedStatesorCanadaonly.
6.ForyourprotectionMclntoshselectsitsdealers
carefully.Onlyonedealerintenqualifiesfora
Mclntoshfranchise.ToreceivetheSERVICE
CONTRACTyourpurchasemustbemadefroma
Mclntoshfranchiseddealer.
7.Yourcompletelyfilledinapplicationfora SERV-
ICECONTRACTmustbepostmarkedwithin30
daysofthedateofpurchaseoftheinstrument.
8.ToreceivetheSERVICECONTRACTallinforma-
tionontheapplicationmustbefilledin.The
SERVICECONTRACTwillbeissuedwhenthe
completelyfilledinapplicationIsreceivedat
MclntoshLaboratoryIncorporatedinBinghamton,
NewYork.Iftheapplicationisnotreceivedat
MclntoshLaboratory,onlytheserviceoffered
underthe90-dayguaranteewillapply.
Copyright© 1972ByMclntoshLaboratoryInc.
1

Adequateventilationextendsthetrouble-freelife
ofelectronicinstruments.Itisgenerallyfoundthat
each10°centigrade(18°F)riseintemperaturere-
ducesthelifeofelectricalinsulationbyonehalf.
Adequateventilationisaninexpensiveandeffective
meansofpreventinginsulationbreakdownthatre-
sultsfromunnecessarilyhighoperatingtempera-
tures.Thedirectbenefitofadequateventilationis
longer,trouble-freelife.
Allowatleast16inchesdeepby16incheswide
by6½incheshighformountingtheMAC1900.Al-
waysallowforairflowbyeitherventilationholes
orspacenexttothebottomofthereceiveranda
meansfora warmairtoescapeatthetop.Withade-
quateventilationtheMAC1900canbemountedin
anyposition.
Toprepareforinstallation,removetheplasticpro-
tectivecoveringfromtheMAC1900.Turnitupside
downsothatitrestsonitstopontheshippingpallet.
Removethefourplasticfeetfastenedtothebottom
ofthechassis.
Thedesignofthemountingtemplateallowsyou
topositionorlocatethecutoutfromthefrontorrear
ofthepaneltowhichtheinstrumentistobemounted.
Positiontheplasticmountingtemplateoverthearea
ofthepaneltobecutoutforinstallation. TheMAC
1900mustrestona shelfbehindthecabinetpanel.
Ifthecutoutistobelocatedfromthefrontofthe
panel,beginatstep2.Ifthecutoutistobelocated
fromtherearofthepanel,beginhere.
1.Onthebackofthecabinetpanel,scribea
verticalcenterlinethroughtheexactcenterofthe
areainwhichthecutoutistobemade.
Placethetemplateagainstthebackofthe
panelandmatchthetemplatecenterlinewiththe
centerlineonthecabinetpanel.
Makesurethatthebottomofthetemplate
restsontheshelfonwhichtheMAC1900willsit.
Markthetwolocatingholes("B"holesonthe
mountingtemplate).
Drillthetwo"B"locatingholes.Becertainthe
drillisperpendiculartothepanel.
Nowpositionthetemplateonthefrontofthe
panelbyaligningthe"C"locatingholesonthe
templatewiththedrillholes.
2.Ifthecutoutistobelocatedfromthefrontof
thepanel:
Withthetemplateinplaceagainstthecabinet
panel,markthefoursmallholes"A"thatidentify
thecornersofthecutout.Jointhecornermarks
witha pencil.Theedgeofthetemplatecanbe
usedasa straightedge.
WiththesawontheINSIDEOFTHEPENCIL
LINEScarefullycutouttherectangularopening.
Thebottomofthecabinetpanelcutoutshouldbe
evenwiththetopoftheshelf.
FoldthePAPERSHELFTEMPLATEalongthe
dottedline.ThePaperShelfTemplateisusedto
locatetheventilationcutoutandmountingholes
intheshelf.Putthedottedlineagainstthefront
ofthecabinetpanel.Alignthecenterlineofthe
PAPERSHELFTEMPLATEwiththecenterofthe
cabinetpanelopening.Marktheventilationcut-
outoutlineandcutitout.SecuretheMAC1900
totheshelfwithtwomachinescrews.
2

HowtoConnect
1.CONNECTINGTAPERECORDERS
ToRecord:
Connecta cablefromtheL TAPEOUT1 jack
tothelefthighlevelinputofa taperecorder.
Connecta cablefromtheR TAPEOUT1 jack
totherighthighlevelinputofthetaperecorder.
Connecta secondtaperecorderinthesame
waytotheL andR TAPE2 OUTputs.
ToPlayback/Monitor:
Connecta cablefromthe"left"channelout-
putofa taperecordertotheL TAPEMONitor1
input.
Connecta cablefromthe"right"channelout-
putofa taperecordertotheR TAPEMONitor1
input.
Connecta secondtaperecorderinthesame
waytotheL andR TAPEMONitor2 INput.
TAPERECORDER#1TAPERECORDER#2
3

2.CONNECTINGRECORDPLAYERSCONNECTINGA 75OHMANTENNA
TheMAC1900hasbeenshippedwithshorting
plugsinthephonoinputs.Removetheshorting
plugsonlyfromthejackthatwillbeused.
Connecta cablefromthe"left"channelofa
recordplayerintothe"L"PHONO1 inputjack.
Connecta cablefromthe"right"channelofa
recordplayerintothe"Ft"PHONO1 jack.
PHONO2 isprovidedfortheuseofa second
recordplayer.
Connecta secondrecordplayerinthesame
waytotheL andR PHONO2 INputjacks.
3.GROUNDCONNECTION:
Agroundpostisprovided.Groundsforturn-
tables,recordchangers,tapedecks,etc.should
beconnectedtothispost.Topreventhum,make
surethegroundwiredoesnotmakeanyconnec-
tionstotheshieldsoftheleftandrightprogram
cablesbetweentheprogramsourceandtheMAC
1900.Theleftandrightprogramcablesandthe
groundwirefromthatsourceshouldbewoundor
twistedtogether.
