McIntosh MC275 User manual

STEREOPOWER
AMPLIFIER
MC275
TABLEOFCONTENTS
GENERALDESCRIPTION1
TECHNICALDESCRIPTION1
ElectricalSpecifications3
MechanicalSpecifications3
PANELFACILITIES4
Input4
Output4
LineOutput4
Fuse4
ACOutlet4
LineVoltage5
INSTALLATION5
CONNECTIONS5
Input—Stereo5
Input—TwinAmp5
Input—Mono5
Speakers—StereoorTwinAmp5
Speakers—Mono6
LineOutput—StereoorTwinAmp6
LineOutput—Mono6
ACPower7
OPERATION7
ADJUSTMENTS7
LineVoltage7
InputSwitch7
GUARANTEE8
FREE3-YEARFACTORYSERVICE
CONTRACTAPPLICATIONcoverpocket
SCHEMATICDIAGRAMcoverpocket
PARTSLISTcoverpocket
RESISTANCEANDVOLTAGECHARTcoverpocket

MC275 STEREO POWERAMPLIFIER
GENERALDESCRIPTION
Overonemillionwattsofamplifieroutput
powercapacityhavebeenmanufacturedby
Mclntoshsince1947.Inthis1,000,000watts
ofaudiopowertherearelessthan10wattsof
distortioncapacityandlessthan1/100watt
ofnoisecapacity!
Thedramaticdifferenceinthequalityof
musicreproductionwhenyoulistenthrough
Mclntoshinstrumentsisdirectlyduetolow
distortionperformance.Careful,devoted
researchisa wayoflifeatMclntosh.The
world'sfinestamplifieristhecreationofper-
severing,resourcefulMclntoshengineers.
MclntoshLaboratoryistheonlymanufac-
turerintheentireindustrytoguaranteethe
lowestdistortionatallaudiofrequencies,at
fullpower.TheU.S.PatentOfficehasrec-
ognizedtheadvancedtechnologyofthe
Mclntoshcircuitbygranting6 patents.1
Longlife,flexibility,highestqualitycon-
structionarecharacteristicdesignsinevery
Mclntoshinstrument.Wideelectricaland
thermalmarginsofsafetyforallcomponents
andtubes,advancedengineering,andcool
operatingdesignaddtothelonglifebuilt
intoeveryMclntoshproduct.Reliabilitypro-
longsyourinvestmentwithoutexpensive
maintenancecosts.
TheMC275hasononechassistwo75
wattpoweramplifiers.Inadditiontouseasa
stereoamplifier,theflexibilityoftheMC275
permitsittobeusedasa monophonic
amplifierthatdelivers150watts,orastwo
separate75wattamplifierswitheachchan-
nelamplifyingcompletelyseparatepro-
grams,orastwoamplifiersforusewithan
electroniccrossovernetwork.Suchflexi-
bilitypermitsmaximumuseforgreatest
returnfromyourinvestment.
TECHNICALDESCRIPTION
ThepatentedMclntoshUnityCoupled
circuitandoutputtransformerhaveestab-
lishedMclntoshamplifiersastheunchal-
lengedleadersintheaudiofield.
Before1947,lowdistortionathighpower
andhighefficiencywasimpossible.A com-
pletelynewengineeringapproachresultedin
anamplifierthatforthefirsttimepermitted
highpowerwithdistortionbelow1%.That
newengineeringproducedtheMclntosh
UnitycoupledcircuitandtheMclntoshbifilar
woundoutputtransformer.Withtheintro-
ductionoftheMclntoshamplifiernewstand-
ardsfordistortion-freeperformancewere
established.
PRIMARYNO.I
turnofprimarynumberoneisnexttothe
sameturnofprimarynumbertwo.Thereis
almostcompletemagneticcouplingbetween
thetwowires.Themagneticcouplingisrein-
forcedbythecapacitancebetweenthe
twowires.
INPUT
OUTPUT
PRIMARYNO.2
Fig.1—Representationofbifilarwinding.
