McIntosh MC 7270 User manual

IMPORTANTSAFETYINSTRUCTIONS
THESEINSTRUCTIONSARETO
PROTECTYOUANDTHEMclNTOSH
INSTRUMENT.BESURETO
FAMILIARIZEYOURSELFWITHTHEM.
1.Readallinstructions- Readthesafetyand
operatinginstructionsbeforeoperatingthe
instrument.
2.RetainInstructions- Retainthesafetyand
operatinginstructionsforfuturereference.
3.Heedwarnings- Adheretowarningsand
operatinginstructions.
4.FollowInstructions- Followalloperatinganduse
instructions.
WARNING:TOREDUCERISKOFFIREOR
ELECTRICALSHOCK,DONOTEXPOSETHIS
INSTRUMENTTORAINORMOISTURE.
5.PowerSources- Connectthepowersupplyonlyto
thetypedescribedintheoperatinginstructionsor
asmarkedontheunit.
6.Power-CordProtection- Routepower-supplycords
sothattheyarenotlikelytobewalkedonor
pinchedbyitemsplaceduponoragainstthem,
payingparticularattentiontocordsatplugs,
conveniencereceptacles,andthepointwherethey
exitfromtheinstrument.
7.Ventilation- Locatetheinstrumentforproper
ventilation.Forexample,theinstrumentshould
notbeplacedona bed,sofa,rug,orsimilar
surfacethatmayblockventilationopenings;or,
placedina built-ininstallation,suchasa bookcase
orcabinet,thatmayimpedetheflowofair
throughtheventilationopenings.
8.Heat- Locatetheinstrumentawayfromheat
sourcessuchasradiators,heatregisters,stoves,or
otherappliance(includingamplifiers)thatproduce
heat.
9.WallorCabinetMounting- Mounttheinstrument
ina wallorcabinetonlyasdescribedinthe
ownersmanual.
10.WaterandMoisture- Donotusetheinstrument
nearwater- forexample,neara bathtub,
washbowl,kitchensink,laundrytub,ina wet
basement,orneara swimmingpool,etc.
11.Cleaning- Cleantheinstrumentbydustingwitha
drycloth.Cleanthepanelwitha clothmoistened
witha windowcleaner.
12.ObjectandLiquidEntry- Donotpermitobjectsto
fallandliquidstospillintotheinstrumentthrough
enclosureopenings.
13.PowerLines- Locateanyoutdoorantennaaway
frompowerlines.
14.OutdoorAntennaGrounding- Ifanoutdoor
antennaisconnectedtotheantennaterminal,be
suretheantennasystemisgroundedtoprovide
someprotectionagainstvoltagesurgesandbuilt
upstaticcharge.
IntheU.S.A.,section810oftheNationalElectrical
Code,ANSI/NFPANo.70-1987,provides
informationonthepropergroundforthemast
andsupportingstructure,groundforthelead-in
wiretoanantennadischargeunit,andsizeof
groundconductors,locationofantenna-discharge
unit,connectiontogroundingelectrodes,and
requirementsforthegroundingelectrode.
Forgroundwire:
a)UseNo.10AWG(5.3mm2)copperNo.8 AWG
(8.4mm2)aluminum,No.17AWG(1.0mm2)
copper-cladsteel,bronzewire,orlargerasground
wire.
b)Secureantennalead-inandgroundwiresto
housewithstand-offinsulatorsspacedfrom4 feet
(1.22meters)to6 feet(1.83meters)apart.
c)Mountantennadischargeunitascloselyas
possibletowherelead-inentershouse.
d)UsejumperwirenotsmallerthanNo.6 AWG
(13.3mm2)copperorequivalentwhenseparate
antennagroundingelectrodeisused.

15.NonusePeriods- Unplugthepowercordfromthe
ACpoweroutletwhen(eftunusedfora long
periodoftime.
16.DamageRequiringService- Servicemustbe
performedbyqualifiedservicepersonnelwhen:
A.Thepowersupplycordortheplughasbeen
damaged;or
B.Objectshavefallen,orliquidhasbeenspilled
intotheinstrument;or
C.Theinstrumenthasbeenexposedtorain;or
D.Theinstrumentdoesnotappeartooperate
normallyorexhibitsa markedchangein
performance;or
E.Theinstrumenthasbeendropped,orthe
enclosuredamaged.
17.Servicing- Donotattempttoservicebeyondthat
describedintheoperatinginstructions.Allother
serviceshouldbereferredtoqualifiedservice
personnel.
18.GroundingorPolarization- Donotdefeatthe
inherentdesignfeaturesofthepolarizedplug,
Non-polarizedlinecordadaptorswilldefeatthe
safetyprovidedbythepolarizedACplug.
19.CAUTION:TOPREVENTELECTRICALSHOCKDO
NOTUSETHIS(POLARIZED)PLUGWITHAN
EXTENSIONCORD,RECEPTACLEOROTHER
OUTLETUNLESSTHEBLADESCANBEFULLY
INSERTEDTOPREVENTBLADEEXPOSURE.
NotetoCATVsysteminstaller:
ThisreminderisprovidedtocalltheCATVsystem
installer'sattentiontoArticle820-22oftheNEC
thatprovidesguidelinesforpropergrounding
and,inparticular,specifiesthatthecableground
shallbeconnectedtothegroundingsystemof
thebuilding,asclosetothepointofcableentry
aspractical.
ATTENTION:POURPREVENIRLESCHOCS
ELECTRIQUESPASUTILISERCETTEFICHE
POLARISEEAVECUNPROLONGATEUR,UNEPRISE
DECOURANTOUUNEAUTRESORTIEDE
COURANT,SAUFSILESLAMESPEUVENTETRE
INSEREES A FOND SANS EN LAISSER AUCUNE
PARTIEA DECOUVERT.
Thelightningflashwitharrowhead,withinan
equilateraltriangle,isintendedtoalerttheuserto
thepresenceofuninsulated"dangerousvoltage"
withintheproduct'senclosurethatmaybeof
sufficientmagnitudetoconstitutea riskofelectric
shocktopersons.
CAUTION:TOPREVENTTHERISKOFELECTRIC
SHOCK,DONOTREMOVECOVER(ORBACK).NO
USER-SERVICABLEPARTSINSIDE.REFERSERVICING
TOQUALIFIEDPERSONNEL.
Theexclamationpointwithinanequilateraltriangle
isintendedtoalerttheusertothepresenceof
importantoperatingandmaintenance(servicing)
instructionsintheliteratureaccompanyingthe
appliance.
Theserialnumber,purchasedate,andMclntosh
LaboratoryServiceContractnumberareimportantto
youforpossibleinsuranceclaimorfutureservice.
Recordthisinformationhere.
SerialNumber
PurchaseDate
ServiceContractNumber
Uponapplication,MclntoshLaboratory
providesa ServiceContracttotheoriginal
purchaser.YourMclntoshAuthorized
ServiceAgencycanexpediterepairswhen
youprovidetheServiceContractwiththe
instrumentforrepair.
Copyright1986© byMclntoshLaboratoryInc.
1