CONNECTINGANFMANTENNA
Oneofthreeantennasystemscanbeused:(1)an
outdoorFMantenna,or(2)a VHF-TVantenna,or(3)
theindoordipolesupplied.
Anoutdoorantennaisrecommendedforoptimum
performanceinallareas.Infringe(outlying)areas,
bestresultswillbeobtainedwitha highlydirectional
FMantennausedinconjunctionwitha rotator.Ro-
tatetheantennauntilthebestreceptionisobtained.
Iftheantennausesa 300ohmlead,connectittothe
300ANT(red)pushconnector.
AVHF-TVantennaisofteneffectivewhenitisde-
signedforbothFMandTVreception.Connectthe
twoleadsfromtheVHF-TVantennatothe300
ANT(red)pushconnector.
CONNECTINGANINDOORDIPOLEANTENNA
Theflexiblefoldeddipoleantenna(300ohm)is
foruseinurbanorhighstrengthsignalareas.
Connectthetwoleadsfromthedipoletothe300
ANT(red)pushconnector.Theflexibilityofthethin
flatwireassemblypermitsittobeplacedundera rug,
tackedbehindthestereo...orplacedinanyother
convenientlocation.Insomecases,itmaybeneces-
saryto"position"theantennaforbestsignalrecep-
tion.Thisshouldbedonebeforeitispermanently
located.
Avoidlocatingtheantennanexttootherwiresor
metalobjects.Thisantennamaynotproveeffective
inhouseshavingmetalsidingormetalfoilinsulation.
Anunbalanced75ohmantennacanbeconnected
totheMAC1900.A "typeF"connectorisusedto
connectthe75ohmcoaxialcabletothebackpanel
FMANT75input.
AMANTENNA
FormostlocalandmoderatelydistantAMrecep-
tionthebuilt-inferriteloopstickantennamaybe
used.TheAMloopstickantennaisona swivelbase.
Itmustberotatedforbestreception.
Betterlongdistantreceptionispossiblewiththe
useofa copperantennawire50to150feetinlength.
Suspendthewireina straightlineashighaspos-
sible.Attachthewireateachendwithsuitableglass
orceramicinsulators.Connecta lead-inwireatany
convenientpointontheantenna.Itisrecommended
thata lightningarresterbeusedwithanoutdoorAM
antenna.Thearrestershouldbewellgroundedtoa
suitablewaterpipeorcopperoraluminumrodsunk
intotheground.
Connectthelead-inwiretotheAMANT(white)
pushconnectorontheantennaterminalstriponthe
backpanel.
CONNECTINGA MAXIMUM
PERFORMANCEINDICATOR
TheMclntoshMAXIMUMPERFORMANCEIN-
DICATORisconnectedtotheTP1 andTP2 jackson
thebackpaneloftheMAC1900.Usetheprocedures
asdirectedintheMAXIMUMPERFORMANCEIN-
DICATORownersmanual.
FUSE
A3.2AMP "SLO-BLO"fuseprotectsthereceiver
circuits.Thisfusedoesnotprotectadditionalequip-
mentconnectedtothebackpanelACoutlets.
ACPOWEROUTLETS
Thereare2 blackandoneredACpoweroutlet.
Thepowertotheblackoutletsiscontrolledbythe
frontpanelACpowerswitchontheVOLUMEcontrol.
Usetheseoutletsfora poweramplifier,ortapere-
corder,etc.Theredreceptacleisonatalltimes.Use
theredoutletfora turntableorrecordchanger.The
turntableisprotectedbythisarrangement.Itisnec-
essarytoturnofftheturntableorrecordchanger
withitsownACpowerswitch.
4

Connecting- RecordPlayers
MaximumPerformanceIndicator
AntennaandGround
FMANTENNA
FMANTENNAINPUT
GROUNDTO
WATERPIPE
ORRODINEARTH
TO
RECORDCHANGER
ORTURNTABLE
MAXIMUM
PERFORMANCE
INDICATOR
TURNTABLERECORDCHANGER
OPTIONAL75
300
5

CONNECTINGLOUDSPEAKERS:
TheMAC1900PowerAmplifierisdesignedfor
stereoconnectiononly.DonotconnecttheMAC
1900formonophonic(oneloudspeaker)operation.
Damagetotheloudspeakerorthepoweramplifier
mayresult.
Bothmainandremotespeakersareconnectedto
thepushconnectorsonthebackpaneloftheMAC
1900.
Whentheoutputofthepoweramplifierandthe
speakershavebeenconnectedtotheproperpush
connectorsonthebackpanel,thefrontpanelpush-
buttonsturnthespeakersONorOFF.
ConnecttheleadfromtheleftMAINloudspeaker
totheLEFTMAINSPEAKERpushconnector.Con-
necttheleadfromtherightMAINloudspeakertothe
RIGHTMAINSPEAKERpushconnector.Uselamp
cord,bellwire,orwirewithsimilartypeofinsulation
toconnectthespeakerstotheamplifier.Forthenor-
mallyshortdistancesofunder50feetbetweenthe
amplifierandspeaker,#18wireorlargercanbe
used.Fordistancesover50feetbetweentheampli-
fierandspeakeruselargerwire.
CONNECTINGMcINTOSHLOUDSPEAKERS
WITHANENVIRONMENTALEQUALIZER
RemovethejumpercablesbetweenthePREAMP-
OUTjacksandAMPINPUTjack.Theenvironmental
equalizerisconnectedusingthesejacks.Connect
PREAMP-OUTtoequalizerin- Connectequalizerout
toAMPINPUT.Followthestandardloudspeakercon-
nectingprocedure.