TheMclntoshoutputtransformeris
unique.Ithastwoprimarywindingswhichare
woundbifilarly.Inthebifilartechniqueboth
primarywiresarewoundsidebyside.EachFig.2—RepresentationofMclntoshoutputcircuit.
1U.S.PatentNo's:2,477,074;2,545,788;2,646,467;2,654,058;2,860,192;2,929,028.
1

IntheMclntoshUnityCoupledcircuitone
ofthebifilarprimarywindingsisconnected
throughthepowersupplytotheplateand
cathodeofoneoftheoutputtubes.Theother
bifilarprimarywindingissimilarlyconnected
totheothertube.
Alllowdistortionhighpoweramplifiers
usepush-pulloutputcircuitsknownasClass
AB1,AB2orB.Twotubesarearrangedina
balancedcircuit.Thispermitseachtubeto
operatealternatelysomewhatoverhalfthe
time.Comparedtofulltimeoperationofthe
tubes,thepush-pullmethodreducesheating
andpermitsmorepowerfroma giventypeof
tube.Despitethisadvantageoftheconven-
tionalpush-pullcircuitoneprobleminpar-
ticularremainedtobesolved.Whencurrent
ineachtubeiscutofftobegintheidleperiod
distortionisproducedattheinstantofcut
offandagainattheinstantwhencurrent
flows.Thisformofdistortionisknownas
NotchDistortionandwaswellillustratedby
Mr.PenTungSahinthe"Proceedingsofthe
I.R.E."Volume24,pp1522-1541in1936.
Imperfectcouplingbetweentheprimary
windingsfoundinallconventionaloutput
transformersproducestheconditionwhich
permitsnotchdistortion.Tryingtoimprove
couplingina conventionaltransformerde-
creasesthepowerresponseatbothlowand
highfrequencies,heatingtheoutputtubes
andloweringtheavailablepoweroutput.
TheMclntoshUnityCoupledoutputcir-
cuitandbifilartransformeristhefirstcom-
mercialbreakthroughthateliminatesnotch
distortionbycouplingbothoutputtubes
almosttoperfection.IntheMclntoshtrans-
formertheextremelyclosecouplingofthe
bifilarwindingsremovestheconditionwhich
permitsnotchdistortion.Furthermorethe
thetwooutputtubesarearrangedaspartial
cathodefollowers.Halfoftheoutputcircuit
isinthecathodeandhalfintheplateofeach
tube.Theoutputtubesnowareoperatingin
alocalfeedbackloopwhich,reducestheir
distortion,reducestheirinternalgenerator
resistance,andreducestheirbalance
requirements.TheMclntoshcircuitin
realityperfectspush-pullhighefficiency
outputcircuits.
TheMC275usesanadvanceddesignof
theMclntoshoutputtransformer.Thenew
transformerdesignincorporatesallofthe
benefitsoftheoriginalMclntoshdesignand
evenfurtherimprovesthepowerbandwidth.
TheMC275outputtransformerhasthree
primarywindings,woundtrifilarly.Thenew
thirdwindingisconnectedbetweenthe
platesofthecathodefollowerdriver.
Leakageinductance(lackofcoupling)
betweentheprimaryandsecondarywind-
ingsof theoutputtransformerlimitsthehigh
frequencyresponseofanamplifier.The
primaryandsecondarywindingsofthe
Mclntoshoutputtransformerareinterleaved
fivetimestoimprovecoupling.Theinter-
leavingisaccomplishedbywindinggroupsof
primarylayers,thensecondarylayersalter-
natelyuntilthetotaltransformeriswound.
InterleavinghelpstoextendtheMclntosh
powerbandwidthtoover100,000cycles.
Since1947thisnewtechnologyhasbeen
builtintoeveryMclntoshpoweramplifier.
Goodvoltageregulationinthepower
supplypermitsoverloadswithoutovershoot
orblocking,goodtransientresponse,and
completestability.Toimproveregulationa
siliconrectifierpowersupplyisusedinthe
MC275.Inadditiontobettervoltageregula-
tion,thesiliconrectifierallowsevenhigher
operatingefficiency,cooleroperation,and
longeramplifierlife."