Contents
INTRODUCTION3,4,5
INSTALLATION6
HOWTOCONNECT1,8
CONNECTIONS9
FRONTPANELCONTROLS10,11,12
REARPANEL
INFORMATION13
PERFORMANCELIMITS14
PERFORMANCECHARTS15,16
TECHNICALDESCRIPTION17,18
BLOCKDIAGRAM19
YourMC7270StereoPowerAmplifierwillgiveyou
manyyearsofsatisfactoryperformance.
Ifyouhaveanyquestions,pleasecontact:
CUSTOMERSERVICE
MclntoshLaboratoryInc.
2ChambersStreet
Binghamton,NewYork13903-9990
Phone:[phone]
TakeAdvantageof3 years
ofContractService...
FillintheApplicationNOW.
MclNTOSHTHREEYEARSERVICECONTRACT
An application forA THREEYEARSERVICE CONTRACT is included with thismanual.
Thetermsofthecontractare:
1.Iftheinstrumentcoveredbythiscontract
becomesdefective,Mclntoshwillprovideallparts,
materials,andlaborneededtoreturnthe
measuredperformanceoftheinstrumenttothe
originalperformancelimitsfreeofanycharge.The
servicecontractdoesnotcoveranyshippingcosts
toandfromtheauthorizedserviceagencyorthe
factory.
2.AnyMclntoshauthorizedserviceagencywill
repairallMclntoshinstrumentsatnormalservice
rates.Toreceivethefreeserviceundertheterms
oftheservicecontract,theservicecontractcer-
tificatemustaccompanytheinstrumentwhen
takentotheserviceagency.
3.Alwayshaveservicedonebya Mclntosh
authorizedserviceagency.Ifthe instrument is
modified or damaged as a result of unauthorized
repair the service contract will be cancelled.
Damagebyimproperuseormishandlingisnot
coveredbytheservicecontract.
4.Theservicecontractisissuedtoyouasthe
originalpurchaser.Toprotectyoufrom
misrepresentationthiscontractcannotbe
transferredtoa secondowner.
5.UnitsinoperationoutsidetheUnitedStatesand
CanadaarenotcoveredbytheMclntoshFactory
ServiceContract,irrespectiveoftheplaceof
purchase.NorareunitsacquiredoutsidetheUSA
andCanada,thepurchasersofwhichshould
consultwiththeirdealertoascertainwhat,ifany,
servicecontractorwarrantymaybeavailable
locally.
2

TheMclntoshMC7270isthefirstandonlyamplifier
speciallydesignedtofulfillDigitalDynamicRange
demands.Itoutperformsallotherswhenlisteningto
soundderivedfromdigitallyrecordedtapes,recordsand
compactdiscs.TheMC7270hasbeendesignedtoper-
formflawlesslybecauseofthiscapacityforoverload:10
decibelsofoverstressatlessthananaverageof0.3%of
distortion!MclntoshamplifierswithPOWERGUARDare
theonlyamplifierswhichcantolerate10decibelsof
dynamicoverload,withoutseveredistortionbreakup.
Thenoiselevelofdigitallyrecordedsoundis30
decibelsbelowthatofconventionalanalogrecordings.
Thirtydecibelsreductionofnoiselevelmeansthatthe
digitaldiscnoisepoweris1000timeslessthanthaton
thebestanalogrecords.Thecompactdisciscapableof
reallifedynamicrangewhilenoisegeneratedfromcom-
pactdiscsisinaudible.Withthenoiserestraintremovedit
isbotheasieranddramaticallymoreenjoyabletolistento
musicatmuchlouderlevels.Tofullyenjoythisnew
capabilityyouramplifiermustbeabletoreceivethreeto
tendecibelsofoverstressfrommusic,anditmustdothis
withoutseverelydistortingthesound!
Foranamplifiertohandlea three-decibeloverload,it
musthavea fulltimecapacityoftwiceitsfullpower.An
overstressdemandof10decibelsisa demandfor10
timesthefullpowercapacityofanamplifier.Toprovide
2500wattsofoverloadfora 250wattamplifierisexpen-
sivebothintheamplifierandintheloudspeakersystem
aswell.ThisistherealworldofDigitalDynamicsRange
demand.Howtoachievetheperformancedemanded,
whichoftenlastsfromminutestoonlya fewthousandths
ofa second,andtoachievethegoaleconomically,isa
realachievement.IneachMclntoshMC7270,thereisa
unique,patented*,digitalsonicoverloadcorrectorwhich
preventstheamplifierfromexceedinganaverageof0.3%
distortionforoverloadstressesupto10decibels!
Mclntoshhasdevelopeda newmethodofmeasuring
amplifierswhichshowsthiscapabilityverydramatically.
Thisisa testoftheSpectralFidelityofanamplifierunder
stress.
SIMULATINGCOMPLEXCRESCENDODEMANDS
Twotones,14kHzand15kHz,arefedtotheinputof
theamplifierundertest.Theoutputoftheamplifierisfed
toa resistivedummyloadacrosswhichisanA.C.
voltmeter,(toallowcomputationoftheamplifier'soutput
power),andtoa spectrumanalyzerwhichdisplaysthe
magnitudeandfrequenciesofsignalsattheamplifierout-
put.
Ideally,onlythetwotesttonesshouldbereproducedat
theamplifieroutput.Someamplifiersgeneratea broadin-
termodulationspectrumoftonescorrespondingtothe
sumanddifferenceofthetesttones.Thesespurious
tonesarecalledIntermodulationDistortion.
Allamplifiers(MclntoshamplifierswithPowerGuardare
theonlyexception),producesucha spectrumwhenthey
aredrivenbeyondtheiroutputcapacity.TheyproduceIn-
termodulationandHarmonicDistortion,theseverityof
whichdependsontheamountofoverdriveandthe
designoftheamplifier.Distortionof30%orgreaterisnot
uncommon.Mostofthespuriousfrequencycomponents
areathighfrequenciesandcaneasilydestroythetweeter
sectionsofloudspeakers,whileseverelydistortingthe
sound.
*U.S.Patent#4048573
INTRODUCTION3