Remote1 LeftRemote1 RightRemote2 LeftRemote2 Right
CONNECTINGLOUDSPEAKERS
WITHOUTENVIRONMENTALEQUALIZER
6

CONNECTING
MCINTOSH
LOUDSPEAKERS
WITHTHE
ENVIRONMENTAL
EQUALIZER
MQ101ENVIRONMENTALEQUALIZER
RIGHTSPEAKERLEFTSPEAKER
7
REMOTE1
LEFT
SPEAKER
REMOTE1
RIGHT
SPEAKER
REMOTE2
LEFT
SPEAKER
REMOTE2
RIGHT
SPEAKER

WhattheControlsDoandHowtoUseThem
VOLUMEON/OFF:
TheVOLUMEcontrolregulatestheloudnessin
bothchannels.TheVOLUMEcontrolhasbeenpre-
cisiontrackedthroughoutthelisteningrange{0to
—65dB)foraccuratestereobalance.Whenrotated
tothecounterclockwisepositiontheACpowertothe
MAC1900isturnedoff.
BALANCE:
TheBALANCEcontroladjustsforunequalloud-
nessineithertheleftorrightchannels.Theloudness
ofthechannelscanbevariedrelativetoeachother
withoutaffectingtheircombinedloudness.
Left. . . turningthecontroltotheleftaccents
theleftchannelbyreducingtherightchanneloutput.
Right. . . turningthecontroltotherightaccents
therightchannelbyreducingtheleftchanneloutput.
TONECONTROLS:
ThetonecontrolsontheMAC1900areslidecon-
trolswitha centerdetenttoindicatetheflatposi-
tions.Slidingtheknobtotheleftdecreasesresponse
andtotherightincreasesresponseinbothchannels.
TREBLE:
SlidingtheTREBLEcontrolleftdecreaseshigh
frequencyresponseupto16dBat20,000Hz.
Slidingthecontrolrightincreaseshighfrequency
responseupto16dBat20,000Hz.
BASS:
SlidingtheBASScontrolleftdecreaseslowfre-
quencyresponseupto16dBat20Hz.
Slidingthecontrolrightincreaseslowfrequency
responseupto16dBat20Hz.
HEADPHONES:
ThefrontpanelHEADPHONEjackhasbeende-
signedtofeedlowimpedancedynamicheadphones.
Electrostaticheadphonesgenerallyrequirehigher
powerthandynamicheadphones.Theymustbecon-
nectedtotheLEFTandRIGHTMAINSPEAKERpush
connectorsonthebackoftheMAC1900.
PlugheadphonesintothefrontpanelHEAD-
PHONEjack.AdjustthefrontpanelVOLUMEcontrol
forcomfortableheadphonelistening.
Theheadphoneoutputisnotaffectedbythe
SPEAKERswitches.
INPUTSELECTOR:
TAPE1:Connectstheoutputfroma taperecorder
thathasitsownplaybackelectronicstothehigh
levelamplifyingstageoftheMAC1900.Inthe
TAPE1 positiontheMAC1900hasflatampli-
fication.
TAPE2:Connectstheoutputfroma taperecorder
thathasitsownplaybackelectronicstothehigh
levelamplifyingstageoftheMAC1900.Inthe
TAPE2 positiontheMAC1900hasflatamplifi-
cation.
AM:ConnectstheAMtunerportionoftheMAC
1900totheamplifiersandloudspeakers.
FM:ConnectstheFMtunerportionoftheMAC
1900totheamplifiersandloudspeakers.
PHONO1:Connectstheoutputofanymagnetic
phonocartridgetothelowlevelamplifying
stages.Theresponsehasbeenshapedtocom-
pensateforRIAAcharacteristics.Theinputim-
pedanceis47,000ohms.
PHONO2:SameasPHONO1.
8

UsingtheTuningDial
TUNINGDIAL
TheMAC1900hasthreedialscales:
1.AM- Marked55to160kHz
2.FM-Marked88to108MHz
3.Loggingscale- Marked0 to100
Theloggingscalecanbeusedtoaccuratelyre-
tuneanystation.Youmayfinditeasiertokeepa
recordofyourfavoritestationsbyuseofthelogging
scale.
Asmallportionofdialpointerhasbeenillumi-
natedtoincreasetheeaseoftuning.
INDICATORS
TherearethreeindicatorsontheMAC1900dial
panel.Theyare:FMMPXindicator,SIGNAL
STRENGTHmeter,andtheTUNINGmeter.
STEREOINDICATOR(FMMPX)
TheFMMPXindicatorlightsredwhenthedial
pointeristunedtoorcrossesa FMstationbroadcast-
ingthe19,000Hzcarrierforstereo.Thespecialcir-
cuitusedwilllightonlywhenthe19,000Hzmultiplex
carrierispresentinthesignal.Theindicatorwillnot
lightonnoisepulsesorinterference.
FMTUNINGMETER
AnFMstationiscorrectlytunedwhenthemeter
needleisintheblackareaoftheFMTUNINGmeter.
SIGNALSTRENGTHMETER
WhentuningFMtheSIGNALSTRENGTHmeter
indicatesthestrengthofthesignalasreceivedfrom
theantenna.Thehighertheindication,thestronger
isthesignal.ForAMtuneforthemaximumindication
onthemeter.
UsingthePushbuttons
MODESELECTOR
Connectstheprogramtotheloudspeakersinthefol-
lowingsevenways:
STEREO:Connectsthe"left"inputtothe"left"
loudspeakerandthe"right"inputtothe"right"
loudspeaker.
S.REV:Connectsthe"left"inputtothe"right"
loudspeakerandthe"right"inputtothe"left"
loudspeaker.
MONO(L+ R):addsthe"left"inputandthe
"right"inputandthenconnectstheL + R pro-
gramtobothamplifiersandloudspeakers.
LtoL & R:Connectsthe"left"inputtobothloud-
speakers.
RtoL & R:Connectsthe"right"inputtoboth
loudspeakers.
L+ R toL:Connectsthe"leftplusright"pro-
gramtothe"left"loudspeakeronly.
L+ R toR:Connectsthe"leftplusright"pro-
gramtothe"right"loudspeakeronly.
