McIntosh MC24O User manual

STEREOPOWER
AMPLIFIER
MC24O
CONTENTS
GENERALDESCRIPTION1
TECHNICALDESCRIPTION1
PANELFACILITIES4
INSTALLATION5
CONNECTIONS5
Input—Stereo5
Input—TwinAmp5
Input—Mono6
Output—StereoorTwinAmp6
Output—Mono6
LineOutput—StereoorTwinAmp6
LineOutput—Mono7
ACPower7
OPERATION7
ADJUSTMENTS7
GUARANTEE8
OWNER'SMANUAL
MC24O

MC240 STEREO POWER AMPLIFIER
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.
TheMC240hasononechassistwo40
wattpoweramplifiers.Inadditiontouseasa
stereoamplifier,theflexibilityoftheMC240
permitsittobeusedasa monophonic
amplifierthatdelivers80watts,orastwo
separate40wattamplifierswitheachchan-
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.
turnofprimarynumberoneisnexttothe
sameturnofprimarynumbertwo.Thereis
almostcompletemagneticcouplingbetween
thetwowires.Themagneticcouplingisrein-
forcedbythecapacitancebetweenthe
twowires.
INPUT
OUTPUT
Fig.1 —Representationofbifilarwinding.
TheMclntoshoutputtransformeris
unique.Ithastwoprimarywindingswhichare
woundbifilarly.Inthebifilartechniqueboth
primarywiresarewoundsidebyside.EachFig.2 —RepresentationofMclntoshoutputcircuit.
1
1U.S.PatentNo's:2,477,074;2,545,788;2,646,467;2,654,058;2,860,192;2,929,028.

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
Notch Distortionandwaswellillustratedby
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.
Leakageinductance(lackofcoupling)
betweentheprimaryandsecondarywind-
ingsoftheoutputtransformerlimitsthehigh
frequencyresponseofanamplifier.The
primaryandsecondarywindingsofthe
Mclntoshoutputtransformerareinterleaved
fivetimesto improvecoupling.Theinter-
leavingisaccomplishedbywindinggroupsof
primarylayers,thensecondarylayersalter-
natelyuntilthetotaltransformeriswound.
InterleavinghelpstoextendtheMclntosh
powerbandwidthtoover100,000cycles.
Since1947thisnewtechnologyhasbeen
builtintoeveryMclntoshpoweramplifier.
Goodvoltageregulationinthepower
supplypermitsoverloadswithoutovershoot
orblocking,goodtransientresponse,and
completestability.Toimproveregulationa
siliconrectifierpowersupplyisusedinthe
MC240.Inadditiontobettervoltageregula-
tion,thesiliconrectifierallowsevenhigher
operatingefficiency,cooleroperation,and
longeramplifierlife.
Togreatlyextendtubeandcomponent
lifea thermistorintheMC240limitscurrent
surgesproducedwhentheequipmentis
turnedon.Thethermistorisa specialtypeof
resistor.Itsresistancedependsonitstem-
perature.Whentheamplifierisoffthetherm-
istorhasa highresistancevalue(about79
ohms).Justaftertheamplifieristurnedon
thecurrentwhichflowsthroughthetherm-
istorheatsitandcausesitsresistanceto
decreasetoa lowvalue(lessthan.1ohms).
CurrentisthuslimitedwhentheMC240is
firstturnedonbutisnotlimitedastheunit
warms.
ELECTRICALSPECIFICATIONS
PowerOutput:
Stereoortwinamplifier—40wattscon-
tinuousperchannel
Monopnonic—80wattscontinuous
HarmonicDistortion:
Lessthan0.5%atratedoutputorless,
20cyclesthrough20,000cycles.Typical
performanceis0.3%orlessat20cycles
and20,000cyclesatfullpower.Typical
performanceinthemidfrequenciesis
0.1%orlessatfullpower.
2