Togreatlyextendtubeandcomponent
lifea thermistorintheMC275limitscurrent
surgesproducedwhentheequipmentis
turnedon.Thethermistorisa specialtypeof
resistor.Itsresistancedependsonitstem-
perature.Whentheamplifierisoffthetherm-
istorhasa highresistancevalue(about79
ohms).Justaftertheamplifieristurnedon
thecurrentwhichflowsthroughthetherm-
istorheatsitandcausesitsresistanceto
decreasetoa lowvalue(lessthan.7ohms).
CurrentisthuslimitedwhentheMC275is
firstturnedonbutisnotlimitedastheunit
warms.
2

ELECTRICALSPECIFICATIONS
PowerOutput:
Stereoortwinamplifier—75wattscon-
tinuousperchannel
Monophonic—150wattscontinuous
HarmonicDistortion:
Lessthan0.5%atratedoutputorless,
20cyclesthrough20,000cycles.Typical
performanceis0.3%orlessat20cycles
and20,000cyclesatfullpower.Typical
performanceinthemidfrequenciesis
0.1%orlessatfullpower.
IntermodulationDistortion:
Lessthan0.5%foranycombinationof
frequenciesfrom20cyclesthrough
20,000cyclesifinstantaneouspeak
powerisbelowtwicetheratedpower.
PhaseShift:
Lessthan±8°20cyclesthrough20,000
cycles
FrequencyRange:
Atratedoutput:
+0,-0.1db16cyclesthrough40,000
cycles
+0,-0.5db16cyclesthrough60,000
cycles
Atone-halfratedoutput:
+0,-1db10cyclesthrough100,000
cycles
NoiseandHum:
90dbormorebelowratedoutput
OutputImpedance:
StereoorTwinAmp:
(Isolatedfromground)4 ohms,8 ohms,
16ohms
(Centertapinternallygrounded)600
ohms
(Onesideinternallygrounded)67ohms,
150ohms
Mono:
(Isolatedfromground)2 ohms,4 ohms,
8ohms,16ohms,32ohms
(Centertapinternallygrounded)300
ohms
(Onesideinternallygrounded)34ohms,
75ohms
OutputVoltage:
StereoorTwinAmp:
(Isolatedfromground)25volts
(Centertapinternallygrounded)230
volts
(Onesideinternallygrounded)70.7
volts,115volts
Mono:
(Isolatedfromground)25volts,70.7
volts
(Centertapinternallygrounded)230
volts
(Onesideinternallygrounded)70.7
volts,115volts
InternalImpedance:
Lessthan10%ofratedloadimpedance
Input:
Inputimpedance—250,000ohms
Inputsensitivity—0.5voltsto30volts
throughgaincontrolwithinputswitchin
MONOorTWINAMPposition.Withthe
switchintheSTEREOposition,sensi-
tivityis2.0voltsthrougha balance
control.
PowerRequirements:
105voltsto130voltsat50/60cycles:
240wattsatzerosignaloutput:400
wattsatratedoutput.Thepowertrans-
formeristappedfor117voltsand125
volts.
TubeComplement:
VoltageAmplifier—One12AX7(drives
bothamps)
PhaseInverter—Two12AU7
DriveAmplifier—Two12BH7
CathodeFollowerDrive—Two12AT7or
12AZ7
Output—FourKT88or6550
MECHANICALSPECIFICATIONS
Dimensions:
Chassisdimensions:17¼"longx 12¼'
widex 8"high.(SeeINSTALLATIONon
Page5 forminimummountingspace.)
Weight:
Chassisonly—67½pounds.Inshipping
carton—76pounds.
Finish:
ChromiumandBlack
AccessoriesSupplied:
OwnersManual
MountingTemplate
MountingFlanges
Octaloutputplugwithcap
3

PANELFACILITIES
Fig.3—TheendpaneloftheMC275showingallinputandoutputfacilities.