LOUD:
TheLOUDNESScontrolautomaticallyprovidesthe
correctamountofbassrequiredtocompensatefor
thechangeinresponseofthehumanearatlow
loudnesslevels.
Whenthevolumeisreduced,themusicwillseem
tolosemuchofitsbassandsomeofitstreble.This
effectisduetothesensitivitycharacteristicofhuman
hearing.Theresponseofthehumaneartobassand
treblepitchdecreasesmorerapidlythanitsresponse
topitchcenteredinthemid-tonalrange.TheLOUD-
nessswitchconvertsthevolumecontroltoa loud-
nesscompensatedcontrol.UseLOUDness-INto
listenatlowvolumeandstillhearfull-frequency
range.
MONITOR
TheMONITORpushbuttonsmakeitpossibleto
instantaneouslycomparerecordedmaterialwiththe
sourcesignalfromeitheroftwotaperecordersused
withtheMAC1900.TherecordersusedwiththeMAC
1900shouldhaveseparaterecordandplayback
headsandseparaterecordandplayamplifiers.
WheneitherTAPEMONITORpushbuttonisinthe
MONITORpositiona rectangleislightedinthedial
area.Whenthelightisononlythetapecanbeheard.
Tolistentoanyotherprogramsourcethelightmust
beoff.
Withthebuttonpushedin,thesignalsourcebe-
comestheprogramasrecordedandisfedthrough
themainpreamplifiertothepoweramplifiersand
loudspeakers.
MONITORTAPE1:Connectstheoutputfroma tape
recorderthathasitsownplaybackelectronicstothe
highlevelinputstagesoftheMAC1900.
MONITORTAPE2:Connectstheoutputfroma tape
recorderthathasitsownplaybackelectronicstothe
highlevelinputstagesoftheMAC1900
9

TheTAPEMONITORswitchesaremechanically
interlockedtopreventsimultaneousmonitoringfrom
twotaperecorders.IfonebuttonisattheINposition,
itmustbepushedagaintoreleaseittotheOUTposi-
tionbeforetheotherbuttoncanbepushed.
IMPORTANT:WhentheMAC1900isoperated
witheitherMONITORpushbuttonintheINposition,
signalfromanyothersourcewillnotbeheardfrom
theloudspeakers.Tohearanyothersource,make
surethepushbuttonisintheOUTposition.
USINGONETAPERECORDER
Theoutputofa taperecordercanbeconnected
toeitherTAPEMONITOR1 orTAPEMONITOR2 in-
put.ThecorrespondingtapeoutputoftheMAC1900
shouldthenbeconnectedtotheinputofthetapere-
corder.Anysourcecanberecordedwithoutbeing
affectedbythetonecontrolorvolumecontrolset-
tings.Theplaybackofthetaperecordingcanbe
monitoredbypushingthecorrespondingtapemon-
itorpushbutton.
TWOTAPERECORDERS
TwotaperecorderscanbeusedwiththeMAC
1900.Recordingscanbemadefromrecorder1 tore-
corder2,orfromrecorder2 torecorder1.
Example:Connecttheoutputofrecorder1 to
TAPEMONITOR1 inputontheMAC1900.Connect
theTAPE1 OUTputontheMAC1900totheinputof
recorder1.Inthesameway,connecttheoutputof
recorder2 toTAPEMONITOR2 inputontheMAC
1900.ConnectTAPE2 OUTputoftheMAC1900to
theinputofrecorder2.
BysettingtheMAC1900INPUTselectorswitchat
TAPE1,a recordingcanbemadeontaperecorder2
froma tapeplayingontaperecorder1.Therecord-
ingcanbemonitoredbypushingtheMONITOR
TAPE2 button.
Thetaperecorderfunctionscanbereversedby
settingtheINPUTselectorswitchatTAPE2.A re-
cordingcanthenbemadeontaperecorder1 froma
tapeplayingontaperecorder2.Therecordingon
taperecorder1 canbemonitoredbypushingthe
MONITORTAPE1 button.
TheMAC1900canalsobeusedwithonerecorder
forrecordingotherprogramsourceswhileplaying
tapesfroma secondrecorder.
backbypushingtheMONITORTAPE2 but-
ton.Atthesametime,theMAC1900canbe
usedtoplaya tapefromtaperecorder1 by
releasingthemonitorbuttonforTAPE2 and
pushingthemonitorbuttonforTAPE1.The
signaloftaperecorder1 willthengotothe
loudspeakerswithoutaffectingtherecording
beingmadeontaperecorder2.
Taperecordingscanbemadesimultane-
ouslyontwotaperecordersbyusingPHONO
1,PHONO2,FM,orAMasa programsource.
Example:
SettheINPUTselectorswitchtothede-
siredsource.Therecordingoneithertapere-
cordercanbemonitoredforplaybackby
pushingtheappropriatetapemonitorbutton.
CAUTION:Whenrecordingwithtwotapere-
cordersatthesametimefromthesamepro-
gramsource,mutualinterferenceofthere-
corderbiasoscillatorscanresult.Thiscan
beheardasa howlorsquealintheback-
groundwhentherecordingsareplayedback.
Thisnoiseiscausedbyinsufficientfilteringof
thebiasoscillatorcircuitsinthetapere-
corders.A testrunshouldbemadeforthe
particularrecordersintendedforthisuse.
FILTERS
LF (LOWFREQUENCY)
UsetheL F filterswitchtoreduceobjection-
ablelow-frequencynoisecreatedbya turntable
orrecordchangeroracousticallycoupledfeed-
back.
OUT. . . filterdisconnected.
IN...low-frequencysignalsbelow50Hzare
reducedwhentheswitchispushedtotheIN
position.
HF (HIGHFREQUENCY)
UsetheH F filterswitchtoreduceobjection-
ablehigh-frequencynoisesuchasrecordscratch.
OUT. . . filterdisconnected.
IN...rollsoffresponsesharplyat7000Hz.
MUTING
Themutingcircuitsuppressesallnoisebetween
FMstations.Itsuppressesallweakerstationsnot
strongenoughtooverridethebackgroundnoise.
