McIntosh C27 User manual

THEMcINTOSHC27SOLIDSTATESTEREOPREAMPLIFIER
ReadingTime:28MinutesPrice:$2-00

WARNING:TOPREVENTFIREORSHOCK
HAZARD,DONOTEXPOSETHISUNITTO
RAINORMOISTURE.
TheMclntoshyouhavepurchasedhasa serialnumberlocatedon
therearpanelofthechassis.
Recordthatserialnumberhere
forfuturereferences.Themodelandserialnumberareimportant
toyouforanyfutureservice.

YourC 27StereoPreamplifierwillgiveyou
manyyearsofpleasantandsatisfactory
performance.Ifyouhaveanyquestions,
pleasecontact:
CUSTOMERSERVICE
MclntoshLaboratoryInc.
2ChambersStreet
Binghamton,NewYork13903
Phone:[phone]
TakeAdvantageof3 years
ofFREEService...
FillintheApplicationNOW.
SERVICECONTRACT. . . . 1
INTRODUCTION. . . . 2
HOWTOINSTALL....2
HOWTOCONNECT....4
WHATTHEFRONTPANELCONTROLS
ANDHOWTOUSETHEM....8
BALANCINGYOURSTEREO....11
PERFORMANCELIMITSANDRATINGS. . . . 12
PERFORMANCECHARTS. . . . 13
TECHNICALDESCRIPTION....14
BLOCKDIAGRAM....15
1.Mclntoshwillprovideailparts,materialsand
laborneededtoreturnthemeasuredperfor-
manceoftheinstrumenttotheoriginalper-
formancelimitsfreeofanycharge.The
SERVICECONTRACTdoesnotcoverany
shippingcoststoandfromtheauthorized
serviceagencyorthefactory.
2.AnyMclntoshauthorizedserviceagencywill
repairallMclntoshinstrumentsatnormal
servicerates.Toreceivethefreeservice
underthetermsoftheSERVICECONTRACT,
theSERVICECONTRACTCERTIFICATEmust
accompanytheinstrumentwhentakento
theserviceagency.
3.Alwayshaveservicedonebya Mclntosh
authorizedserviceagency.Iftheinstrument
ismodifiedordamaged,asa resultofun-
authorizedrepairtheSERVICECONTRACT
willbecancelled.Damagebyimproperuseor
THREEYEARSERVICECONTRACT
Anapplicationfora FREETHREEYEARSERVICECONTRACTisincludedwiththismanual.
Thetermsofthecontractare:mishandlingisnotcoveredbytheSERVICE
CONTRACT.
4.TheSERVICECONTRACTisissuedtoyouas
theoriginalpurchaser.Toprotectyoufrom
misrepresentationthiscontractcannotbe
transferredtoa secondowner.
5.ForyourprotectionMclntoshselectsonly
dealerswhohavetechnicalcompetenceto
guidepurchasersfairly,andprovideservice
whennecessary.ToreceivetheSERVICE
CONTRACTyourpurchasemustbemade
froma Mclntoshfranchiseddealer.
6.Yourcompletelyfilledinapplicationfora
SERVICECONTRACTmustbepostmarked
within30daysofthedateofpurchaseofthe
instrument.
7.ToreceivetheSERVICECONTRACTallin-
formationontheapplicationmustbefilled
in.TheSERVICECONTRACTwillbeissued
whenthecompletelyfilledinapplicationis
receivedatMclntoshLaboratoryIncorporated
inBinghamton,NewYork.
Copyright© 1977byMclntoshLaboratoryInc.
Contents