INPUT
TheinputoftheMC275hasa threeposi-
tionleverswitchtopermit theamplifiertobe
usedinanyoneofthreeways:
(1)Asa monophonic150wattamplifier
(2)Astwin75wattamplifiersusedwithan
electroniccrossovernetwork,ortwo
completelyseparateamplifiers,orin
stereoapplicationsrequiring0.5volt
sensitivityinthepoweramplifiers
(3)Asa stereoamplifierusedwitha
Mclntoshpreamplifierorotherhigh
outputstereoprogramsources.
TheinputmarkedSTEREOhasa sensi-
tivityof2.0voltscontrolledbya BALANCE
control.Theinputimpedanceis250,000
ohms.
TheinputmarkedTWINAMPhasa sensi-
tivityof0.5volt.EachTWINAMPinputis
controlledbya GAINcontrol.TheGAINcon-
trolallowssignalsourceswithoutputfrom
0.5voltupto30voltstobeconnectedwith-
outoverloadingtheinputtotheamplifier.
Theinputimpedanceis250,000ohms.
TheinputmarkedMONOhasasensitivity
of0.5volt.TheMONOpositionoftheinput
switchparallelstheinputstobothampli-
fiers.Whentheoutputsareproperlycon-
nectedtheMC275becomesa 150watt
monophonicamplifier.TheMONOinputhas
aGAINcontrolthatpermitsconnectingsignal
sourcesupto30voltswithoutoverloading
theamplifier'sinput.Theinputimpedance
is250,000ohms.
OUTPUT
Thetwobarrierterminalstripsmarked
OUTPUTprovidestereoconnectionsforthe
normalspeakerimpedancesof4 ohms,
8ohms,and16ohms.Formonophonicoper-
ationconnectionsfor2 ohms,4 ohms,8
ohms,16ohms,and32ohmsareprovided.
Theterminalstripsmayalsobeconnected
fora constantvoltageoutputof25voltsin
eitherstereoormono,orfor70.7volts
inmono.
ThesecondaryoftheMclntoshoutput
transformermaybeoperatedeithergrounded
ornotgrounded.Assupplied,thesecondary
oftheMC275isnotgrounded.Ifgrounding
isdesirablea wiremaybeconnectedbe-
tweenchassisandcommonofeitherorboth
outputterminalstrips.
Thebarrierstripoutputconnectionsare
fedfromthesecondaryoftheoutputtrans-
former,
LINEOUTPUT(Seealsopages5,6 and7)
TheoctalsocketmarkedLINEOUTPUT
hasconnectionsfor70.7volts,115volts,
230volts,or600ohmsforstereooperation.
Formonophonicoperationtheoctalsocket
providesconnectionsfor70.7volts,115
volts,230volts,or300ohms.
The600ohmwindingandthe300ohm
windingarebalancedtoground.Pin1 ofthe
octalsocketisthecentertapforthesewind-
ingsandisgrounded.
Theconnectionsattheoctalsocketare
fedfromtapsonthecathodewindinginthe
primaryoftheoutputtransformer.
FUSE
TheMC275usesa 5.0ampereslo-blo
typefuse.TheauxiliaryACsocketisnot
fused.
ACOUTLET
TheauxiliaryACoutletcanbeusedto
supplypowertootherequipmentinthe
4

system.Theoutletwillprovidea maximumof
300wattsofpower.TheACoutletisnotfused.
LINEVOLTAGE
TheMC275operatesonanylinevoltagebe-
tween105voltsand130volts,50cyclesto60
cycles.TheswitchmarkedLINEVOLTAGE
changesthetapontheprimaryofthepower
transformer.Theswitchcanbeadjustedat
any time.
INSTALLATION
Adequateventilationextendsthetrouble-
freelifeofelectronicinstruments.Itis
generallyfoundthateach10°centigrade
(18°F)riseintemperaturereducesthelifeof
electricalinsulationbyonehalf.Adequate
ventilationisaninexpensiveandeffective
meansofpreventinginsulationbreakdown
thatresultsfromunnecessarilyhighoperat-
ingtemperatures.Thedirectbenefitofade-
quateventilationislonger,trouble-freelife.