Themutingthresholdsettingdeterminesthe
strengthofthesignalwhichcanbeheardwithmuting
inoperation.Themutingthresholdiscarefullyad-
10
Example:
ArecordingfromFM,AM,PHONO1 or
PHONO2 canbemadeontaperecorder2 if
theINPUTselectorswitchissettothecor-
respondingsourceposition.Therecordingon
taperecorder1 canbemonitoredforplay-

justedtooptimumatthefactoryusingprecisiontest
instruments.
OUT. . . MUTINGdisconnected.
IN...betweenstationnoisesuppressedonFM.
SPEAKER
Whenthespeakershavebeenconnectedtothe
properpushconnectorsonthebackpanel,thepush-
buttonsturnthespeakersONorOFF.
Iftheprogramistobeheardfromthemainspeak-
ersonly,theREMOTEpushbuttonsarepushedIN.
Thisturnsofftheremoteloudspeakers.
iftheprogramistobeheardfromtheremote
speakersonlytheMAINpushbuttonispushedIN.
Thisturnsoffthemainspeakers.
Tohearprogramfrombothmainandremote
speakers,boththeMAINandREMOTEpushbuttons
mustbeintheOUTposition.
MAIN
PUSHBUTTONOUT...theprogrammate-
rialisheardfromtheMAINspeakers.
IN...theMAINloudspeakersareturned
OFF.
REMOTE1 or2
PUSHBUTTONOUT...theprogrammate-
rialisheardfromtheREMOTEloudspeakers.
IN...theREMOTEloudspeakersareturned
OFF.(Thesepushbuttonsdonotaffectthe
headphonejack.)
BalancingyourStereo
Theperformanceandenjoymentofa stereosys-
temisgreatlyincreasedwhenthesoundisproperly
balanced.Thebalanceofthestereosystemisaf-
fectedbymanythingsincludingroomacoustics,
furnitureplacement,roomshape,smalldifferencesin
loudspeakers,etc.Factorsthateffectproperstereo
balancearecorrectphaseforbothchannelsand
equal programloudness.
TOADJUSTPHASE
1.PresstheMODEpushbuttontotheMONOposi-
tion.
2.TurntheBALANCEcontroltothe12o'clock
position.
3.Standabout10feetinfrontofandmidwaybe-
tweentheloudspeakers.Thesoundshouldap-
peartocomefromdirectlyinfrontofyou.Ifthe
soundisnotdirectlyinfrontofyou,reversethe
leadsononeoftheloudspeakersonly.When
thesoundcomesfromthemidpointbetweenthe
speakerstheyareinPHASE.
TOBALANCELOUDNESS
1.PresstheMODEpushbuttontoMONO.
2.Playa familiarrecording.
3.TurntheBALANCEcontroltothe12o'clock
position.
Whiletheprogramisplaying,standbetween
thetwoloudspeakers.Listenfora differencein
loudnessbetweenspeakers.Ifthereisthena dif-
ferenceinloudnessturntheBALANCEcontrol
towardthespeakerthatisnotasloud.Adjustthe
BALANCEcontroluntilthesoundisbalancedbe-
tweenbothspeakers.
ListeningtoYourStereo
LISTENINGTOA STEREORECORD
TurntheINPUTSELECTORtoPHONO1 or
PHONO2,whicheverisconnectedtotherecord
playeryouwishtohear.
MakecertaintheMODEpushbuttonisinthe
STEREOposition.
AdjusttheVOLUMEcontroltodesiredvolume.
LISTENINGTOA MONOPHONICRECORD
TurntheINPUTSELECTORto PHONO1 or
PHONO2,whicheverisconnectedtotherecord
playeryouwishtohear.
PushtheMODEpushbuttonINtoMONO.
AdjusttheVOLUMEcontroltodesiredvolume.
LISTENINGTOA STEREOTAPERECORDER
TurntheINPUTSELECTORtoTAPE1 orTAPE
2,whicheverisconnectedtotherecordplayeryou
wishtohear.
SettheMODEpushbuttontoSTEREOorMONO,
dependingontheprogramonthetape.
AdjusttheVOLUMEcontroltodesiredvolume.
TwotaperecorderscanbeusedwiththeMAC
1900.Recordingandmonitoringcanbedonewith
bothtaperecorders.
Tomonitorwhilerecordingyourtaperecorder
musthaveseparaterecordandplaybackheadsand
electronicsforeach.TheTAPEMONitorpushbutton
letsyoumonitorthequalityoftaperecordingsmade
duringtherecordingprocess.WhentheTAPEMON-
itorpushbuttonisintheINpositionitwillplaythe
soundfromthetapeasitpassestheplaybackhead,
amomentafteritisrecorded.Therecordingprocess
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continuesasusual.WhentheswitchisintheOUT
positionnormalprogramfromthesourceisheard.
HOWTOCOPYTAPE
1.Putthetapetobecopiedontherecordercon-
nectedtoTAPE1 input.
2.TurntheinputselectortoTAPE1.
3.ThesignalavailableattheTAPEOUTPUT
jacksistheplaybackofTAPE1.
4.RecordontherecorderconnectedtoTAPE
2.Therecordingcanbemonitoredbypressing
intheTAPE2 pushbutton.Instantaneous
comparisonoftherecordedprogramwiththe
originalcanbeheard.
LISTENINGTOAM
TurntheINPUTSELECTORtoAM.
Rotatethetuningknobofthestationofyour
choice.
Adjustthevolumetoa comfortablelevel.
LISTENINGTOFMorFMSTEREO
TurntheINPUTSELECTORtoFM.
PushtheSTEREOMODEselectorpushbuttonin.
Rotatethetuningknobtothestationofyour
choice.
Adjustthevolumetoa comfortablelevel.
TheMAC1900usesa Mclntoshdevelopedauto-
maticmono-stereoswitchingcircuit.Theswitching
iselectronicwithoutswitchingclicksortransients.