Intheseoscillograms,youcanseethedifferencein
SpectralFidelitybetweena stressedMclntosh,andother
amplifiers.
1.TheMclntoshstressed10dBabovefullpower
2.Animportedamplifierstressed10dBabovefullpower
3.A domesticallymanufacturedamplifierwhichhadtobe
tested"under-stressed"sinceitwouldnotperformwith
10dBofoverdrive.
TheMclntoshshowsonly3 components,whichare44
foroneand50dBdownforallothers,roughlyequivalent
to0.3%distortion.Theotheramplifiershows17discords,
someofwhichareonly10dBdown,or30%distortion,
withmanylessthan30dBdown,or3%distortion.
ThisiswhyMclntoshcustomersconsistentlyfindthat
theirMclntoshalonemeetsthetestforaccuracy,thetest
forclarity,thetestformusicalityevenwhena peakoften
timesthepowerdemandsuddenlysmashesintothe
poweramplifier.
Whenotheramplifiersaresimilarlystressedthey
generatelargequantitiesofdiscordantsoundsdestroying
therealmusicalityofthereproducedinstruments.
Noteinoscillogramnumber3 thecompletefailureof
oneofthemostpopularofAmericanamplifiers.If14and
15kHzareamplifiedatthesametimetheamplifiershuts
downby"motorboating".Itwasnecessarytolowerthe
frequencyofthetestsignalsto10and11kHztoeven
completethetestortoperformthetestatlessthanhalf
ratedpoweronthisdomesticallymanufacturedamplifier.
ItisnoaccidentthatMclntoshamplifierssoundbetter.
Itisnoaccidentthata Mclntoshisa betterinvestment.
•Itsoundsbetter
•Itismorereliable
•Itlastslonger
•Itsresalevalueisthehighest
Ifgoodenoughwilldo,thereareatleast100answers
foryou.Butifthebestiswhatyouneedthenthereison-
lyonerealanswer.
Mclntosh—theamplifierthatin40yearshasoutlived60
otherswhohavesimplyfadedaway.
4INTRODUCTION

ANEWAMPLIFIERDESIGNEDFOR
ANEWAGEOFHOMEENTERTAINMENT
TheMC7270stereopoweramplifierisdesignedto
operatewithloudspeakershavinga nominalimpedance
of1,2,4,or8 ohms.Theamplifierisruggedandreliable.
ThemechanicalandelectricaldesignoftheMC7270is
theresultofthemanyyearsofengineeringandmanufac-
turingexperiencebythedesignersatMclntosh.This
"knowhow",combinedwithmeticulousattentionto
designandproductiondetails,makestheMC7270oneof
thefinestproductsproducedbyMclntoshLaboratory.
Somemanufacturersofpoweramplifiersadvertisethat
theirproductsdonotuseprotectioncircuitsandthatsuch
circuitscompromiseperformance.Itisindeedpossiblefor
suchcircuitstocausesubstantialamountsofdistortion
andundesirablelisteningeffectswhichistrueofalmost
anyprotectivecircuitdesign.Therealgeniusofgood
designrecognizestheseproblemsandcircumventsthem
whileretainingtherealmeritsoftheprotectivecircuits.
Thesearejustsomeoftheextravaluesyoureceivewhen
youinvestinMclntoshequipment.Itispreciselyforthis
reasonthatittakeslongertocompletea Mclntosh
engineeringdesigntask.Fromsuchengineeringdedication
comestheMclntoshreputationforhighestsoundquality
withgreatestlongtermreliability.
TheMC7270incorporatessevenprotectioncircuits
whichinsureitstotalreliability,sevenprotectioncircuits
thatprotectthemusicandyourlistening,notinterfere
withthem.ThesecircuitsaredescribedintheTechnical
Description.
INTRODUCTION5