HowtoConnect
CONNECTINGA TURNTABLETOPHONO1
Connectthecablefromthe"left"channeloftheturn-
tableintotheL PHONO1 INPUTjack.
Connectthecablefromthe"right"channeloftheturn-
tableintotheR PHONO1 INPUTjack.
PHONO2 isprovidedfortheuseofa secondturntable.
Connectthecablefromthe"left"channeloftheturn-
tableintotheL PHONO2 INPUTjack.
Connectthecablefromthe"right"channeloftheturn-
tableintotheR PHONO2 INPUTjack.
PlugtheACpowercablefromtheturntableintothered
unswitchedPOWERoutletontherearpanel.Thered
receptacleisonatalltimes.Itisnecessarytoturnoffthe
turntablewithitsownACpowerswitch.Thisarrangement
protectstheturntable.
ThefourblackACpoweroutletsarecontrolledbythe
frontpanelPOWERswitch.Usetheseoutletsfora tuner,
taperecorder,etc.
GROUNDCONNECTION
Asinglegroundpostisprovided.Groundsforturntables,
recordchangers,tapedecks,etc.aretobeconnectedto
thispost.Theleftandrightprogramcablesandtheground
wirefromthatsourceshouldbewoundortwistedtogether.
Toavoidhummakesurethegroundwiredoesnotmake
anyconnectionstotheshieldsoftheleftandrightprogram
cablesbetweentheprogramsourceandtheC 27.
CONNECTINGA STEREOTUNER
Connectthecablefromthe"left"channeltuneroutput
totheL TUNERINPUTjack.
Connectthecablefromthe"right"channeltuneroutput
totheR TUNERINPUTjack.
AUX—Anyhighlevelprogramsourcesuchasa tuner,
aTVsetora taperecordercanbeconnectedtotheAUX1
and2 INPUTjacks.Theconnectingprocedureisthesame
asforthetunerinput.
CONNECTINGTAPERECORDERS
ToRecord:
Connecta cablefromtheL TAPE1 OUTPUTjacktothe
lefthighlevelinputofthetaperecorder.
Connecta cablefromtheR TAPE1 OUTPUTjacktothe
righthighlevelinputofthetaperecorder.
Connecta secondrecorderinthesamemannertothe
TAPE2 OUTPUTjacks.
ToPlayback/Monitor:
Connecta cablefromtheleftchanneloutputofa tape
recordertotheL TAPE1 INPUTjack.
Connecta cablefromtherightchanneloutputofa tape
recordertotheR TAPE1 INPUTjack.
Connecta secondrecorderinthesamemannertothe
TAPE2 INPUTjacks.
4

ACPOWERTO
TUNERS,TAPE
RECORDERSAND
POWERAMPLIFIERS
CONNECTIONSFORTUNERS,TURNTABLES,ANDTAPERECORDERS
TAPE
RECORDER
2
TAPE
RECORDER
1
ACPOWER
TOTURNTABLES
5
TURNTABLE#1
FMANTENNA
TUNERTURNTABLE#2

SPEAKERSYSTEM#1
CONNECTIONSFORPOWERAMPLIFIERANDLOUDSPEAKERS
SPEAKERSYSTEM#2
6
LOUDSPEAKER
ENVIRONMENTAL
EQUALIZER
POWERAMPLIFIER

CONNECTINGTHEC 27TOPOWERAMPLIFIERS
ConnecttheMAINOUTPUTjackstotheinputofa
stereopoweramplifier.TheLeftMAINjackisconnected
totheleftamplifierinputjack.TheRightMAINjackis
connectedtotherightamplifierinputjack.
AsecondpairofMAINOUTPUTjackshavebeensupplied
tofeeda secondpoweramplifier.Intheeventitisdesired
totaperecordafterthetonefilterandvolumecontrols
thesecondsetofMAINOUTPUTjackscanbeconnected
tothetaperecorderinput.Thecontrolsarethenallavail-
abletoalterthesignalbeforerecording.Remember,any
changeinthesettingofthecontrolswillchangethepro-
gramtothetaperecorder.Sincetherecordingsignal
availableattheMAINOUTPUTismuchhigherthanthe
outputavailablefromtheTAPE1 orTAPE2 OUTPUT
jacks,besurenottooverloadthetaperecorderinput.It
willnotbepossibletomonitorfromthetaperecordervia
theTAPEMONITORpushbuttonswhilefeedingtherecorder
fromtheMAINOUTPUTjacks.DepressingtheTAPE
MONITORpushbuttonwilldisconnecttheprogramsource
andmaycausefeedbackoscillation.
CablesaresuppliedwiththeC 27tointerconnectto
thepoweramplifier.Longercablescanbeusedifneeded.
Thelengthofthecableislimitedbythecapacityofthe
cable.Thetotalcapacitymustnotexceed1,000pF.For
instance:cableswitha capacityof25pFperfootmaybe
40feetlong.13.5pFperfootcablemaybe75feetlong.
Theinputimpedanceoftheamplifiersshouldbe22k
ohmsorgreater.
CTRCHAN
UsetheCenTeRCHANneloutputtofeedleftplusright
signaltoa separatepoweramplifierformonophonicback-
groundmusicorfora centerchannel.
CONNECTINGFORSPEAKERSWITCHINGAND
HEADPHONE
Whenthepoweramplifieroutputisproperlyconnected
totherearpanelFROMAMPLIFIERpushconnectors,
speakersintwoseparateareascanbeturnedonoroff
andtheC 27frontpanelHEADPHONEjackwillreceive
theprogramsignal.
Connecta wirefromtheproperimpedanceoutput
connectiononthepoweramplifiertotheFROMAMPLIFIER
pushconnectorsontheC 27rearpanel.Connectthefirst
pairofspeakerstotherearpanelSPEAKER1 pushcon-
nectorsanda secondpairofspeakerstotheSPEAKER2
pushconnectors.Whenconnectingthespeakersandpower
amplifieritisimportanttoconnectthepoweramplifier
outputimpedancetapfortheparallelimpedancecom-
binationofthetwospeakersystemsused.Thefrontpanel
SPEAKERswitchpermitsroutingprogramtoeither
SPEAKER1,SPEAKER2,both1 and2,orOFF.Inthe
SPEAKEROFFposition,theprogramisfedonlytothe
HEADPHONEjackforprivatelistening.
ACOUTLETS
Thereare4 blackACoutletsandoneredACoutlet.The
powertotheblackACoutletsiscontrolledbythefront
panelACpowerswitch.Usetheseoutletsfora tuner,tape
recorder,etc.ThetotalcapacityofallACpoweroutletsis
600watts.Theredreceptacleisonatalltimestofeed
ACpowertoa turntablesothatitcanbeturnedonoroff
withitsownACswitch.
FUSE
A0.5AMPfuseprotectsthepreamplifiercircuits.The
fusedoesnotprotectadditionalequipmentconnectedto
thebackpanelACoutlets.
7

