McIntosh MC 502 User manual

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THEMclNTOSHMC502POWERAMPLIFIER

VARIOUSREGULATORYAGENCIESREQUIRETHATWEBRINGTHE
FOLLOWINGINFORMATIONTOYOURATTENTION.PLEASEREADIT
CAREFULLY.
WARNING:TOPREVENTFIREORSHOCK
HAZARD,DONOTEXPOSETHISUNITTO
RAINORMOISTURE.
TheMclntoshyouhavepurchasedisa ModelMC
502.Ithasa serialnumberlocatedontherearpanel
ofthechassis.Recordthatserialnumberhere:
SerialNumber
Themodel,serialnumberandpurchasedateare
importanttoyouforanyfutureservice.Recordthe
purchasedatehere:
PurchaseDate
Uponapplication,MclntoshLaboratoryprovidesa
Three-YearServiceContract.YourMclntosh
authorizedServiceAgencycanexpediterepairs
whenyouprovidetheServiceContractwiththein-
strumentforrepair.Toassist,recordyourService
Contractnumberhere:
ServiceContractNumber

YourMC502StereoPowerAmplifier
willgiveyoumanyyearsofpleasantand
satisfactoryperformance.Ifyou
haveanyquestions,pleasecontact:
Contents
CUSTOMERSERVICE
MclntoshLaboratoryInc.
2ChambersStreet
Binghamton,NewYork13903
Phone:[phone]
TakeAdvantageof3 years
ofContractService...
FillintheApplicationNOW.
HOWTOINSTALL...2,3
HOWTOCONNECT...4
THEFRONTPANELCONTROLS
ANDHOWTOUSETHEM...7
REARPANELINFORMATION...8
PERFORMANCELIMITS...9
PERFORMANCECHARTS...10,11
TECHNICALDESCRIPTION...12,13
BLOCKDIAGRAM...14
1.Mclntoshwillprovideallparts,materials
andlaborneededtoreturnthemeasured
performanceoftheinstrumenttothe
originalperformancelimits.TheSER-
VICECONTRACTdoesnotcoverany
shippingcoststoandfromtheauthoriz-
edserviceagencyorthefactory.
2.AnyMclntoshauthorizedserviceagency
willrepairMclntoshinstrumentsatnor-
malservicerates.Toreceiveservice
underthetermsoftheSERVICECON-
TRACT,theSERVICECONTRACTCER-
TIFICATEmustbepresentedwhenthe
instrumentistakentotheserviceagency.
3.Alwayshaveservicedonebya
Mclntoshauthorizedserviceagency.If
theinstrumentismodifiedordamaged
asa resultofunauthorizedrepair,the
SERVICECONTRACTwillbecancelled.
Damagebyimproperuseormishandl-
ingisnotcoveredbytheSERVICE
CONTRACT.
4.TheSERVICECONTRACTisissuedto
youastheoriginalpurchaser.Topro-
tectyoufrommisrepresentation,this
contractcannotbetransferredtoa se-
condowner.
5.ToreceivetheSERVICECONTRACT,
yourpurchasemustbemadefroma
Mclntoshfranchiseddealer.
6.Yourcompletelyfilledinapplicationfor
theSERVICECONTRACTmustbepost-
markedwithin30daysofthedateof
purchaseoftheinstrument.
7.ToreceivetheSERVICECONTRACT,all
informationontheapplicationmustbe
filledin.TheSERVICECONTRACTwill
beissuedwhenthecompletelyfilledin
applicationisreceivedbyMclntosh
LaboratoryIncorporatedinBinghamton,
NewYork.
8.UnitsinoperationoutsidetheUnited
StatesandCanadaarenotcoveredby
theMclntoshFactoryServiceContract,
irrespectiveoftheplaceofpurchase.
Norareunitsacquiredoutsidethe
U.S.A.andCanada,thepurchasersof
whichshouldconsultwiththeirdealer
toascertainwhat,ifany,servicecon-
tractorwarrantymaybeavailablelocally.
Copyright1979© byMclntoshLaboratoryInc.
MclNTOSHTHREEYEARSERVICECONTRACT
AnapplicationforA THREEYEARSERVICECONTRACTisincludedwiththismanual.
Thetermsofthecontractare:
1