Thesuggestedminimumspaceformount-
ingtheMC275is20"longx 14"widex 10"
high.Alwaysallowforairfloweitherby
ventilationholesorspacenexttothebottom
oftheamplifieranda meansforthewarm
airtoescapeatthetop.
TheMC275canbemountedinanyposi-
tionexceptupsidedown.Iftheamplifieristo
beinstalledona verticalsurfaceitisrecom-
mendedthatthetransformersbeonthe
downside.Theadvantageofthispositionis
thattheflowofheatfromthetubesrises
verticallyanddoesnottendtoheatthe
transformers.
IftheMC275istobepermanentlymounted
usethetwomountingflangessuppliedwith
theamplifier.Theflangesareshippedsepa-
ratelyandmustbeattachedtotheamplifier
bottomateachend.
TurntheMC275overwiththetransform-
ersdownandplaceitona pieceofclothor
cardboardtopreventscratches.Theampli-
fierisshippedwith4 plasticfeetonthebot-
tomcover.Removethe4 plasticfeet.Re-
movethethreeselftappingscrewsatone
endoftheamplifierthatholdthebottom
covertothechromiumchassis.Placethe
squareedgeofthemountingflangewiththe
threeroundholespositionedoverthethree
holesinthebottomcover.Thethreeholesin
themountingflangewilllineupoverthe
threeholesinthechassis.Replacethethree
selftappingscrewsthroughthemounting
flangeandbottomcover.Themounting
flangeisnowfirmlyattachedtotheampli-
fier.Themountingflangeontheotherendof
theamplifierisinstalledinthesameway.
Fig.4—BottomofMC275withmountingflangesattached.
PlacetheMC275mountingtemplateon
theareawheretheamplifieristobemounted.
Centerpunchthemountingshelfthrough
the4 slottedholesinthemountingflanges
shownonthetemplate.Removethetemplate
andinstallfour#10roundheadscrewsin
thecenterpunchmarks.Donottightenthese
screws.Placetheamplifieroverthescrew
headsandslidetheamplifiertoeitherside
intheslottedholesofthemountingflanges.
The#10woodscrewscanthenbetightened
securely.
CONNECTINGTHEMC275
INPUT-STEREO
Theshieldedcablefromtheleftoutputof
theMclntoshpreamplifierispluggedinto
thejackmarkedSTEREOINPUT-LThe
shieldedcablefromtherightoutputofthe
Mclntoshpreamplifierispluggedintothe
jackmarkedSTEREOINPUT-R.
INPUT-TWINAMP
Theshieldedcablefromtheleftprogram
sourceispluggedintothejackmarked
TWININPUT-LTheshieldedcablefromthe
rightprogramsourceispluggedintothe
jackmarkedTWININPUT-R.
INPUT-MONO
Theshieldedcablefromtheprogram
sourceispluggedintothejackmarked
MONOINPUT.
OUTPUT-STEREOORTWINAMPLIFIERS
Forstereoortwinchanneloperationitis
notnecessarytousethesameimpedance
loadoneachoutput.Simplyconnecteach
outputfortheimpedancedesired.
5

Warning:Donotparalleltheamplifierout-
putswhenusingtheamplifierasa stereo
amplifierortwinchannelamplifier.Damage
totheoutputtubesmayresultifparallel
operationisattemptedwhentheoutputvolt-
agesarenotthesamephaseandamplitude.
Speakersareconnectedatthebarrier
stripsmarkedOUTPUTontheleftendof
thepanel.
IncompliancewiththeNationalElectrical
Code,ClassIIwiringcanbeusedbetween
thespeakerandtheamplifieratthe4 ohm,
8ohm,or16ohmconnection.ClassIIwiring
islampcord,bellwire,orotherwirewith
thistypeofinsulation.Forthenormallyshort
distancesofunder100feetbetweenthe
amplifierandspeaker,#18wireorlarger
canbeused.Fordistancesover100feet
betweentheamplifierandspeakeruse
largerwire.