Thecircuitswitchessmoothlyandsilentlywhen
the19,000Hzmultiplexpilotsignalispresent.When
theFMMPXindicatorislit,thestationisbroadcast-
inga 19,000Hzpilotcarrier.Thissignalcausesthe
automaticcircuittoswitchtheMAC1900tostereo.If
astationisnotbroadcastinga 19,000Hzpilotsignal
forstereo,theFMMPXindicatorwillremainoffand
thetunerwillautomaticallyswitchtomono.
Themutingcircuitsuppressesallnoisebetween
stations.Itsuppressesallweakerstationsnotstrong
enoughtooverridethebackgroundnoise.Themuting
thresholdsettingdeterminesthestrengthofthesig-
nalwhichcanbeheardwithmutinginoperation.The
mutingthresholdiscarefullyadjustedtooptimumat
thefactory.
Whiletuningyoumaynoticethatthetuningin-
dicatorwillshowa stationyetnoprogramisheard
fromthespeakers.Themutingcircuitinthetunerhas
rejectedthestationbecausethereisobjectionable
noisewiththeweaksignalfromthestation.Pushthe
MUTINGpushbuttontotheOUTpositionandthesta-
tionwillbeheard.Mostprogramsthatcanbetuned
inthismannerareofpoorqualityduetointerfering
noise.
PerformanceLimits
PREAMPLIFIERANDPOWERAMPLIFIERSECTION
POWEROUTPUT:55RMSwattscontinuousper
channelinto4 or8 ohmsbothchannelsoperating
20Hzto20,000Hz.
30RMSwattscontinuousperchannelinto16
ohmsbothchannelsoperating20Hzto20,000Hz.
HARMONICDISTORTION:Doesnotexceed0.20%
atratedpoweroutputfrom20Hzto20,000Hzwith
bothchannelsoperating.Typicalperformanceisless
than0.1%atratedpower.Distortiondecreasesas
outputpowerisreduced.
INTERMODULATIONDISTORTION:Doesnotexceed
0.20%ifinstantaneouspeakpoweroutputistwice
ratedpowerorlessperchannelwithbothchannels
operatingforanycombinationoffrequencies20Hz
to20,000Hz.
DAMPINGFACTOR:50across8 ohmload
FREQUENCYRESPONSE:±0.5dB20Hzthrough
20,000Hz
INPUTSENSITIVITYANDIMPEDANCE:
PowerAmplifier:2.5volts,100kohms
Phono1 andPhono2:2.0mV,47kohms
Tape1 andTape2:250mV,250kohms
TOTALNOISE:
PowerAmplifier:95dBbelowratedoutput
TapeInput:90dBbelowratedoutput
PhonoInput:76dBbelow10mVinput
TAPEOUTPUT:
Tuner:1.0volt
Tape:250mVwithratedinputfromlowlevelin-
puts
Phono:1.2voltswith10mVinputat1,000Hz
PreampOutput:2.5voltswithratedinput
BASSCONTROLS:±16dBat20Hz
TREBLECONTROLS:±16dBat20,000Hz
L.F.FILTER:Activefilter,12dBperoctaverolloff
below50Hz,down18dBat20Hz
H.F.FILTER:Activefilter,12dBperoctaverolloff
above7,000Hz,down18dBat20,000Hz
AMTUNERSECTION
SENSITIVITY:75µVIHF(externalant.)
SIGNALTONOISERATIO:45dBminimumIHF;55
dBat100%modulation
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HARMONICDISTORTION:Doesnotexceed1%at
30%modulation
FREQUENCYRESPONSE:3500Hzat—6dBdown
ADJACENTCHANNELSELECTIVITY:30dBmini-
mumIHF
IMAGEREJECTION:65dBminimum,540kHz-1600
kHzFMTUNERSECTION
USEABLESENSITIVITY:2.5microvoltsat100%
modulation(±75kHzdeviation)for3%totalnoise
andharmonicdistortion
SIGNALTONOISERATIO:70dBbelow100%modu-
lation
HARMONICDISTORTION:Mono:Doesnotexceed
0.3%at100%modulation±75kHzdeviation
Stereo:Willnotexceed0.7%
AUDIOFREQUENCYRESPONSE:±1dB20Hzto
15,000Hzwithstandardde-emphasis(75µsec.)and
19,000Hzpilotfilter
CAPTURERATIO:1.8dB
SELECTIVITY:55dBalternatechannelselectivity
IHMminimum
SPURIOUSREJECTION:90dBIHFminimum
IMAGEREJECTION:80dB;88to108kHz(IHF)
STEREOSEPARATION:34dBat1,000Hz
SCAFILTER:50dBrejectionfrom67kHzto74kHz.
275dBperoctaveslope
POWERREQUIREMENTS:120volts,50/60Hz,40
wattsatzerosignaloutput,300wattsatratedoutput
FACILITIESANDFEATURES
BASS:Slidecontrolwithmechanicaldetentforflat
-16dBto+16dBat20Hz
TREBLE:Slidecontrolwithmechanicaldetentfor
flat-16dBto+16dBat20,000Hz
LOUDNESS:Pushbutton...forloudnesscompen-
satedorflatresponse
BALANCE:Naturalbalanceatcenterposition,attenu-
ationofleftorrightchannelbyrotatingcontrol.
VOLUME:Precision"tracked"atalllisteninglevels.
(0to-65dB.)Doesnotchangestereobalanceas
loudnessischanged.TheACpowerON/OFFswitch
iscoupledwiththiscontrol.
INPUT:Sixpositions- TAPE1,TAPE2,AM,FM,
PHONO1,andPHONO2.
MODE:Pushbutton- Leftchannelonlytobothspeak-
ers.Rightchannelonlytobothspeakers,StereoRe-
verse,Stereo,Mono,L+R,L+Rtorightspeakeronly,
andL+Rtoleftspeakeronly.
TAPEMONITOR:Twopushbuttonswitches.Eitherof
twotaperecorderscanbemonitoredbyselecting
theTAPEMON.1 pushbuttonorTAPEMON.2 push-
button.Theyaremechanicallyinterlockedtoaccept
onlyonepushbuttonattheINpositionatonetime.