WhattheFrontPanelcontrolsDo
andHowtouseThem
INPUTSELECTOR
TheINPUTSELECTORisa fivepositionswitchthat
connectsthechoseninputprogramtothepropercircuits
intheC 27.
AUX1:Connectstothehighlevelinput,anyoutput
froma highlevelprogramsourcerequiringflatamplifica-
tion.Sucha sourcecouldbea televisionset,playbackfrom
anothertaperecorder,anadditionaltuner,etc.Inthe
AUX1 positionthegainis20dBtotheMAINoutputs,
0dBtotheTAPEoutputs.Theinputimpedanceis100k
ohms.
AUX2:SameasAUX1 butfora secondhighlevel
programsource.
TUNER:ConnectstheoutputfromanyAM,FMorMPX
FMtunertothehighlevelinputstage.IntheTUNERposi-
tiontheC 27hasflatamplification.Thereis20dBof
gaintotheMAINoutputs,0 dBtotheTAPEoutputs.The
inputimpedanceis100kohms.
PHONO1:Connectstheoutputofanymagneticphono
cartridgetothelowlevelinputstage.Theresponsehas
beenshapedaccordingtotheRIAAstandardtocompensate
forthecharacteristicsofa magneticphonocartridge.The
gainat1KHzis62dBtotheMAINoutputs,42dBtothe
TAPEoutputs.Theinputimpedanceis47kohmsand
100pF.
PHONO2:SameasPHONO1 butfora secondphono
system.
MODESELECTOR
LtoL & R:Connectstheleftinputtobothloudspeak-
ers.R toL & R:Connectstherightinputtobothloud-
speakers.StereoRev:Connectstheleftinputtotheright
loudspeakerandtherightinputtotheleftloudspeaker.
Stereo:Connectstheleftinputtotheleftloudspeaker
andtherightinputtotherightloudspeaker.
Mono(L+ R):Addstheleftinputandtherightinput
andthenconnectstheL + R programtobothloud-
speakers.
L+ R toL:Addstheleftandrightprogramtogether
andconnectsthecombinedprogramtotheleftloud-
speakeronly.
L+ R toR:Addstheleftandrightprogramtogether
andconnectsthecombinedprogramtotherightloud-
speakeronly.
PRECISETRACKINGVOLUMECONTROL
TheVOLUMEcontrolisa precisionstepattenuator
whichhaslefttorighttrackingaccuracywithin1 dB
throughoutitsentirerange.Suchextremelyaccuratematch-
ingisachievedthroughelectronicallycontrolledtrimming
oftheresistancematerialdepositedonpairsofprinted
circuits.Sincetheswitchcommutatortouchesonlycontact
padsandnottheactualresistanceelement,trackingac-
curacyisnotdegradedwithuseasinordinaryvolume
controls.Clockwiserotationincreasesvolume.TheVOLUME
controlintheC 27hasbeendesignedandmanufactured
forMclntoshLaboratory.
FILTERS/SPEAKERS
ThesmallcenterknoboftheconcentricFILTER/
SPEAKERScontrolshapestheC 27frequencyresponseto
reduceorfilterobjectionablelowfrequencynoiseand/or
objectionablehighfrequencynoise.Thelargeouterknob
8