Theloudspeakerimpedanceisusually
identifiedontheloudspeakeritself.Connect
oneoftheleadsfromtheleftloudspeakerto
thescrewmarkedCOMontheleftbarrier
strip.Connecttheotherleadfromtheleft
loudspeakertothescrewmarkedwiththe
numbercorrespondingtothespeakerim-
pedanceontheLEFTbarrierstrip.
Ifthespeaker
impedanceis:
4ohms
8ohms
16ohms
Connectoneleftspeaker
leadtoscrewLEFT-COM
andtheotherto:
LEFT-4
LEFT-8
LEFT-16
Connectonerightspeaker
leadtothescrewmarked
RIGHT-COMandtheotherto:
RIGHT-4
RIGHT-8
RIGHT-16
Theonlyadverseeffectontheoperation
ofa Mclntoshamplifierwhenitisimproperly
matchedisa reductionintheamountof
distortion-freepoweravailabletotheloud-
speaker.Closeimpedancematchingisde-
sirableformaximumdistortion-freepower.
Usethistabletodetermineproperspeaker
connections:
OUTPUT-MONOPHONIC
WhentheMC275istooperateasa 150
wattmonophonicamplifier,theoutputsof
thetwochannelscombinetoproducea
single150wattoutput.Thischartliststhe
properconnectionsandinterconnectionsfor
monophonicoperation.
Ifthe
speaker
impedance
is:
2ohms
4ohms
8ohms
16ohms
32ohms
Connectonespeaker
leadtothescrew
markedLEFT-COM
andtheotherto:
LEFT-4
LEFT-8
LEFT-16
RIGHT-8
RIGHT-16
Connecta
wirebetween:
LEFT-COMand
RIGHT-COM
LEFT-COMand
RIGHT-COM
LEFT-COMand
RIGHT-COM
RIGHT-COMand
LEFT-8
RIGHT-COMand
LEFT-16
Connectanother
wirebetween:
LEFT-4and
RIGHT-4
LEFT-8and
RIGHT-8
LEFT-16and
RIGHT-16
—
—
WhenconnectedasoutlinedtheMC275
operatesasa 150wattmonophonicamplifier.
LINEOUTPUT-STEREOorTWINAMPLIFIERS
TheoctalsocketmarkedLINEOUTPUT
hasconnectionsfor70.7volts,115voltsand
600ohms.The25voltlineisfedfromthe
barrierstrips.
For25voltlineoperationconnectoneof
theleftleadstothescrewmarkedCOMon
theLEFTbarrierstrip.Theotherleftleadis
connectedtothescrewmarked8 onthe
LEFTbarrierstrip.Connecttherightleads
inthesamemannerontheRIGHTbarrier
strip.
The67ohmand70.7voltconnectionsare
thesame.Toconnectfor67ohmor70.7
voltoperationtheoctalplugsuppliedinthe
hardwarebagisused.Solderoneoftheleft
leadstopin#7andtheothertopin#1.
Solderoneoftherightleadstopin#8and
theothertopin#1.Pin#1istheground
side.
The600ohmconnectionsfortheleftside
aremadebetweenpin#2andpin#3.The
600ohmconnectionsfortherightsideare
madebetweenpin#4andpin#5.The600
ohmwindingisbalancedtoground.Pin#1
ontheoctalsocketisthecentertapofthe
600winding.Pin#1isgrounded.
Forallapplicationsusing600ohmsuse
ClassI wiringiftheinstallationmustmeet
therequirementsoftheNationalElectrical
Code.
LINEOUTPUT-MONOPHONIC
TheoctalsocketmarkedLINEOUTPUT
hasconnectionsforoperatingtheMC275
monophonicallyat70.7volts,115voltsor
300ohms.Foroperatingat25voltsconnec-
tionsaremadeatthebarrierstrips.
Tofeeda 25-voltline,connectonesideto
thescrewmarkedCOMontheLEFTbarrier
strip.Connecttheothersidetothescrew
marked8 ontheLEFTbarrierstrip.Then
connectwitha wirethescrewmarkedCOM
ontheLEFTbarrierstriptothescrewmarked
Ifthespeakerimpedance
isbetween:
3.2to6.5ohms
6.5to13ohms
13to26ohms
Connectthe
speakerleads
betweenCOMand:
4ohms
8ohms
16ohms
6

COMontheRIGHTbarrierstrip.Connect,
withanotherwire,thescrewmarked8 onthe
LEFTbarrierstriptothescrewmarked8 on
theRIGHTbarrierstrip.