IntermodulationDistortion:
Lessthan0.5%foranycombinationof
frequenciesfrom20cyclesthrough
20,000cyclesifinstantaneouspeak
powerisbelowtwicetheratedpower.
PhaseShift:
Lessthan+6°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)125ohms
Mono:
(Isolatedfromground)2 ohms,4 ohms,
8ohms,16ohms,32ohms
(Centertapinternallygrounded)300
ohms
(Onesideinternallygrounded)62ohms
OutputVoltage:
StereoorTwinAmp:
(Isolatedfromground)25volts
(Centertapinternallygrounded)140
volts
(Onesideinternallygrounded)70.7
volts
Mono:
(Isolatedfromground)25volts
(Centertapinternallygrounded)140
volts
(Onesideinternallygrounded)70.7
volts
InternalImpedance:
Lessthan10%ofratedloadimpedance
Input:
Inputimpedance—250,000ohms
Inputsensitivity—0.5voltsto30volts
throughgaincontrolwithinputswitchin
MONOorTWINAMPposition.Withthe
switchintheSTEREOposition,sensi-
tivityis2.0voltsthrougha balance
control.
PowerRequirements:
105voltsto130voltsat50/60cycles:
145wattsatzerosignaloutput:270
wattsatratedoutput.Thepowertrans-
formeristappedfor117voltsand125
volts.
TubeComplement:
VoltageAmplifier—One12AX7(drives
bothamps)
PhaseInverter—Two12AU7
DriveAmplifier—Two12BH7
CathodeFollowerDrive—Two12AX7
Output—Four6L6GCor7027A
MECHANICALSPECIFICATIONS
Dimensions:
Chassisdimensions:17¼"longx 10¾"
widex 8"high.(SeeINSTALLATIONon
Page5 forminimummountingspace.)
Weight:
Chassisonly—56pounds.Inshipping
carton—65pounds.
Finish:
ChromiumandBlack
AccessoriesSupplied:
OwnersManual
MountingTemplate
MountingFlanges
Octaloutputplugwithcap
3

PANELFACILITIES
Fig.3—TheendpaneloftheMC240showingallinputandoutputfacilities.
INPUT
TheinputoftheMC240hasa threeposi-
tionleverswitchtopermittheamplifiertobe
usedinanyoneofthreeways:
(1}Asa monophonic80wattamplifier
(2)Astwin40wattamplifiersusedwithan
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.
TheinputmarkedMONOhasa sensitivity
of0.5volt.TheMONOpositionoftheinput
switchparallelstheinputstobothampli-
fiers.Whentheoutputsareproperlycon-
nectedtheMC240becomesan80watt
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.
ThesecondaryoftheMclntoshoutput
transformermaybeoperatedeithergrounded
ornotgrounded.Assupplied,thesecondary
oftheMC240isnotgrounded.Ifgrounding
isdesirablea wiremaybeconnectedbe-
tweenchassisandcommonofeitherorboth
outputterminalstrips.
Thebarrierstripoutputconnectionsare
fedfromthesecondaryoftheoutputtrans-
former.
LINEOUTPUT(Seealsopages6 and7)
TheoctalsocketmarkedLINEOUTPUT
hasconnectionsfor70.7volts,125ohms,
140volts,or600ohmsforstereooperation.
Formonophonicoperationtheoctalsocket
providesconnectionsfor70.7volts,62
ohms,140volts,or300ohms.
The600ohmwindingandthe300ohm
windingarebalancedtoground.Pin1 ofthe
octalsocketisthecentertapforthesewind-
ingsandisgrounded.
Theconnectionsattheoctalsocketare
fedfromtapsonthecathodewindinginthe
primaryoftheoutputtransformer.
4