willturnmainorremoteloudspeakersonoroffwhena
poweramplifierhasbeenproperlyconnectedtotheC 27
rearpanel.
FILTEROPERATION
OUT:Filterdisconnected,programnotmodifiedby
eitherhighorlowfrequencyfilters.
LF(LOWFREQUENCYFILTER)
UsetheLFpositiontoreduceobjectionablelow-
frequencynoisecreatedbya turntableorrecordchanger
oracousticallycoupledfeedback.Responseisrolledoff
below50Hz.
HF(HIGHFREQUENCYFILTER)
UsetheHFpositiontoreduceobjectionablehigh-
frequencynoisesuchasrecordscratch.Responseis
rolledoffabove5,000Hz.
LF-HF
Bothhighandlowfrequencyfiltersareactivated.
SPEAKERSOPERATION
OUT:Allspeakersaredisconnected.Programisfedonly
totheHEADPHONEjackforprivatelistening.
1:ProgramisfedonlytotherearpanelSPEAKER1 con-
nectorsandtheHEADPHONEjack.SPEAKER2 is
disconnected.
1& 2:ProgramisfedtotherearpanelSPEAKER1 and
2connectorsandtheHEADPHONEjack.
2:Programmaterialisfedonlytotherearpanel
SPEAKER2 connectorsandtheHEADPHONEJACK.
SPEAKER1 isdisconnected.
BASS
Concentric,11position,tonecontrolswitchesadjust
thebass.Theouterknobadjuststherightchannelbass
response.Thecenterknobadjuststheleftchannelbass
response.
Left:Adjuststhebassloudnessfromtheleftloudspeaker.
Clockwiserotationincreasesthebassloudnesswhilecoun-
terclockwiserotationdecreasesthebassloudness.Each
stepofthetonecontroladjuststhebassloudnessabout4
dB.
Right:Hasthesameeffectonthesoundfromtheright
loudspeaker.
TREBLE
Concentric,11position,tonecontrolswitchesadjust
thetreble.Theouterknobadjuststherightchannelre-
sponse.Thecenterknobadjuststheleftchanneltreble
response.
Left:Adjuststhetrebleloudnessfromtheleftloud-
speaker.Clockwiserotationincreasesthetrebleloudness
whilecounterclockwiserotationdecreasesthetrebleloud-
ness.Eachstepofthetonecontroladjuststhetreble
loudnessabout4 dB.
Right:Hasthesameeffectonthesoundfromtheright
loudspeaker.
LOUD/BALANCE
LOUDNESS:
Usetheloudnesscontrolforfullfrequencyrangelisten-
ingateventhesoftestlisteninglevels.TheLOUDness
control(thesmallcenterknoboftheconcentriccontrol)
provideslow-frequencyboosttocompensatefora hearing
characteristicofthehumanearatlowlisteninglevels.
Theearbecomeslesssensitivetolowfrequenciesasvolume
isreduced.Increasedloudnesscompensationistherefore
desirableatlowerlisteninglevels.Theloudnesscontrol
booststhelowfrequenciessothattheearhearsapproxi-
matelythesamemusicalbalanceasitwouldathigher
listeninglevels.
FLAT:(Maximumcounterclockwiserotation)Fullvolume
withflatfrequencyresponse.Theloudnesscompensation
isinoperativeinthisposition.
TurningthecontrolclockwisetowardMAXreducesthe
listeningvolume,andautomaticallyincreasesthecompen-
sationbyboostingbass.
BALANCE
TheBALANCEcontroladjustsforunequalvolumein
eithertheleftorrightchannels.Thevolumeofthechan-
nelscanbevariedrelativetoeachotherwithoutaffecting
theircombinedvolume.
Turningthecontroltotheleftaccentstheleftchannel
9