The34ohmandthe70.7voltconnec-
tionsarethesame.Toconnectfor34ohms
or70.7voltssolderoneoftheleadstopin
#1intheoctalsocket.Theotherleadis
solderedtopin#7.A wireisconnectedbe-
tweenpin#7andpin#8.
For150wattmonophonicoperation70.7
voltsisavailablefromthesecondaryofthe
outputtransformers,isolatedfromground.
Connectoneleadtotheterminalstripscrew
markedLEFT-COMandtheotherleadtothe
screwmarkedRIGHT-16.Connecta wire
betweenRIGHT-COMandLEFT-16.
Toconnectfor300ohmsoneleadis
solderedtopin#2.Theotherleadissol-
deredtopin#3.Connectpin#2topin#4
witha wire.Withanotherwireconnectpin
#3topin#5.Pin#1isthecentertapofthe
300ohmwinding.The300ohmwindingis
balancedtoground.
ACPOWER
TheMC275operateson105voltto130
volt,50to60cyclepower.Theamplifierwill
beturnedonandoffifitspowercordis
pluggedinoneoftheauxiliaryACoutletson
theprogramsource.
OPERATINGTHEMC275
BeforeturningtheMC275oncheckall
connectionsandplugstoseethattheyare
firmlyandcorrectlyconnected.Checkto
makesurethatthetubesarefirmlyseated
inthepropersockets.Afterthefollowing
adjustmentshavebeencompletedtheMC-
275willoperatewithoutanyfurtherattention.
ADJUSTMENTS
LINEVOLTAGE
TheLINEVOLTAGEswitchshouldbeset
tothepositionthatmorecloselymatchesthe
averagelinevoltageinyourarea.Thelocal
electricpowercompanycanusuallytellyou
theaveragelinevoltageinyourarea.Ifthe
linevoltageisbetween105voltsand120
voltsputtheLINEVOLTAGEswitchinthe117
voltposition.Ifthelinevoltageisbetween
120voltsand130voltsputtheswitchinthe
125voltposition.Ifthelinevoltageisun-
known,itissafertooperatewiththeswitch
inthe125voltposition.
INPUTSWITCH
STEREO
Withtheinputswitchintheleftposition
markedSTEREOtheinputoftheMC275is
connectedtothetwojacksmarkedSTEREO
INPUTlocatedbelowtheBALANCEcontrol.
Theamplifierhasaninputsensitivityinthis
positionof2.0volts.Thispositionhasbeen
providedtobeusedwithMclntoshpreampli-
fiersoranyotherhighoutputpreamplifiers.
TheBALANCEcontrolassistsinbalancing
theoverallstereosystemforequalloudness.
Equalloudnessisaffectedbyroomacous-
tics,furnitureplacement,speakersand
manyotherthings.Witha monophonicsignal
adjustthiscontroluntilbothloudspeakers
areequalinsoundoutput.
TWINAMP
Withtheinputswitchinthecenterposi-
tionmarkedTWINAMP,theinputofthe
MC275isconnectedtothetwojacksmarked
TWININPUTlocatedbelowthetwocenter
controlknobsmarkedGAIN.Theinputsensi-
tivityinthispositionis0.5volt.Thisposition
hasbeenprovidedforusewitha lowoutput
stereopreamplifier,anelectroniccrossover
network,orastwocompletelyseparated
monophonic75wattamplifiers.
Thetwocentercontrolknobsmarked
GAINshouldbeadjustedtomatchtheinput
sensitivityoftheMC275totheoutputvoltage
oftheprogramsourceequipment.