SPEAKER:Main-SwitchtheMAINloudspeakersys-
temONorOFFwithoutaffectingtheperformanceof
REMOTEspeakers.
Remote1 - SwitchoneREMOTEloudspeaker
systemONorOFFwithoutaffectingtheperformance
ofMAINspeakers.
Remote2 - Switcha secondREMOTEloud-
speakersystemONorOFFwithoutaffectingtheper-
formanceofMAINspeakers.
HEADPHONEJACK:Forlisteningwithlowimpedance
dynamicstereoheadphones.
TRANSISTORCOMPLEMENT:53siliconfieldeffect
orbipolartransistors,39diodes,3 integratedcir-
cuits,4 thyristors.
MECHANICALINFORMATION
SIZE:Frontpanelmeasures16incheswide(40.64
cm)by5½incheshigh(13.97cm).Chassismeasures
15incheswide(38.1cm)by51/8inchhigh(13.02cm)
by15inchesdeep(38.1cm)includingbackpanel
connectors.Knobclearancerequiredis1½inches
(3.81cm)infrontofmountingpanel.
FINISH:Frontpanelisanodizedgoldandblack.
WEIGHT:33pounds(14.97kg)net,46pounds(20.87
kg)inshippingcarton.
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TechnicalDescription
AUDIOSECTION
EachchanneloftheMAC1900hasfourbasic
sections.Theyare:phonoorlowlevelamplifier,tape
orhighlevelamplifier,filteramplifier,andpower
amplifier.
PHONOAMPLIFIER
Therearethreetransistorsineachchannelofthe
phonoamplifier.Theinputtransistorsarecarefully
selectedforlownoiseandhighgain.Highgaintran-
sistorsareusedintheotherstagesofthephono
amplifiertoachievehighoverallopen-loopgain.With
highopen-loopgaina largeramountofnegative
feedbackcanbeusedaroundthephonoamplifier
whichreducesnoiseanddistortion.Thefeedback
networkalsoprovidesprecisionRIAAfrequencycom-
pensationrequiredformagneticphonographcart-
ridges.
Feedbackremainsineffectevenat20Hzwhere
highergainisrequiredforRIAAcompensation.The
outputofthephonoamplifierprovidesa lowim-
pedanceoutputforthetapeoutputjackandthein-
putofthehighlevelpreamplifier.
Inputsensitivityofthephonoamplifieris2.0milli-
volts.Thegainoftheamplifieris42dBat1000Hz.
Thephonoamplifierhasverywidedynamicrange.
At 1000Hz thephonoinputcircuitwillaccept100
millivoltswithoutoverload,a voltagefargreaterthan
theoutputofanycurrentmagneticphonocartridge.
Phonoinputoverloadisvirtuallyimpossible.
Asignallevelof10millivoltsatthephonoinput
at1000Hzwillproduce1.2voltsatthetapeoutput.
Thetapeoutputhasa sourceimpedanceof200
ohms.Formostefficientsignaltransferthetapeout-
putshouldoperateintoa loadimpedanceof47k
ohmsorgreater.
HIGHLEVEL
Attheinputtothehighlevelamplifierthesignal
passesthroughthemodeselectorswitchthroughthe
balancecontrolandvolumecontrolintotheamplifier.
Loudnesstapsareprovidedonthevolumecontrol.
Withtheloudnessswitchinthecircuit,thelowfre-
quencyoutput(below400Hz)isincreasedwhenthe
volumecontrolisat50%ofrotationorbelow.
Adifferentialamplifierconfigurationisusedatthe
inputofthetapeamplifier.Thetonecontrolsareina
negativefeedbacklooparoundtheamplifierstages
togiveexcellentlownoiseandlowdistortionchar-
acteristics.Withthetonecontrolsinthefeedback
aroundthetapeamplifier,thenegativefeedbackre-
mainsineffectevenatthemaximumboostsettingof
eitherbassortreblecontrols.
Theslidetonecontrolsandtheirassociated
componentshavebeenmountedona doublesided,
platedthrough,printedcircuitboard.Thiscomplete
tonecontrolmoduleismountedtothefrontpanelfor
easeinoperationandmaintenance.A mechanicalde-
tenthasbeenprovidedattheelectricalandmechan-
icalcenteroftheslidingbassandtreblecontrols.
Thetonecontrolsprovidesmoothgradualboostor
cutofoutputlevelsgivingminimumchangeatmid
frequenciesandmaximumchangeattheaudiospec-
trumextremes20Hzand20,000Hz.Withthetone
controlsinthecenterpositionthegainofthetape
amplifiersis20dB.Theoutputofthetapeamplifier
feedsdirectlytothefilteramplifier.
FILTERAMPLIFIER
Eachchannelofthefilteramplifierhastwotran-
sistorsconnectedincompoundemitterfollowercon-
figuration.Tomaintainlowdistortionandnoiseand
presenta uniforminputimpedancetheresistiveand
reactiveelementswhichmakeupthefiltersareina
feedbackpath.TheLowFrequency(rumble)filter
startstorolloffat50Hzata rateof12dBperoctave,
reducingtheoutputat20Hzby18dB.
TheHighFrequency(scratch)filterstartstoroll
offat7,000Hzata rateof12dBperoctave,reducing
theoutputat20,000Hzby18dB.Filtercomponents
havebeenmathematicallyselectedinvaluetogive
thedesiredfrequencyrolloffcharacteristics.Close
tolerancecomponentsusedassureuniformityinfilter
response.Thefilteramplifieroutputisanemitterfol-
lowerwitha sourceimpedanceof200ohms.The
outputisfedtojacksontherearattheMAC1900.In
thenormaloperation,bothfiltersout,thefilteram-
plifierhasa gainof0 dB.