byreducingtherightchanneloutputandturningthecon-
troltotherightaccentstherightchannelbyreducingthe
leftchanneloutput.
PANLOC
DepressingthetwoPANLOCbuttonsonthefrontpanel
permitstheC 27tobeeitherlockedfirmlyinplaceor
unlockedsothatthechassiscanbeslidforward,untilit
isstoppedbythemid-positionlatches.Pressthelatches
onthesideoftheC 27inwardtopermittheinstrumentto
slideinoroutofthecabinet.WiththeC 27frontpanel
flushwiththecabinetpanel,pressingthePANLOCbuttons
willlocktheunitfirmlyinthecabinet.
THEPUSHBUTTONS
TheC 27isdesignedtobeusedwithoneortwotape
recorders.Thefrontpanelpushbuttonspermitnormal
playbackofeitherrecorder,monitorasrecordingsare
beingmadeoneitherrecorder,orcopyingtapesfromone
recordertoanotherwhilelisteningtoa separateprogram.
TAPEMONITOR
Tomonitorwhilerecordingfromanysourcechosenon
theINPUTSELECTOR,yourtaperecordermusthavesepa-
raterecordandplaybackheadsandseparateelectronics.
TheMONITORswitchesletyoumonitorthequalityoftape
recordingsmadeduringtherecordingprocess.Whenthe
chosenTAPEMONITORpushbuttonisintheINposition
thesoundfromthetapeasitpassestheplaybackheadis
hearda momentafteritisrecorded.Therecordingprocess
continuesasusual.WhentheswitchisintheOUTposition,
theprogramthatisbeingrecordedisheard.
TAPE1 MONITOR
PUSHBUTTONOUT...Theprogramsourceselectionby
theINPUTSELECTORisfedtothepoweramplifiersand
heardthroughtheloudspeakers.
PUSHBUTTONIN...Theprogramsourcebecomesthe
recordedtapeontherecorderconnectedtoTAPE1 INPUT.
Therecordedprogramfromtaperecorder1 isfedtothe
poweramplifiersandheardfromtheloudspeakers.
InthePUSHBUTTONINposition,a rectangleislighted
abovethepushbutton.Whenthelightisononlythetape
willbeheard.Tolistentoothersourcesthepushbutton
mustbeoutandthelightoff.
TAPE2 MONITOR
Asecondtaperecordercanbeoperatedinthesame
mannerusingpushbuttonTAPE2 MONITOR.
UsethefrontpanelpushbuttonTAPE2 toplaybackfrom
ataperecorderpluggedintotherearpanelTAPE2 INPUT
jacks.
HOWTOCOPYTAPE,TAPECOPY1 ->2
Putthetapetobecopiedontherecorderconnectedto
TAPE1 INPUT.PressintheTAPECOPY1 ->2 pushbutton.
ThesignalavailableattheTAPE2 OUTPUTjacksisthe
playbackofTAPE1.Recordontherecorderconnectedto
TAPE2 OUTPUT.Thesourcetapeontaperecorder1 can
beheardbypressingintheTAPE1 MONITORpushbutton.
TherecordingcanbemonitoredbypressingintheTAPE2
MONITORpushbutton.Instantaneouscomparisononthe
recordedprogramwiththeoriginalcanbeheard.
TAPECOPY2 ->1
WhenpushedINtheTAPE2 OUTPUTjacksarecon-
nectedtotheTAPE1 INPUTjackswithoutaffectingthe
programheardfromthespeakers.Inthispositiona copy
oftheprogramontaperecorder2 canbemadeontape
recorder1.
TomonitorthetapecopyproceduresusetheTAPE
MONITORpushbuttons.
POWER
ThePOWERswitchenergizestheC 27circuitsandcon-
nectsACpowertotheblackrearpanelACpoweroutlets.
TheblackACpoweroutletsarenotfused.A lightedrec-
tangleabovetheONPOWERswitchindicatestheAC
powertothesystemisturnedon.
HEADPHONES
ThefrontpanelHEADPHONEjackhasbeendesignedto
feedlowimpedancedynamicheadphones.Electrostatic
headphonesgenerallyrequirehigherpowerthandynamic
headphones.Theymustbeconnectedtotheoutputcon-
nectorsonthepoweramplifier.
PlugheadphonesintothefrontpanelHEADPHONE
jack.AdjusttheVOLUMEcontrolforcomfortablehead-
phonelistening.Theoutputofanexternalpoweramplifier
mustbeproperlyconnectedtotheC 27backpanelfor
programmaterialtobeavailableattheHEADPHONEjack.
10