Tobalancea stereosystemusingthe
TWINAMPinputpositionusea monophonic
signalfromtheprogramsource.A mono-
phonicsignalsuppliesthesamevoltageto
bothchannels.Turnthevolumecontrolon
thesourceequipmenttothe12o'clockposi-
tion(halfrotation).TurnuptheGAINcon-
trolontheLEFTinputuntiltheloudnessin
theroomisnotquiteasloudasyouliketo
listen.ThenturntheGAINcontrolonthe
RIGHTinputuntilitisequallyasloudasthe
leftchannel.Thesystemisnowbalancedfor
loudnessandprovidesthemostconvenient
amountofloudnesschangewhenusingthe
volumecontrolonthesourceequipment.
MONO
Withtheswitchintherightposition
markedMONOtheinputsoftheMC275are
internallyconnectedinparalleltothejack
7

markedMONOINPUT.Thisjackislocated
belowtherightcontrolknobmarkedGAIN.
Theinputsensitivityinthispositionis0.5
volt.TheGAINcontrolshouldbeadjustedto
matchtheinputvoltagesensitivityofthe
MC275totheoutputvoltageoftheprogram
source.
FormaximumflexibilitysettheGAINcon-
trolasfollows:turnthevolumecontrolon
theprogramsourceequipment(i.e.tape
recorder,tuner,preamplifier,etc.)tothe
12o'clock(halfrotation)position.Turnthe
GAINcontrolontheMC275untiltheloud-
nessofthesoundfromthespeakerisjusta
littlelouderthanyounormallyliketolisten.
Thevolumecontrolontheprogramsource
equipmentnowhasthemostconvenient
amountofloudnesschangeineitherdirec-
tionfromthe12o'clockposition.
IMPORTANT
Theexcellentperformancethatisinher-
entinallMclntoshamplifiersdoesnot
dependonthecriticaladjustmentofbiasor
balancecontrolsintheoutputcircuit.The
patentedMclntoshcircuitdeliversitsadver-
tisedspecificationswithoutanyneedfor
thesecontrolsandisnotdependentoncare-
fullybalancedtubesforitsperformance.
WithMclntoshyoucaninstalltheamplifier
andforgetit.
TheMC275youhavepurchasedwillgive
youyearsofpleasantandsatisfactoryper-
formance.Ifyouhaveanyquestionscon-
cerningtheoperationormaintenanceofthis
amplifierpleasecontact:
CustomerService
MclntoshLaboratory,Inc.
2ChambersStreet
Binghamton,NewYork
Ourtelephonenumberis723-5491.
Thedirectdialareacodeis607.
GUARANTEE
MclntoshLaboratoryIncorporatedguar-
anteesthisequipmenttoperformasadver-
tised.Wealsoguaranteethemechanicaland
electricalworkmanshipandcomponentsof
thisequipmenttobefreeofdefectsfora
periodof90daysfromdateofpurchase.
Thisguaranteedoesnotextendtocompo-
nentsdamagedbyimproperusenordoesit
extendtotransportationtoandfromthe
factory.
3-YEARFACTORYSERVICECONTRACT
Anapplicationfora FREE3-YEARFAC-
TORYSERVICECONTRACTisincludedinthe
pocketinthebackcoverofthismanual.The
FREE3-YEARFACTORYSERVICECONTRACT
willbeissuedbyMclntoshLaboratoryupon
receiptofthecompletelyfilledoutapplica-
tionform.Thetermofthiscontractisdefined
inthe3-yearfactoryservicecontract.Ifthe
applicationisnotmailedtoMclntoshLabo-
ratoryonlytheservicesofferedunderthe
standard90-dayguaranteewillapplyonthis
equipment.TAKEADVANTAGEOF3 YEARS
OFFREEFACTORYSERVICEBYFILLING
INTHEAPPLICATIONNOW.
InCanada:Manufacturedunderlicenseby:
McCurdyRadioIndustries,Ltd.
22FrontStreetWest
Toronto,Canada
Designsubjecttochangewithoutnotice.
8

LABORATORY,INC.
2CHAMBERSSTREET,BINGHAMTON,N.Y.
MadeInU.S.A.
Phone-AreaCode607-723-5491
PrintedinU.S.A.