FUSE
TheMC240usesa 3.2ampereslo-blo
typefuse.TheauxiliaryACsocketisnot
fused.
ACOUTLET
TheauxiliaryACoutletcanbeusedto
supplypowertootherequipmentinthe
system.Theoutletwillprovidea maximumof
300wattsofpower.TheACoutletisnotfused.
LINEVOLTAGE
TheMC240operatesonanylinevoltages
between105voltsand130volts,50cyclesto
60cycles.TheswitchmarkedLINEVOLTAGE
changesthetapontheprimaryofthepower
transformer.Theswitchcanbeadjustedat
anytime.
INSTALLATION
Adequateventilationextendsthetrouble-
freelifeofelectronicinstruments.Itis
generallyfoundthateach10°centigrade
(18°F)riseintemperaturereducesthelifeof
electricalinsulationbyonehalf.Adequate
ventilationisaninexpensiveandeffective
meansofpreventinginsulationbreakdown
thatresultsfromunnecessarilyhighoperat-
ingtemperatures.Thedirectbenefitofade-
quateventilationislonger,trouble-freelife.
Thesuggestedminimumspaceformount-
ingtheMC240is20"longx 14"widex 10"
high.Alwaysallowforairfloweitherby
ventilationholesorspacenexttothebottom
oftheamplifieranda meansforthewarm
airtoescapeatthetop,
TheMC240canbemountedinanyposi-
tionexceptupsidedown.Iftheamplifieristo
beinstalledona verticalsurfaceitisrecom-
mendedthatthetransformersbeonthe
downside.Theadvantageofthispositionis
thattheflowofheatfromthetubesrises
verticallyanddoesnottendtoheatthe
transformers.
IftheMC240istobepermanentlymounted
usethetwomountingflangessuppliedwith
theamplifier.Theflangesareshippedsepa-
ratelyandmustbeattachedtotheamplifier
bottomateachend.
TurntheMC240overwiththetransform-
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—BottomofMC240withmountingflangesattached.
PlacetheMC240mountingtemplateon
theareawheretheamplifieristobemounted.
Centerpunchthemountingshelfthrough
the4 slottedholesinthemountingflanges
shownonthetemplate.Removethetemplate
andinstallfour#10roundheadscrewsin
thecenterpunchmarks.Donottightenthese
screws.Placetheamplifieroverthescrew
headsandslidetheamplifiertoeitherside
intheslottedholesofthemountingflanges.
The# 10woodscrewscanthenbetightened
securely.
INPUT-STEREO
Theshieldedcablefromtheleftoutputof
theMclntoshpreamplifierispluggedinto
thejackmarkedSTEREOINPUT-L.The
shieldedcablefromtherightoutputofthe
Mclntoshpreamplifierispluggedintothe
jackmarkedSTEREOINPUT-R.
CONNECTINGTHEMC240
INPUT-TWINAMP
Theshieldedcablefromtheleftprogram
sourceispluggedintothejackmarked
TWININPUT-L.Theshieldedcablefromthe
rightprogramsourceispluggedintothe
jackmarkedTWININPUT-R.
5