BalancingyourStereo
Theperformanceandenjoymentofa stereosystemis
greatlyincreasedwhenthesoundisproperlybalanced.
Thebalanceofthestereosystemisaffectedbymany
thingsincludingroomacoustics,furnitureplacement,room
shape,smalldifferencesinloudspeakersetc.
TOBALANCELOUDNESS
1.SettheMODEswitchtoMONO(L+ R]
2.Playa familiarrecording.
3.TurntheBALANCEcontroltothe10o'clockposition.
Whiletheprogramisplaying,standbetweenthetwo
loudspeakers.Listenfora differenceinloudnessbetween
speakers.Ifthereisa differenceinloudnessturnthe
BALANCEcontroltoadjustforequalloudness.
Clockwiserotationbalancesforlackofloudnessonthe
right—conterclockwisebalancesforlackofloudnesson
theleft,Next,settheMODEselectortoSTEREO.Ifthere
isthena differenceinloudnessturntheBALANCEcontrol
towardthespeakerthatisnotasloud.Adjustuntilthe
soundisbalancedfrombothspeakers.
11

PerformanceLimitsandRatings
PERFORMANCELIMITSarethemaximumdeviationfrom
perfectionpermittedfora Mclntoshinstrument.Wepromise
youthatyourC 27mustbecapableofperformanceator
exceedingtheselimitsoryougetyourmoneyback.Mc-
lntoshistheonlymanufacturerthatmakesthisguarantee.
FREQUENCYRESPONSE
+0 -0.5 dB 20 Hz to 20k Hz
DISTORTION
Willnotexceed0.05%atratedoutputlevel,20Hzto20k
Hz
INPUTSENSITIVITYANDIMPEDANCE
AUXILIARYi and2,TUNER,TAPE1 and2:250millivolts
at100kohms.PHONO1 and2:2 millivoltsat47kohms
and100pF.
HUMANDNOISE
AUXILIARY1 and2,TUNER,TAPE1 and2:85dBun-
weighted;90dBIHFA weighted.
PHONO1 and2:80dBbelow10mVinput,unweighted;
85dBIHFA weighted.
OUTPUTLEVELANDIMPEDANCE
MAINOUTPUT2.5voltswithratedinput,lessthan1k
ohmssourceimpedance,tooperateinto22kohmsload
orhigher.Maximumoutputisgreaterthan10volts
TAPEOUTPUT0.25voltswithratedinput,lessthan1.5k
ohmssourceimpedance,tooperateinto22kohmsload
orhigher.Maximumoutputisgreaterthan10volts
CENTERCHANNELOUTPUT(L+ R)2.5voltswithrated
inputtobothchannels,lessthan1.2kohmssourceim-
pedance,tooperateinto22kohmsloadorhigher.
VOLTAGEAMPLIFICATIONINDECIBELS
AUXILIARY1 and2,TUNER,TAPE1 and2
toMAINOUTPUT20dB
toTAPEOUTPUT0 dB
PHONO1 and2
toMAINOUTPUT62dB
toTAPEOUTPUT42dB
SEMICONDUCTORCOMPLEMENT
18Silicon—Planartransistors
4SiliconDiodes
5LightEmittingdiodes
ACPOWEROUTLETS
1unswitched(Red)
4switched
POWERREQUIREMENT
120volts,50/60Hz,15watts
MECHANICALINFORMATION
SIZE:FrontPanelmeasures16incheswide(40.6cm)by5
7/16incheshigh(13.8cm).Chassismeasures143/4in-
cheswide(37.5cm)by4 13/16incheshigh(12.2cm)by13
inchesdeep(30.0cm),includingPANLOCshelfandback
panelconnectors.Knobclearancerequiredis1 1/4inches
(3.2cm)infrontofthemountingpanel.
FINISH:Frontpanelisanodizedgoldandblackwith
specialMclntoshgold/tealpanelnomenclatureillumina-
tion.Chassisisblack.
MOUNTING:ExclusiveMclntoshdevelopedprofessional
PANLOC.
WEIGHT:20pounds(9.1kg)net,32pounds(14.5kg)in
shippingcarton.
FACILITIESANDFEATURES
BASS
Separate11positionrotaryswitchesforeachchannel.
-17dBto+16dBat20Hz
TREBLE
Separate11positionrotaryswitchesforeachchannel
-20dBto+20dBat20kHz
LOUDNESS
Flatresponse,orcontinuouslyvariableloudnessequaliza-
tion.
BALANCE
Naturalbalanceatcenterposition,attenuationofleftor
rightchannelbyrotatingcontrol
VOLUME
Precisionstepattenuatorwithmatchedelementsforproper
trackingatalllisteninglevels.Doesnotchangestereo
balanceasvolumeischanged.
INPUT
Fivepositions—AUXILIARY1 and2,TUNER,PHONO1
and2
MODE
Sevenpositions—Leftchannelonlytobothspeakers.Right
channelonlytobothspeakers,STEREOREVerse,STEREO,
MONO(L+ R),L + R toLeftspeakeronly,andL + R to
Rightspeakeronly
TAPEMONITOR
Twopushbuttonswitches.Eitheroftwotaperecorderscan
bemonitored.Theyaremechanicallyinterlockedtoaccept
onlyonepushbuttonattheINpositionatonetime
TAPECOPYSWITCH
Twopushbuttonswitches.Eitheroftwotaperecorderscan
beconnectedtocopyfromonetotheotherorviceversa.
Theyaremechanicallyinterlockedtoacceptonlyone
pushbuttonattheINpositionatonetime.
LFFILTER(RumbleFilter)
Flatorroll-off12dBperoctavebelow50Hz,down12dBat
20Hz
HFFILTER(ScratchFilter)
Flatorroll-off12dBperoctaveabove5kHz.down12dBat
20k Hz
SPEAKERSWITCH
Facilitiesareprovidedtoconnectthepoweramplifierto
therearpaneloftheC 27topermitswitchingtwopairs
ofspeakersonandoff.
HEADPHONEJACK
Forlowimpedanceheadphones;poweredbyproperlyin-
terconnectedexternalpoweramplifier.
12