TheMC7270maybeinstalledina Mclntoshcabinetor
custominstalledinfurnitureofyourchoice.Alwayspro-
videadequateventilation.Makesurethereis1½inches
(3.8cm)ofspaceabovetheamplifiersoasnottoin-
terferewitha coolingflowofair.
CUSTOMINSTALLATION
ThePANLOCsystemofinstallingequipmentconvenient-
lyandsecurely,isa productofMclntoshresearch.Turned
clockwise,thePANLOCbuttonsonthefrontpanellock
thechassisfirmlyinplace.A counterclockwiseturnofthe
PANLOCbuttons unlocksthechassisfromitsmounting.
Toinstalltheinstrumentina Mclntoshcabinet,follow
theinstructionsthatareenclosedwiththecabinet.For
anyothertypeofinstallationfollowtheseinstructions:
1.UnpackfromCarton
OpenthecartonandremovethePANLOCbrackets,hard-
warepackage,andmountingtemplate.Removethein-
strumentfromitsplasticbagandplaceitupsidedownon
theshippingpallet.Unscrewthefourplasticfeetfromthe
bottomofthechassis.
2.MarktheCabinetPanel.
Tapethemountingtemplateinpositiononthecabinet
panelwheretheinstrumentistobeinstalled.Thebroken
linesthatrepresenttheoutlineoftherectangularcutout
alsorepresenttheoutsidedimensionsofthechassis.
Makesuretheselinesclearshelves,partitions,orany
equipment.Withthetemplateinplace,firstmarkthesix
AandB holesandthefoursmallholesthatlocatethe
cornersofthecutout.Then,jointhefourcornermarkings
withpencillines,usingtheedgeofthetemplateasa
straightedge.
3.DrillHoles
Usea drillwitha 3/16inch(5mm)bitheldperpendicular
tothepanelanddrillthesixA andB holes.Then,usinga
drillbitslightlylargerthanthetipofyoursawblade,drill
oneholeateachoftwodiagonallyoppositecorners.The
holesshouldbarelytouchtheinsideedgeofthepenciled
outline.Beforetakingthenextstep,makesurethatthesix
AandB holeshavebeendrilled.
4.SawthePanelCutout
Saw,carefully,ontheinsideofthepenciledlines. First
makethetwolongcutsandthenthetwoshortcuts.
Aftertherectangularopeninghasbeencutout,usea file
tosquarethecornersandsmoothanyirregularitiesinthe
cutedges.
5.InstalltheMountingStrips
Inthehardwarepackageyouwillfindtwomounting
strips,andtwosetsofmachinescrews.Forpanelsthat
arelessthan1/2inch(12.7mm)thick,usethe3/4inch
(19.1mm)screws;forpanelsthataremorethan1/2inch
(12.7mm)thick,usethe1-1/4inch(31.8mm)screws.
Startingattheright-handsideofthepanel,inserta
screwoftheproperlengthintothecenterholeinthe
panel,markedB onthetemplate.Onthebackofthe
panel,aligna mountingstripwiththeholesinthepanel
andtightenthescrewuntilthescrewheadispulledinto
thewood.
Repeatthisproceduretoattachthemountingstripto
theleftsideof thepanel.
6.AttachthePANLOCBrackets
UsingtwoscrewsoftheproperlengthintheA holeson
eachside,attachthePANLOCbracketstothecabinet
panel;theshortflangeismountedagainstthefront(face)
ofthecabinetpanel.Thescrewspassthroughthe
PANLOCbracketflange,thecabinetpanel,andthen
throughthemountingstripspreviouslymounted.
7.InstalltheInstrument
GuidetheACpowercordthroughthepanelopeningto
thebackofthecabinet;then,slidetheinstrumentinto
theopeningcarefullysothattherailsonthebottomof
eachsideofthechassisengagethetracksonthemoun-
tingbrackets.Continuetoslidetheinstrumentintothe
cabinetuntilthefrontpanelisflushwiththecabinet
panel.TurnthePANLOCbuttonsatthelowerleftand
rightcornersoftheinstrumentpanelclockwisetolock
theunitfirmlyinthecabinet.TurnthePANLOCbuttons
counterclockwisetounlocktheinstrument.Itcanthen
slideoutwardtopermittheremovaloftheinstrument
fromthecabinet.
6INSTALLATION
MOUNTINGTEMPLATE

Useshieldedcablestoconnectthesignalfromthe
preamplifierorsignalsourcetothepoweramplifier.To
minimizethepossibilityofhum,theshieldedcables
shouldbeofparallelconstructionorlooselytwisted
together,locatedawayfromspeakerconnectingcables
andACpowercords.Becertaintousegoodquality
shieldedcablesforallinterconnections.Yourdealercan
adviseyouonthekindandlengthofcablethatwillbest
suityourinstallation.
Theappropriatelengthandsizeofloudspeakercablefor
yourinstallationwillhelptopreservethequalityofsound
forwhichtheloudspeakershavebeendesigned.Ifunder-
sizewireisuse,resistanceisaddedtotheamplifier/
loudspeakerscombinationwhichadverselyaffectstheper-
formance.Addedresistancereducesthedampingfactor,
modifiesthefrequencyresponseandreducesthepower
output.Yourdealer'sadvicewillserveyoubestforyour
installation.Thecablestoandfromthespeakershouldbe
ofparallelconstructionorbelooselytwistedtogether.
Thechartshowstherecommendedminimumwiresize
forthelengthofwirebetweentheamplifierandthe
loudspeakers.
SPEAKERCABLELENGTHS
AMPLIFIERTOSPEAKER
For4 OhmLoad
Feet
15
25
40
60
100
Meters
4.6
7.6
12.2
18.3
30.48
For8 OhmLoad
Feet
30
50
80
120
200
Meters
9.1
15.2
24.4
36.6
60.0
Wire
Gauge
18
16
14
12
10
Thesespeakercablelengthsrepresenta wire
resistanceequalto5%ofthespeakerimpedance.
CONNECTINGFORSTEREOOPERATION
PlugtheleftoutputofthepreamplifierintotheLeftinput
jackofthepoweramplifier.Plugtherightoutputofthe
preamplifierintotheRight(Mono)inputjackofthepower
amplifier.
SettheMODEswitchtotheSTEREOposition.
Connecta speakercablefromthecommonterminalof
theleftspeakertotheamplifierLEFTOUTPUTterminal
stripCOMmonscrew.Connectanotherspeakercable
fromtheotherloudspeakerterminaltothecorrect
speakerimpedanceterminalontheLEFTOUTPUTter-
minalstrip.Therightchannelspeakerisconnectedinthe
samemannertotheRIGHTOUTPUTterminalstrip.
Whenmultiplespeakersaretobeconnectedtoeither
orbothoutputs,thecombinedimpedancemustbe
calculated.Thecombinedimpedancemustbeconnected
totheappropriateimpedancetap.Usethistabletoaidin
selectingthecorrectimpedancematch:
Combinedimpedanceinohms:
Connectfor:
0.8to1.61 ohmoutput
1.6to3.22 ohmoutput
3.2to6.44 ohmoutput
6.4andup8 ohmoutput
Iftheloadimpedanceusedislowerthantheoutputim-
pedancetap,reducedpowerandpossibledistortionwill
result.Iftheloadimpedanceusedishigherthantheout-
putimpedancetap,neitherthequalityofsoundnorthe
amplifierwillbeharmed,onlytheavailablepoweris
reduced.Formultiplespeakeroperation,runseparate
speakercablesfromtheamplifiertoeachspeaker.
TheMC7270willfeeda constantvoltagelineasisoften
usedinbackgroundmusicapplications,pagingsystemsand
thelike.Forconstantvoltageof25voltsconnectoutputleads
tothe4 ohmconnectorsoneachchannel.
Becausethecrosstalkbetweenchannelsisalmostnon
existent,eachchannelcanbeusedasa separate
amplifier.(Example;useonechannelformono
backgroundprograminoneareaandtheotherchannel
forpagingina separatearea.)
CONNECTINGFORMONOPHONICBRIDGEORSINGLE
CHANNELOPERATION
WhentheMC7270isusedasa monophonicorsingle
channelpoweramplifier,thetwochannelsareaddedina
bridgeconfigurationtoproduceoutputupto540watts.
FormonophonicoperationusingtheMONO-BRIDGE
mode,theamplifiercansupplyfullpowertooutputim-
pedancesof2,4,8 and16ohms.
Pluga shieldedcablefromthesignalsourceor
preamplifiertotheRIGHT(MONO)inputjackonly.
ChangetheMODEswitchonthebackpanelofthe
amplifiertotheMONO-BRIDGEposition.TheMONO-
BRIDGEswitchinternallyconnectstheoutputoftheright
channelinputamplifiertobothleftandrightchannel
poweramplifierswiththephaseoftheleftchannelin-
vertedtoachievebridgeoperationandincreasedpower
output.
BECERTAINTHE MC7270ISNEVEROPERATEDWITHTHE
BACKPANELMODESWITCHINTHESTEREOPOSITION
WITH THE AMPLIFIER CONNECTED FOR MONO BRIDGE.
OperatedwiththeMODEswitchintheMONO-BRIDGE
position,thecircuitsubtractstheleftchannelfromthe
rightchannelL-R).Theremainingdifferencesignalisall
thatisthenfedtothespeaker.
HOWTOCONNECT7