ThePANLOCsystemofinstallingequipmentcon-
venientlyandsecurelyisa productofMclntosh
research.BydepressingthetwoPANLOCbuttons
onthefrontpanel,theinstrumentslidecanbelock-
edfirmlyinplaceoritcanbeunlockedsothatthe
chassiscanslideforward,givingyoueasyaccessto
thetopandrearpanels.
Thetrouble-freelifeofanelectronicinstrumentis
greatlyextendedbyprovidingsufficientventilation
topreventthebuildupofhighinternaltemperatures
thatcausedeterioration.Allowenoughclearanceso
thatcoolaircanenteratthebottomofthecabinet
andbeventedfromthetop.Withadequateventila-
tiontheinstrumentcanbemountedinanyposition.
Therecommendedminimumspaceforinstallation
is15inches(38.1cm)deep,17inches(43.2cm)wide,
and4-1/2inches(11.4cm)high.
Toinstalltheinstrumentina Mclntoshcabinet,
followtheinstructionsthatareenclosedwiththe
cabinet.Foranyothertypeofinstallationfollow
theseinstructions:
1.OpenthecartonandremovethePANLOC
brackets,hardwarepackage,andmounting
templatefromthecarton.RemovetheMC502from
itsplasticbagandplaceitupsidedownontheship-
pingpallet;unscrewthefourplasticfeetfromthe
bottomofthechassis.
2.Markthecabinetpanel.
Placethemountingtemplateinthepositiononthe
cabinetpanelwheretheinstrumentistobeinstall-
ed,andtapeitinplace.Thebrokenlinesthatrepre-
senttheoutlineoftherectangularcutoutalsorepre-
senttheoutsidedimensionsofthechassis.Make
suretheselinesclearshelves,partitions,orany
equipment.Withthetemplateinplace,firstmarkthe
sixA andB holesandthefoursmallholesthat
locatethecornersofthecutout.Then,jointhefour
cornermarkingswithpencillines,usingtheedgeof
thetemplateasa straightedge.
3.DrillHoles
Usea drillwitha 3/16inchbitheldperpendicularto
thepanelanddrillthesixA andB holes.Then,using
adrillbitslightlywiderthanthetipofyoursaw
blade,drilloneholeateachoftwodiagonallyop-
positecorners.Theholesshouldbarelytouchthein-
sideedgeofthepenciledoutline.Beforetakingthe
nextstep,makesurethatthesixA andB holeshave
beendrilled.
4.SawthePanelCutout
Sawcarefullyontheinsideofthepenciledlines.
Firstmakethetwolongcutsandthenthetwoshort
cuts.Aftertherectangularopeninghasbeencutout,
usea filetosquarethecornersandsmoothanyir-
regularitiesinthecutedges.
2

5.InstalltheMountingStrips
Inthehardwarepackageyouwillfindtwomounting
strips,andtwosetsofmachinescrews.Forpanels
thatarelessthan1/2inchthick,usethe3/4inch
screws;forpanelsthataremorethan1/2inchthick,
usethe1-1/4inchscrews.
Startingattheright-handsideofthepanel,insert
ascrewoftheproperlengthintothecenterholein
thepanel,markedB onthetemplete.Onthebackof
thepanel,aligna mountingstripwiththeholesin
thepanelandtightenthescrewuntilthescrewhead
ispulledintothewood.
Repeatthisproceduretoattachthemounting
striptotheleftsideofthepanel.
6.AttachthePANLOCBrackets
UsingtwoscrewsoftheproperlengthintheA holes
oneachside,attachthePANLOCbracketstothe
cabinetpanel;theshortflangeismountedagainst
thefront(face)ofthecabinetpanel.Thescrews
passthroughthePANLOCbracketflange,the
cabinetpanel,andthenthroughthemountingstrips
previouslymounted.
7.InstalltheInstrument
GuidetheACpowercordthroughthepanelopening
tothebackofthecabinet;then,slidetheinstrument
intotheopeningcarefullysothattherailsonthe
bottomofeachsideofthechassisengagethe
tracksonthemountingbrackets.Continuetoslide
theinstrumentintothecabinetuntilitisstoppedby
theadjustpositionlatches.Pressthelatchesin-
ward,thispermitstheinstrumenttoslideintothe
cabinetuntilitsfrontpanelisflushwiththecabinet
panel.DepressthePANLOCbuttonsatthelowerleft
andrightcornersoftheinstrumentpaneltolockthe
unitfirmlyinthecabinet.DepressingthePANLOC
buttonsagainwillunlocktheinstrumentsothatit
canslideforwardtotheadjustposition;ifyoupress
inwardontheadjustpositionlatchesthenyoucan
removetheinstrumentfromthecabinet.
3