POWERAMPLIFIER
Theinputimpedanceofthepoweramplifieris
100,000ohmsandrequires2.5V RMStodrivethe
amplifiertoratedoutput.Theshortjumpercablesin
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therearoftheMAC1900connectthefilteramplifier
output(PREAMPout)andpoweramplifierinput
(AMPinput).
Attheinputofthepoweramplifier,twotransistors
areconnectedasa differentialamplifier.Thetwoin-
putsignalstothedifferentialamplifieraretheinput
signal(outputofthefilteramplifier),andthenegative
feedbacksignalfromtheoutputofthepoweram-
plifier.Theuseofa differentialamplifierpermitsthe
bestuseoflargeramountsnegativefeedbackto
maintainlownoiseanddistortion.Thecombinedout-
putofthedifferentialamplifierfeedsa linearvoltage
amplifierwhichinturndrivestwomediumpower
drivertransistors.Thedrivertransistorsdrivetheout-
putstages.
Theoutputsectionisarrangedasa directcoupled
seriespush-pullamplifier.Thepowertransistorsused
intheoutputcircuitareselectedfortheirhighpower
dissipationcapability,widefrequencyresponseand
large"safeoperatingarea."
Thepowertransistorsaremountedonlargeblack
anodizedheatsinkstoassurethatundernormalop-
erationthetransistorswilloperateata lowtempera-
ture.Ifoperatingtemperaturesshouldincreasedue
toa shortedspeaker,orrestrictedventilation,an
automaticsensingdeviceturnstheACpoweroffto
theMAC1900.TheMAC1900willturnonagainwhen
thetemperaturehasreturnedtonormallimits.This
additionalprotectionassuresyouofthecomplete
reliabilityoftheMAC1900evenunderthemostex-
tremeoperatingconditions.
Tofurtherinsurereliabilitya specialpowerout-
putSENTRYMONITORINGCIRCUITpreventsfailure
ofthepoweroutputtransistorsduetoexcessivemis-
matchorshortingoftheoutput.WhentheMAC1900
operatesnormallytheSENTRYMONITORINGCIR-
CUIThasnoeffectonsignalspassingthroughthe
poweramplifier.Ifthepowerdissipationintheout-
puttransistorsshouldriseabovenormaldesignlim-
its,theSENTRYMONITORINGCIRCUITrestrictsthe
drivetotheoutputstagewhichreducesthedissipa-
tionintheoutputtransistors.TheSENTRYMON-
ITORINGCIRCUITactsinstantaneouslyforanyinput
signalorloadcombination.Thisarrangementas-
surescircuitreliability.OnlyMclntoshgivesyouthis
degreeofprotection.
Thedirectcoupledseriespush-pullamplifiercir-
cuitplacestheoutputatgroundpotentialwhenthere
isnosignalinputwhicheliminatestheuseofa
couplingcapacitorintheoutputcircuit.Theavailable
lowfrequencypoweroutputisrestrictedinothercir-
cuitsthatrequirea couplingcapacitorintheoutput.
TheMAC1900PowerAmplifierisalldirectcoupled
toinsuremaximumlowfrequencyperformance.In
directcoupledcircuitsfailureofanytransistorinthe
poweramplifiercouldcausea DCpotentialtoap-
pearintheoutput.Toassurethatnodamagingor
interferingDCappearsacrosstheoutputterminals,
aprotectorcircuit(Patented)hasbeendesignedinto
theMAC1900.Thiscircuitconstantlymonitorsthe
outputcircuitandifatanytimea constantDClevel
appears,thecircuit"crowbars"a shortacrossthe
outputtoground.Theprotectivecircuitisveryfast(it
reactsinmilliseconds}andtheshortwillremain
acrosstheoutputuntilthecausehasbeenfixed.This
extraprotectionpreventsdamagetoyourspeakers.
Undernormaloperatingconditionstheprotective
circuithasnoeffectontheoperationoftheoutput
circuit.Thisisanotherexampleofthecontinuous
protectionMclntoshprovides.
POWERSUPPLIES
Twohighcurrentpowersupplies,a positive40
voltandnegative40voltDC,areusedtodrivethe
outputpoweramplifier.Verylargecapacitors(9300
microfarads)areusedtostorea largeamount-of
energytoprovidegoodfiltering,andexcellentvolt-
ageregulation.Goodlowfrequencyresponseand
negligiblelowfrequencydistortioninthepoweram-
plifierstagesdependsontheregulationofthepower
supply.
Asecondpowersupplyprovidesthepowerfor
thepoweramplifierdrivers.Itisa highvoltagefull
waverectifiedandfilteredsupply.
Twoadditionalelectronicallyregulatedpower
suppliesareusedinyourMAC1900.A regulated16
voltpowersupplyfurnishesvoltagetotheAMand
FMcircuits.A regulated75voltpowersupplywith
anaccessoryfilterchainprovides75VDCand20VDC
tothephonoandtapeamplifiercircuits.Bothelec-
tronicallyregulatedpowersuppliesarelowimped-
anceseriesregulatedtransistorcircuitsusingtight
tolerancezenerdiodesasreferences.Humandripple
reductionhasbeena majordesignconsiderationon
theMAC1900powersupplies.
FMSECTION
TheRadioFrequency(RF)Sectionhousesthe
completeFM-RFfrontendandpartoftheAM-RF
circuitry.A sevensectionvariablecapacitoristhe
heartoftheRFsection.Foursectionsofthevari-
ablecapacitorareusedforFMandtheremaining
threeareintheAMsection.Byinterleavingthesec-
tions(AM-FM-AM-FM,etc.),spuriousresponseshave
beensignificantlyreduced.Theuseofthelatest
"stateoftheart"infieldeffecttransistorsanda well
designedvariabletuningcapacitorhasprovided
ahighdegreeofRFselectivityandexcellentspuri-
ousresponserejection.Imagerejectionandspurious
responseshasbeenbroughttoa minimuminthe
RFsectionoftheMAC1900.
Adualinsulatedgatemetaloxidesiliconfieldef-
fecttransistor(MOS-FET)isusedasthefirstRFam-
18