TheMONO-BRIDGEoperationoftheMC7270amplifier
alwaysrequiresthespeakerloadtobeconnectedbe-
tweentwooppositechannelimpedanceterminals.The
commonterminalsarenotused.Donotconnectleadsin-
dependentlybetweenleftandrightchannelimpedance
tapsasthiswillcausehighcirculatingcurrentsand
overheating.Connectthespeakerleadstotheoutputter-
minalsaslistedinthischart:
LoadImpedanceConnecttheConnectthe
InOhms-SpeakerCablesto:+SpeakerCablesto:
2Left1 OhmTerminalRight1 OhmTerminal
4Left2 OhmTerminalRight2 OhmTerminal
8Left4 OhmTerminalRight4 OhmTerminal
16Left8 OhmTerminalRight8 OhmTerminal
Neitheroutputterminalisatgroundpotential.
TousetheMC7270tofeeda linewhoseconstant
voltageis25volts,connectthespeakercablesfor2 ohm
MONO-BRIDGEoutput.
8HOWTOCONNECT

STEREOPHONIC
CONNECTIONS
MUSTBEIN
STEREOPOSITION
FORSTEREOPROGRAMS
RIGHT
SPEAKER
MONO-BRIDGED
CONNECTIONS
(CONNECTIONSFOR
8OHMSPEAKERILLUSTRATED)
CONNECTIONSSHOWNFORMONO
WHENMODESWITCH
ISINMONO-BRIDGEPOSITION
CONNECTIONS9
PROGRAMSOURCE
PROGRAMSOURCE
SPEAKER
LEFT
SPEAKER

METERS
Outputpowermonitormeterscalibratedinbothwatts
anddecibels,indicatetheoutputpowerofeachchannel.
Theupperscaleonthemeterhasbeencalibratedto
showaveragewattsandthelowerscale,decibels.
Readingfromrighttoleft,themarksbetweenthe
numbersindicatingwattsare(fromtheindicated270
watts):firstmark,108watts.Second54watts,thein-
dicated27watts,10.8watts,5.4watts,theindicated2.7
watts,1.08watts,0.54watts,theindicated0.027watts,
0.0108watts,.0054watts,theindicated0.0027watts,
0.00108wattsand0.00054watts.
Themetersrespondtothepeakoutputofeachchannel.
Ordinarymeterslackthecapabilityofindicatingtheshort
intervalinformationina soundwave.Themassofthe
metermovementistoogreattorespondtothenearlyin-
stantaneouschangesinmusicprogrammaterial.Shortin-
tervalinformationcanhavea durationasshortashalfa
thousandthofa second.Ordinarily,a meterpointermov-
ingoveritsscaleinsucha shorttimecouldnotbeseen.
Mclntoshhasdevelopedcircuitsthatdrivethemetersto
respondtotheshortintervalinformationina soundwave
toanaccuracyof90%.Theelectricalpulsethatdrivesthe
meterpointeristimestretchedsothatthepeakposition
ofthepointercanregisterinthepersistenceofvision
characteristicoftheretinaofthehumaneye.Thepointer
canbelockedatthepeakindicationbyswitchingtothe
HOLDpositionoftheMETERrangeswitch.
LEFTCAIN
Usethe LEFT GAINcontrol to adjust the output inthe
leftchanneltothedesiredlisteninglevel.Turnthecontrol
clockwisetoincreasetheoutput.
RIGHT/MONOGAIN
UsetheRIGHT/MONOGAINcontroltoadjusttheout-
putintherightchanneltothedesiredlisteninglevel.Turn
thecontrolclockwisetoincreasetheoutput.
Whenconnectedformonophonicoperationusethe
RIGHT/MONOGAINtoadjustthetotaloutputtothe
desiredlisteninglevel.TherearpanelMODESWITCH
mustbeintheMONO-BRIDGEposition.
10FRONTPANELCONTROLS