INPUT-MONO
Theshieldedcablefromtheprogram
sourceispluggedintothejackmarked
MONOINPUT.
OUTPUT-STEREOORTWINAMPLIFIERS
Forstereoortwinchanneloperationitis
notnecessarytousethesameimpedance
loadoneachoutput.Simplyconnecteach
outputfortheimpedancedesired.
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
OUTPUT-MONOPHONIC
WhentheMC240istooperateasan80
wattmonophonicamplifier,theoutputsof
thetwochannelscombinetoproducea
-single80watt output.Thischartliststhe
properconnectionsandinterconnectionsfor
monophonicoperation.
Warning:Donotparalleltheamplifierout-
putswhenusingtheamplifierasa stereo
amplifierortwinchannelamplifier.Damage
totheoutputtubesmayresultifparallel
operationisattemptedwhentheoutputvolt-
agesarenotthesamephaseandamplitude.
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
—
—
WhenconnectedasoutlinedtheMC240
operatesasan80wattmonophonicamplifier.
LINEOUTPUT-STEREOorTWINAMPLIFIERS
TheoctalsocketmarkedLINEOUTPUT
hasconnectionsfor70.7volts,125ohmsand
140voltsor600ohms.The25voltlineisfed
fromthebarrierstrips.
For25voltlineoperationconnectoneof
theleftleadstothescrewmarkedCOMon
theLEFTbarrierstrip.Theotherleftleadis
connectedtothescrewmarked16onthe
LEFTbarrierstrip.Connecttherightleads
inthesamemannerontheRIGHTbarrier
strip.
The125ohmand70.7.voltconnectionsare
thesame.Toconnectfor125ohmor70.7
voltoperationtheoctalplugsuppliedinthe
hardwarebagisused.Solderoneoftheleft
leadstopin#2andtheothertopin#1.
Solderoneoftherightleadstopin#4and
theothertopin#1.Pin#1istheground
side.
The600ohmconnectionsfortheleftside
aremadebetweenpin#2andpin#3.The
600ohmconnectionsfortherightsideare
madebetweenpin#4 andpin#5.The600
ohmwindingisbalancedtoground.Pin#1
ontheoctalsocketisthecentertapofthe
600winding.Pin#1isgrounded.
Ifthespeakerimpedance
isbetween:
3.2to6.5ohms
6.5to13ohms
13to26ohms
Connectthe
speakerleads
betweenCOMand:
4ohms
8ohms
16ohms
6
Theonlyadverseeffectontheoperation
ofa Mclntoshamplifierwhenitisimproperly
matchedisa reductionintheamountof
distortion-freepoweravailabletotheloud-
speaker.Closeimpedancematchingisde-
sirableformaximumdistortion-freepower.
Usethistabletodetermineproperspeaker
connections:

Forallapplicationsusing600ohmsuse
ClassI wiringiftheinstallationmustmeet
therequirementsoftheNationalElectrical
Code.
LINEOUTPUT-MONOPHONIC
TheoctalsocketmarkedLINEOUTPUT
hasconnectionsforoperatingtheMC240
monophonicallyat70.7volts,62ohmsor
300ohms.Foroperatingat25voltsconnec-
tionsaremadeatthebarrierstrips.
Tofeeda 25voltline,connectonesideto
thescrewmarkedCOMontheLEFTbarrier
strip.Connecttheothersidetothescrew
marked16ontheLEFTbarrierstrip.Then
connectwitha wirethescrewmarkedCOM
ontheLEFTbarrierstriptothescrewmarked
COMontheRIGHTbarrierstrip.Connect,
withanotherwire,thescrewmarked16on
theLEFTbarrierstriptothescrewmarked16
ontheRIGHTbarrierstrip.
The62ohmandthe70.7voltconnec-
tionsarethesame.Toconnectfor62ohms
or70.7voltssolderoneoftheleadstopin
#1intheoctalsocket.Theotherleadis
solderedtopin#2.A wireisconnectedbe-
tweenpin#2andpin#4.
Toconnectfor300ohmsoneleadis
solderedtopin#2.Theotherleadissol-
deredtopin#3.Connectpin#2topin#4
witha wire.Withanotherwireconnectpin
#3topin#5.Pin#1isthecentertapofthe
300ohmwinding.The300ohmwindingis
balancedto ground.
ACPOWER
TheMC240operateson105voltto130
volt,50to60cyclepower.Theamplifierwill
beturnedonandoffifitspowercordis
pluggedinoneoftheauxiliaryACoutletson
theprogramsource.
OPERATINGTHEMC240
BeforeturningtheMC240oncheckall
connectionsandplugstoseethattheyare
firmlyandcorrectlyconnected.Checkto
makesurethatthetubesarefirmlyseated
inthepropersockets.Afterthefollowing
adjustmentshavebeencompletedtheMC-
240willoperatewithoutanyfurtherattention.
ADJUSTMENTS
LINEVOLTAGE
TheLINEVOLTAGEswitchshouldbeset
tothepositionthatmorecloselymatchesthe
averagelinevoltageinyourarea.Thelocal
electricpowercompanycanusuallytellyou
theaveragelinevoltageinyourarea.Ifthe
linevoltageisbetween105voltsand120
voltsputtheLINEVOLTAGEswitchinthe117
voltposition.Ifthelinevoltageisbetween
120voltsand130voltsputtheswitchinthe
125voltposition.Ifthelinevoltageisun-
known,itissafertooperatewiththeswitch
inthe125voltposition.
INPUTSWITCH
STEREO
Withtheinputswitchintheleftposition
markedSTEREOtheinputoftheMC240is
connectedtothetwojacksmarkedSTEREO
INPUTlocatedbelowtheBALANCEcontrol.
Theamplifierhasaninputsensitivityinthis
positionof2.0volts.Thispositionhasbeen
providedtobeusedwithMclntoshpreampli-
fiersoranyotherhighoutputpreamplifiers.
TheBALANCEcontrolassistsinbalancing
theoverallstereosystemforequalloudness.
Equal loudnessisaffectedbyroomacous-
tics,furnitureplacement,speakersand
manyotherthings.Witha monophonicsignal
adjustthiscontroluntilbothloudspeakers
areequalinsoundoutput.
TWINAMP
Withtheinputswitchinthecenterposi-
tionmarkedTWINAMP,theinputofthe
MC240isconnectedtothetwojacksmarked
TWININPUTlocatedbelowthetwocenter
controlknobsmarkedGAIN.Theinputsensi-
tivityinthispositionis0.5volt.Thisposition
hasbeenprovidedforusewitha lowoutput
stereopreamplifier,anelectroniccrossover
network,orastwocompletelyseparated
monophonic40wattamplifiers.
Thetwocentercontrolknobsmarked
GAINshouldbeadjustedtomatchtheinput
sensitivityoftheMC240totheoutputvoltage
oftheprogramsourceequipment.
7

