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