METERRANGE
TheMETERRANGEswitchhastwopositions:WATTS
andHOLD.
WATTS
IntheWATTSposition,themeterneedleindicatesthe
variationsinprogramloudness.Althoughtheprimaryout-
putcalibrationofthemetersisfrom0.0027watts(2.7
milliwatts)upto270watts,theratedpoweroutputofthe
MC7270,theadditionalindicationtotherightofthe270
wattsmark,is540watts.WhiletheMC7270cannot
reachthispowerlevelcontinuouslyitispossibleforshort
intervalpeakstoconsiderablyexceedthe270wattscon-
tinuousrating.
HOLD
IntheHOLDposition,themeterneedlelockstothe
highestpowerpeakina sequenceofpeaks.Themeteris
driventomaximumpower,electronicallyheldthereuntil
ahigherpeakpassesthroughtheamplifier,whichmoves
themeterneedletoa newindication.Ifnofurtherpeaks
arereachedthemeterneedlewillveryslowlyreturnto
itsrestposition(decayrate:6 dBperminute).
POWER
ThepowerswitchturnstheMC7270ONorOFF.The
switchdoesnotcontrolthepoweroutletontheback
panel.IfyouwishtocontroltheACpowerfroma
preamplifiercontrolcenter,leavetheswitchintheON
position.BesuretheACcordoftheMC7270isplugged
intothecontrolledoutletsontherearofthepreamplifier
controlcenter.
OFF:IntheOFFpositiontheACpowertotheamplifier
isturnedoff.
FRONTPANELCONTROLS11

THEMclNTOSH(EXCLUSIVE)POWERGUARD
DIGITALDYNAMICSPROTECTIONCIRCUIT
Improvedrecordingsandrecordingtechniqueshaveim-
posedhigherpowerdemandsontoday'samplifiers.Poor-
lydesignedamplifierscanpresentmusiclistenerswitha
formofharshunpleasantdistortionduetoamplifier
overload(hardclipping).Clipping,whichlooksandacts
likenonmusicalsquarewaves,iscausedwhenthe
amplifierisaskedtoproducemorepoweroutputwith
lowdistortionthanitiscapableofordesignedtodeliver.
Amplifiers,whendriventoclipping,candeliverupto40%
harmonicandintermodulationdistortionthatdecreases
thepleasureandenjoymentyougetfromlistening.This
formofdistortion(clippedsignal)alsoproducesextraheat
energywhichwilldamagemostspeakers.Mclntosh
leadershipinengineeringhasdevelopedthePowerGuard
circuitwhich- (1)dynamicallypreventspoweramplifiers
frombeingoverdrivenintohardclipping- (2)assuresthat
theamplifierwillproduceitsmaximumoutputwithoutin-
creaseddistortion- (3)protectsyourspeakerfromex-
cessiveheating.PowerGuardisa patentedMclntosh
design(U.S.patent#4048573).
TheMC7270hasa circuitthatcomparesthewave
shapeoftheoutputsignaltotheinputsignal.Ifthe
disparitybetweenthetwosignals,duetooverdrive,ex-
ceedsanaverageof0.3%(equivalentto0.3%totalhar-
monicdistortion)anamberPOWERGUARDindicatoril-
luminates.Withanyfurtherincreaseindistortionthe
POWERGUARDcircuitoperatestolimittheamplifierin-
putdynamicallysothattheamplifiercannotbeover-
driven.POWERGUARDeliminatesamplifieroutputclip-
ping.POWERGUARDonlyoperateswhentheamplifieris
askedtodelivermorepowerthanthatforwhichitwas
designed.Whilethepoweroutputremainswithinthese
limitsthePOWERGUARDindicatorsdonotilluminate.
12FRONTPANELCONTROLS

MODESWITCH
TheMC7270willoperateintwomodes,Stereoand
MonoBridge.
LEFTANDRIGHTOUTPUTTERMINALS
Forstereooperation,outputimpedancesof1,2,4 and
8ohmshavebeenprovidedona secure,screwtypebar-
rierstrip.Forrnonophonicoperationproperinterconnec-
tionprovides2,4,8,and16ohmsatthesamebarrier
strips.Seepage7 forconnectinginstructions.
INPUTJACKS
Inthestereomodeofoperation,bothLeftandRightin-
putjacksfunction.Inthemonomodeofoperationonly
theRight(MONO)channelinputjackacceptssignal.In
mono,theLeftchannelinputjackisdisconnected.
INPUTLEVEL
TheinputsensitivityoftheMC7270is0.75voltsor
2.5Vdepending onthe position ofthe INPUT LEVEL
switch.ForthebestsignaltonoiseratiowhenusingMcln-
toshsource equipment, place the INPUT LEVEL switch in
the 2.5Vposition andthe frontpanel LEFT and
RIGHT/MONOGAINcontrolsinthefullyclockwiseposi-
tion.Ifmoregainisdesiredthe0.75Vpositionmaybe
used.AllMclntoshpreamplifiershavebeendesignedto
deliver2.5voltsoutputwithratedinput.Forsource
equipmentotherthanMclntosh,settheswitchinthe
positionnearesttothestatedoutputratingofthesource
equipment.
ACPOWER
TheinputtotheMC7270is120volts50/60Hzatupto
12amps.Theprimarycircuitisprotectedbya 15Amp
Fuse.
REARPANELINFORMATION13