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
markedMONOtheinputsoftheMC240are
internallyconnectedinparalleltothejack
markedMONOINPUT.Thisjackislocated
belowtherightcontrolknobmarkedGAIN.
Theinputsensitivityinthispositionis0.5
volt.TheGAINcontrolshouldbeadjustedto
matchtheinputvoltagesensitivityofthe
MC240totheoutputvoltageoftheprogram
source.
FormaximumflexibilitysettheGAINcon-
trolasfollows:turnthevolumecontrolon
theprogramsourceequipment(i.e.tape
recorder,tuner,preamplifier,etc.)tothe
12o'clock(halfrotation)position.Turnthe
GAINcontrolontheMC240untiltheloud-
nessofthesoundfromthespeakerisjusta
littlelouderthanyounormallyliketolisten.
Thevolumecontrolontheprogramsource
equipmentnowhasthemostconvenient
amountofloudnesschangeineitherdirec-
tionfromthe12o'clockposition.
IMPORTANT
Theexcellentperformancethatisinher-
entinallMclntoshamplifiersdoesnot
dependonthecriticaladjustmentofbiasor
balancecontrolsintheoutputcircuit.The
patentedMclntoshcircuitdeliversitsadver-
tisedspecificationswithoutanyneedfor
thesecontrolsandisnotdependentoncare-
fullybalancedtubesforitsperformance.
WithMclntoshyoucaninstalltheamplifier
andforgetit.
TheMC240youhavepurchasedwillgive
youyearsofpleasantandsatisfactoryper-
formance.Ifyouhaveanyquestionscon-
cerningtheoperationormaintenanceofthis
amplifierpleasecontact:
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.
GUARANTEE
CustomerService
MclntoshLaboratory,Inc.
2ChambersStreet
Binghamton,NewYork
Ourtelephonenumberis723-5491.
Thedirectdialareacodeis607.
8

2CHAMBER SSTREET,BINGHAMTON ,N.Y.
Phone-Are aCode6077235491
LABORATORYINC.
MadeinU.S.A.
PRINTEDINU.S.A.
1M038-002