13

TechnicalDescription
TheaccompanyingBLOCKDIAGRAMindicatesthe
varioussignalpathswithintheC 27.Circuitsrepresented
byeachblockarediscussedbelow.
INPUTSELECTORSWITCH
TheINPUTSELECTORroutesthevariousinputsignals
withintheC 27.Theswitchisrobustinconstructionand
haslargedoublewipingsilvercontacts.Isolatingnetworks
ateachhighlevelinputandshortingswitchactionground
allunusedinputspreventingundesiredprogramfeed-
through.ThesameisolatingnetworksblockanyDCthat
mightbepresentona highlevelinputtoeliminatepops
orclickswhenchanginginputs.
PHONOPREAMPLIFIER
Eachchannelofthephonopreamplifierusesthree
discretetransistorsespeciallyselected,andcircuitop-
timized,toproducelownoiseanddistortionfreeoperation.
Ofparticularbenefitistheuseoflowimpedancecom-
ponentsintheRIAAfeedbackequalizationnetworks.The
networkdriveisprovidedbythefinalstagetransistor
whichoperatesatanadequatecurrentleveltodrivethe
lowimpedancewithoutslewratelimiting.
MONITORANDTAPECOPYSWITCHES
Tapeswitchingusespushbuttonswitches.Theswitches
aremounteddirectlytoa printedcircuitboardthatin-
corporatesthenecessaryinter-connectionwiring.Isolating
networksareincludedateachtapeinputandoutput
whicheliminatesthepossibilityofDCleakagefromasso-
ciatedtaperecorderswhichpreventpopsorclickswhen
switching.Thepushbuttonswitcheshavereliablepush-
pushactionratchetstopreventswitchhangup.Light
emittingdiode(LED)lampsareactivatedbytheswitches
toindicatetheparticularswitchfunctionbeingused.
MODESWITCH
Aseven-positionMODESELECTORswitchallowsrouting
oftheleftandrightchannelsignalsinvariouscombina-
tionsforsystembalancingandoperation.TheMODESE-
LECTORfunctionsonlyinthesignalpathdirectedthrough
theMAINOUTPUTjacksanddoesnotcontroltheTAPE
OUTPUT.
VOLUMECONTROL
TheVOLUMEcontrolisa precisionstepattenuator
designedandmanufacturedforMclntoshLaboratory.Itis
a32-stepcontrolwitha 70dBrange,plusvolumeoff.
Leftandrightchanneltrackingarewithin1 dB.Thisex-
tremeaccuracyisobtainedthroughspecialelectronically
controlledresistanceelementtrimming.Sincetheswitch
commutatortouchesonlycontactpadsandnotthepreci-
sionresistorelements,trackingaccuracyispermanently
maintainedandisnoisefree.
VOLTAGEAMPLIFIER
Avoltageamplifierwith14dBgainfollowstheVOLUME
control.Thisamplifierplusthefollowingamplifierwith
6dBgamconstitutethemainamplifiersectionoftheC 27.
Theamplifierhashighinputimpedance,lowoutputim-
pedance,verylownoiselevelandwidedynamicrange.
HIGHFREQUENCYANDLOWFREQUENCYFILTERS
TheHFandLFFILTERnetworksareswitchcontrolled.
Thehigh-frequencyfilternetworkreducestrebleresponse
above5,000Hz.Thelow-frequencyfilterreducesbass
responsebelow50Hz.Theslopeofthefilterfrequency
responseisselectedformaximumrejectionofobjectionable
noises.Carefuldesignhaskeptthelossofusableprogram
materialtoa minimum.
LOUDNESSCONTROL
TheLOUDnesscontroliscontinuouslyvariable.Full
volumeandflatfrequencyresponseisobtainedinthe
counterclockwiseorFLATpositionofthiscontrol.Asthe
controlisadvancedclockwisethevolumelevelreduces
butloudnesscompensationisintroducedwhichmaintains
thelow-frequencyoutputata higherlevelthanthemid
andhighfrequencyoutput.Thisactioncomplementsthe
humanearresponsecharacteristics,theFletcherMunson
effect.Loudnesscompensationisprovidedfora widerange
oflisteninglevels.
BALANCECONTROL
RotatingtheBALANCEcontroltotheleftorrightallows
youtobalancethelevelofthetwochannels.Atthede-
tentedcenterpositionthegainofthetwochannelsisequal.
BASSANDTREBLECONTROLSANDASSOCIATED
AMPLIFIER
Ahighgainthreestagelinearamplifierisusedinthe
bassandtreblecontrolcircuits.Negativefeedbackreduces
thegamto6 dBwhenthecontrolsaresetflat.Thefeed-
backpathincorporatesthecontrolresponseshapingnet-
14