PERFORMANCEGUARANTEE
PerformanceLimitsarethemaximumdeviationfrom
perfectionpermittedfora Mclntoshinstrument.Wepro-
miseyouthatwhenyoupurchasea newMC7270froma
Mclntoshfranchiseddealer,itwillbecapableoforcanbe
madecapableofperformanceatorexceedingtheselimits
oryoucanreturntheunitandgetyourmoneyback.
Mclntoshistheonlymanufacturerthatmakesthisstate-
ment.
PERFORMANCE
Mclntoshaudiopowerratingsareinaccordancewith
theFederalTradeCommissionRegulationofNovember4,
1974concerningpoweroutputclaimsforamplifiersused
inhomeentertainmentproducts.
POWEROUTPUT
STEREO
270wattsminimumsinewavecontinuousaveragepower
output,perchannel,bothchannelsoperatinginto1 ohm,
2ohms,4 ohms,or8 ohmsloadimpedance,whichis:
16.4voltsRMSacross1 ohm
23.2voltsRMSacross2 ohms
32.9voltsRMSacross4 ohms
46.5voltsRMSacross8 ohms
MONO-BRIDGE
540wattsminimumsinewavecontinuousaverage
poweroutputinto2 ohms,4 ohms,8 ohms,or16ohms
loadimpedance,whichis:
32.9voltsRMSacross2 ohms
46.5voltsRMSacross4 ohms
65.7voltsRMSacross8 ohms
93.0voltsRMSacross16ohms
OUTPUTLOADIMPEDANCE
STEREO
1ohm,2 ohms,4 ohms,and8 ohms;separateterminals
areprovidedforeachoutput.
MONO-BRIDGED
2ohms,4 ohms,8 ohms,or16ohms,balancedto
ground.
RATEDPOWERBAND
20Hzto20,000Hz
TOTALHARMONICDISTORTION
STEREO
0.02%maximumharmonicdistortionatanypowerlevel
from250milliwattsto270wattsperchannelfrom20Hz
to20,000Hz,bothchannelsoperating.
MONO
0.02%maximumharmonicdistortionatanypowerlevel
from250milliwattsto540wattsfrom20Hzto20,000
Hz.
INTERMODULATIONDISTORTION
STEREO
0.02%maximumifinstantaneouspeakpower
outputis540wattsorlessperchannelwithboth
channelsoperatingforanycombinationoffre-
quencies,20Hzto20,000Hz,
MONO
0.02%maximumifinstantaneouspeakpower
outputis1080wattsorlessforanycombinationof
frequencies,20Hzto20,000Hz.
FREQUENCYRESPONSE(ATONEWATTOUTPUT)
20Hzto20,000Hz+0-0.25dB.
10Hzto100,00Hz+0- 2 dB.
NOISEANDHUM
100dBbelowratedoutput.
RATINGS
DAMPINGFACTOR
Greaterthan30
INPUTIMPEDANCE
20,000ohms.
INPUTSENSITIVITY
Switchable:0.75voltor2.5volts-levelcontrolprovided
forhigherinputvoltages.
POWERGUARD
ClippingispreventedandTHDdoesnotexceed2%with
upto20dBoverdrivea 1 kHz.
GENERALINFORMATION
POWERREQUIREMENTS
120volts50/60Hz;13amps.
SEMICONDUCTORCOMPLEMENT
79silicontransistors
31siliconrectifiersanddiodes
7integratedcircuits
MECHANICALINFORMATION
SIZE
163/16incheswide(41.1cm)by7 1/8incheshigh(18.1
cm)by141/2inchesdeep(36.8cm),includingconnec-
tors.Knobclearancerequiredis1 1/4inches(3.2cm)in
frontofmountingpanel.
FINISH
Thefrontpanelisa combinationofglassandblack
anodizedaluminum.Thechassisisblack.
WEIGHT
82pounds(37.2kg)net,96pounds(43.5kg)inshipping
carton.
14PERFORMANCELIMITS

PERFORMANCECHARTS15
20
FREQUENCYINHz
1001k 10k 20k
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
FREQUENCYINHz
1001k10k
FREQUENCYINHz
1001k
MC7270
HARMONICDISTORTIONVSFREQUENCY
ATRATEDPOWER
MC7270
FREQUENCYRESPONSEAT1 WATTOUTPUT
MC7270
CHANNELSEPARATIONVSFREQUENCY
OUTPUTIMPEDANCE:8
INPUTIMPEDANCE:1k
0.05
0.04
0.03
0.02
0.01
0
HARMONIC
DISTORTION
IN%
20
10k20k
0
-1
-2
-3
-4
-5
-6
-7
-8
-9
-10
10
OUTPUT
INdB
100k
OUTPUT
INdB

0.1
POWEROUTPUTINWATTS
1000
0-1
POWEROUTPUTINWATTS
1000
16PERFORMANCECHARTS
1.010100
0,06
0.05
0.04
0.03
0.02
0.01
0
IM
DISTORTION
IN%
MC7270
INTERMODULATIONDISTORTIONVSPOWEROUTPUT
BOTHCHANNELSDRIVEN
FREQUENCYINHz
1001k10k
MC7270POWEROUTPUTVSFREQUENCY
0.02%HARMONICDISTORTION
BOTHCHANNELSOPERATINGINTO8 OHMS
110100
0.05
0.04
0.03
0.02
0.01
0
HARMONIC
DISTORTION
IN%
MC7270
HARMONICDISTORTIONVSPOWEROUTPUT
BOTHCHANNELSOPERATINGINTO8 OHMS
20kHz
1kHz
20Hz
300
400
300
200
100
0
10
POWER
OUTPUT
IN
WATTS
100k