HowtoConnect
INPUT
STEREOORTWINAMPLIFIEROPERATION
Useshieldedcablestoconnectthesignalfrom
thepreamplifierorsignalsourcetothepower
amplifierinput.Tominimizethepossibilityofhum
theshieldedcablesshouldberunparalleltoeach
otherorlooselytwistedtogether.Locatethecables
awayfromspeakerleadsandACpowercords.All
connectionsaremadeonthebackpaneloftheMC
502.
Forstereooperation,theleftoutputofthe
preamplifiershouldbepluggedintotheLeftinput
jackofthepoweramplifier.Therightoutputofthe
preamplifiershouldbepluggedintotheRight
(MONO)inputjackofthepoweramplifier.
MONOPHONICORSINGLECHANNELOPERATION
Ashieldedcablefromthesignalsourceisplug-
gedintotheRight(MONO)inputjackoftheMC502
only.TheMODESWITCHonthebackpanelofthe
amplifiermustbeplacedintheMONOposition.In
theMONOpositiontherightchannelinputjackcon-
nectstobothamplifiers.TheLeftINPUTisdiscon-
nected.
OUTPUT
Selectionofthepropergaugewiretoconnectthe
loudspeakerspreservesthequalityofsoundrepro-
ductionforwhichtheloudspeakershavebeende-
signed.Ifundersizewireisused,resistanceisadded
totheamplifier/loudspeakercombinationwhich
adverselyaffectstheperformance.Addedresis-
tancecausesreductionofdampingcharacteristics,
modificationoffrequencyresponseandreductionin
poweroutput.
Uselampcord,bellwire,orwirewithsimilartype
ofinsulationtoconnectthespeakerstotheampli-
fier.Inallcases,theleadstoandfromthespeaker
shouldbetwinconductorortwistedtogether.When
using8 ohmspeakersandforthenormallyshortdis-
tancesofunder30feetbetweentheamplifierand
speaker,No.18wireorlargercanbeused.For
distancesover30feetbetweentheamplifierand
speakeruselargerdiameterwire.Selectthecorrect
sizewireforthewirelengthfromthechart.Itis
recommendedthattheDCresistanceofthespeaker
leadsbelessthan5%ofthespeakerimpedance.Up
to10%canbetolerated.Resistanceoftheleads
shouldbecomputedforthelengthofwirebothto
andfromthespeakerorspeakers.
Wire
Gauge
22
20
18
16
14
12
10
MAXIMUMWIRELENGTHS
For4 OhmLoad
Feet
6
10
15
25
40
60
100
Meters
1.8
3.1
4.6
7.6
12.2
18.3
30.5
For8 OhmLoad
Feet
12
20
30
50
80
120
200
Meters
3.7
6.1
9.1
15.2
24.4
36.6
66.0
Wirelengthsaboverepresentthewireresistanceequalto5%
ofthespeakerimpedance.
Formultiplespeakeroperation,runseparate
leadsfromtheamplifiertothespeakers.
CONNECTINGLOUDSPEAKERSFORSTEREO
Connecttheleadsfromtheleftloudspeakertothe
LeftandCommonOUTPUTterminalsontheMC502.
Connecttheleadsfromtherightmainloudspeaker
totheRightandCommonterminals.
CONNECTINGONELOUDSPEAKERFORMONO
ConnectleadsfromtheloudspeakertotheLeft
andRightOUTPUTterminalsontheMC502.Check
tobesuretheMODEswitchisintheMONOposi-
tion.
InMONOoperationneitheroutputterminal(Left
orRight)isatgroundpotential.Theoutputisbalanc-
edtoground.Dueprecautionisrequiredwhencon-
nectingtestinstrumentssinceneitherterminalcan
begrounded.
ACPOWER
TheamplifierACpowercordispluggedintoa 120
volt50/60Hzwalloutlet.
FUSE
A4-ampfuseprotectstheMC502circuits.The
fusedoesnotprotectadditionalequipmentcon-
nectedtotherearpanelACpoweroutlets.
4