15

works.SeparateBASSandTREBLEcontrolsroutethe
feedbacksignaltoshapethefrequencyresponsebyvarying
theamountoffeedbackusedatdifferentfrequencies.
Accuratecontrolisaccomplishedbyusingprecisioncapa-
citorsandresistors.Theoutputofthisamplifierisfedto
theMAINOUTPUTjacks.
CENTERCHANNELAMPLIFIER
TheCTRCHAN(centerchannel)amplifierisanemitter
followerarrangedforveryhighinputimpedance.Unity
gainisprovidedattheCTRCHANnelOUTPUTjackwhen
boththeLEFTandRIGHTchannelshaveequaloutput
signals.
POWERSWITCHANDPOWEROUTLETS
ThepushbuttonPOWERswitchcontrolsACpowerto
theC 27andthefourblackswitchedoutlets.Powerto
theredunswitchedoutletispresentallthetimeandis
notcontrolledbythepowerswitch.Theredoutletisin-
tendedtopowerturntables.Theswitchedoutletsareused
topowertuners,poweramplifiers,andsimilarcomponents.
POWERTRANSFORMERANDPOWERSUPPLY
TheC 27powertransformerusesa grainorientedsilicon
steelcorepluscopperandmagneticsteelshieldbandsto
eliminateanyradiatedmagnetichumfield.Theextracare
inshieldingassuresthatthepowertransformerwillnot
radiateanexternalmagneticfieldthatcouldintroduce
humintotheamplifiercircuitsoranadjacentphonopickup
cartridgeortaperecorderplaybackhead.
Thepowersupplyuseszenerdiodereferencevoltage
regulationandelectronicfiltering.Normalpowerline
variationsdonotaffecttheperformanceoftheC 27.The
turnonandofftimecharacteristicsofthepowersupply
arecontrolledtopreventturnonandofftransients.
SPEAKERSWITCHANDHEADPHONEJACK
TheSPEAKERswitchisarrangedtocontroltwosetsof
loudspeakerspoweredbyanexternalpoweramplifier.
Whenconnectingthespeakersandpoweramplifieritis
importanttoconnectthepoweramplifieroutputimpe-
dancetaptotheparallelimpedanceofthetwospeaker
systemsused.Theheadphonejackisalsofedfromthe
externalpoweramplifier.Thepoweramplifierisrequired
tomaketheHEADPHONEjackoperate.
16

MclNTOSHLABORATORYINC.
2CHAMBERSST.BINGHAMTON,N.Y.13903
607-723-3512
Thecontinuousimprovementofitsproductsisthepolicyof
MclntoshLaboratoryIncorporated,whoreservetherightto
improvedesignwithoutnotice.
PrintedinU.SA.
339091