INPUTAMPLIFIER
Eachchannelinputcontainsa completeseventransistor
lowpoweramplifier.A differentialtransistorpairprovides
highinputimpedancewithlownoise.Thedifferential
signalsarecombinedina currentmirrorcircuitwhich
drivesa classA amplifierstage.Thefollowingoutput
stageisa complimentarypairoftransistorswithclassAB
biasing.Theoutputsignaldrivesthemeteringcircuitand
thehighpoweroutputamplifier.Thisdiscretetransistor
inputamplifierdesignhaslownoise,lowdistortionand
freedomfromturn-on/turn-offtransients.
TheINPUTLEVELselectorandGAINcontrols(aspassive
attenuators)whichprecedetheinputamplifier,preventin-
putoverloadedwhenthecontrolsarecorrectlyset.
IntheMONOBRIDGEmodeofoperationtheinput
signalfeedsonlytherightinputamplifierusingthe
RIGHT/MONOGAINcontrol.Theoutputoftherightinput
amplifierfeedsbothoutputpoweramplifiersectionswith
theleftinputamplifierasa phaseinvertertotheleftout-
putamplifier.Thephaseinversionputstheoutput
amplifiers180°outofphasewithrespecttoeachother
toprovidea bridgeoutput.Thechannels,ofcourse,
operateinphasefortheSTEREOmode.
OUTPUTPOWERAMPLIFIER
Theoutputoftheinputamplifierisfedtothepower
amplifierinputthrougha junctionFETelectronicswitch.
Thecontrolsignaltotheswitchisheldforaboutonese-
condtoeliminateturn-on/turn-offtransientsandprevent
transientsthatmayoriginateinthesourceequipment
frombeingamplifiedandfedtotheloudspeakers.
Thefirststageoftheoutputpoweramplifierisa dif-
ferentialtransistorpairbiasedforbestlinearity.Theoffset
tothedifferentialpairisadjustable.Correctadjustment
assuresthelowestpossibledistortionatlowfrequencies.
Ineachchannel,currentmirrorcircuitcombinesthedif-
ferentialoutputsintoonesignalwhichisamplifiedbya
classA voltageamplifier.Boththedifferentialtransitors
andthevoltageamplifieraresuppliedbyactivecurrent
sources.Theresultsarelowerdistortionandcleanerturn-
oncharacteristics.
Thedriverstageconsistsofa complimentarypairof
powertransitorsbiasedclassAB.Threecomplimentary
pairsofruggedpowertransistorsmountedonover-sized
anodizedaluminumheatsinks,makeupthepowerout-
putstage.A uniquedesignof thebiasnetworkpermits
theoutputtransistorstooperateclassB butpreventsthe
crossoverdistortionassociatedwithclassB operation.The
heatsinksremaincoolwhenthereisnooutput.
Theamplifieroutputsignalisfedtotheoutputterminals
throughtheoutputautotransformer.TheMclntoshdesign-
edinterleavedmultifilarwoundautotransformerisusedto
properlymatchtheamplifiertostereooutputloadtaps
for1,2,4 and8 ohms.TheMC7270willdeliverfull
powerovertheentireaudiofrequencyrangeatanyof
theseimpedances.Theautotransformeralsoprotects
speakersfromdamageintheeventofamplifierfailure.
Shoulda directcurrentcomponentappearintheoutputit
isshuntedtogroundbytheautotransformer.DCcannot
damagethespeaker.
AMclntoshpatentedSentryMonitoringcircuitconstantly
monitorstheoutputsignalandinstantlyreactstoprevent
overloadoftheoutputtransistors.Atsignallevelsupto
designmaximumthecircuithashighimpedanceandhas
noeffectupontheoutput.Shouldthepoweroutputex-
ceeddesignmaximum,theSentryMonitoringcircuit
operatestolimitthesignaltotheoutputtransistors.In
theeventofa shortcircuitacrosstheamplifieroutputor
severeimpedancemismatchtheSentryMonitoringcircuit
willprotecttheoutputtransistorsfromfailure.Positive
andnegativehalvesoftheoutputsignalareindependent-
lymonitoredandprotected.
POWERGUARDPROTECTIONCIRCUIT
TheMclntoshpatentedPowerGuardcircuiteliminates
amplifierclippingduetooverdrive.Thecircuitilluminates
amberPOWERGUARDindicatorlampswhenthe
amplifierdrivetrystoexceedthemaximumoutputcapaci-
ty.PowerGuardpreventsloudspeakerdamageand
eliminatesharshoutputdistortioncausedbyamplifierclip-
ping.
InthePowerGuardcircuit,theoutputwaveformiscom-
paredtotheinputwaveform.Aslongasthereisn'tany
disparitybetweenthesesignals,thecircuithasn'tanyin-
fluenceontheperformanceoftheamplifier.Shouldthe
amplifierdrivetrytoexceedtheamplifier'smaximum
powercapacity,a differencewilldevelop.Ifthedisparity
exceeds0.03%ontheaverage,(equivalentto0.3%total
harmonicdistortion)thedifferencecausestheamber
POWERGUARDindicatorstolight.A furtherincreasein
thedisparity,controlsanelectronicattenuatoratthe
amplifierinputtoreducetheamplifiergain,thusholding
theamplifieroutputtoitsmaximumundistortedvalue
regardlessofthedegreeofoverdrivetotheamplifier.
Thedifferencesignalisfedtoa speciallycompensated
operationalamplifierintegratedcircuit.Itsoutputis
detectedbya fullwavebridgethatfeedssignalstothe
controlcircuitryforthePOWERGUARDindicatorsandto
theelectronicattenuatorattheamplifierinput.Theat-
tenuatorisa lightemittingdiode/lightdependentresistor
networkselectedspeciallyforitslowdistortionandtime
constantcharacteristics.
TECHNICALDESCRIPTION17

METERCIRCUIT
Themetercircuithasthreebasicsections:a logarithmic
amplifier,a fullwaverectifier,anda DCamplifier.
Thelogarithmicamplifierconsistsofa highgainopera-
tionalamplifierwitha bipolarconnectedsilicondiode
pairasfeedbackelements.Thesediodeshavea uniform
logarithmiccharacteristicoveran 80 dB range.Only60 dB
ofthislogarithmicrangeisusedintheMC7270.
Thefullwaverectifiercircuitusesanoperationalampli-
fierwithsilicondiodefeedbacknetworks.Thisamplified
diodecircuithasnearlyperfectrectificationcharacteristics.
Onerectifierdetectsonlypositivesignals.Theother
respondsonlytonegativesignalsandproducesa positive
output.Theoutputsoftherectifiersarecombinedatthe
operationalamplifieroutput,so thehighestsignal,either
positiveornegative,istheonethatisindicatedbythe
meters.Gatediodesareusedtochargea lowleakage
capacitorwhichattainsandholdsa chargeduringsignal
peaks.Theoperationalamplifierprovidesa largeamount
ofcurrentsothepeakholdingcapacitorcanchargesud-
denly.Thechargeonthepeakholdingcapacitoris
amplifiedina twotransistorDCamplifierwhichisusedto
drivethemeter.Theoutputoftheamplifierhasa DC
feedbacknetworkconnectedtothedetectortoassureex-
cellentoveralllinearityandfrequencyresponse.Thecur-
rentdrivetothemetershasa peakingcapacitortoac-
celeratetheupscaleresponseofthemeterneedle.The
metersalsohavea parallelshuntresistortocorrectly
damptheiraction.IntheWATTSmodethedischargeof
thepeakholdingcapacitoriscontrolledbya resistorcur-
rentsource.InWATTSHOLD,theresistorisdisconnected
sothepeakreadingisretained.Therateofdecayin
WATTSHOLDisabout6 dBperminute.
POWERSUPPLY
Thepowersupplyisa fullwavebridgerectifierarrange-
mentprovidingplusandminus52voltsDC.Electronic
regulatorsstepdownandregulateplusandminus15volt
sourcesforlowlevelcircuitsandplus5 voltsforthelogic
circuits.Thermistorsareusedinthepowertransformer
primarycircuittolimittheturn-oncurrent.
18TECHNICALDESCRIPTION