Stereophonic
Connections
PLACESWITCH IN
STEREOPOSITION
FOR
STEREOOPERATION
MODE
STEREO
MONO
RIGHT
SPEAKER
LEFT
SPEAKER
PROGRAMSOURCE

Monophonic Connections
SPEAKER
STEREO
MONO
MODE
PLACESWITCH IN
STEREO
POSITION
FOR
STEREOOPERATION
PROGRAM
SOURCE

FrontPanelControlsand
HowtoUseThem
LEFTGAIN
Usetheleftgaincontroltoadjustthevolumein
theleftchanneltothedesiredlisteninglevel.Turn
thecontrolclockwisetoincreasethevolume.
RIGHT/MONOGAIN
Usetheright/monogaincontroltoadjustthe
volumeintherightchanneltothedesiredlistening
level.Turnthecontrolclockwisetoincreasethe
volume.WhentheMC502isconnectedformono-
phonicoperationandtherearpanelModeSwitchin
MonothevolumeiscontrolledbytheRight/Mono
Gaincontrolonly.
HEADPHONES
TheoutputofthefrontpanelHEADPHONEjack
hasbeendesignedtofeedlowimpedancedynamic
stereoheadphones.Plugdynamicheadphonesinto
thefrontpanelHEADPHONEjack.Adjustthefront
panelLEFTGAINandRIGHT/MONOGAINcontrol
forcomfortableheadphonelistening.Electrostatic
headphonesgenerallyrequirehigherpowerthan
dynamicheadphones.ConnectthemtotheLEFT
andRIGHTOUTPUTcontactsonthebackoftheMC
502.
SPEAKERS
OFF:Theloudspeakersareturnedoffwhenthe
SPEAKERswitchisintheOFFposition.Youcan
listentoheadphonesinprivate.
ON:Musicwillbeheardthroughthe
loudspeakers.Usethisasthenormallisteningposi-
tion.
THESPEAKERSWITCHMUSTBEINTHE"ON"
POSITIONTOHEARMUSICFROMTHE
LOUDSPEAKERS.
POWER
ThepowerswitchturnstheMC502ONorOFF.
Theswitchdoesnotcontrolthepoweroutletonthe
backpanel.IfyouwishtocontroltheACpowerfrom
apreamplifiercontrolcenterleavetheswitchinthe
ONposition.BesuretheACcordoftheMC502is
pluggedintothecontrolledoutletsontherearofthe
preamplifiercontrolcenter.
OFF:IntheOFFpositiontheACpowertothe
amplifieristurnedoff.
POWERGUARDINDICATORLAMPS
POWERGUARDassuresthatthepoweramplifier
sectionoftheMC502cannotbeoverdriven,thus
amplifieroutputclippingiseliminated.Clippingis
causedwhentheamplifierisaskedtoproducemore
poweroutputthanitcandeliverwithlowdistortion.
Amplifiersarecapableofdeliveringlargequantities
ofpowerwhentheyaredriventoclippingandcan
havemorethan40%harmonicdistortion.Theextra
energycontentoftheclippedsignalwilldamage
mostspeakers.A Mclntoshadvancementhelpsto
protectyourspeakerfromthiskindofdamage.The
MC502hasa builtin"waveformcomparator"that
comparesthewaveshapeoftheoutputsignaltothe
inputsignal.Ifthenon-linearitybetweenthetwo
signalsexceeds0.5%thePOWERGUARDcircuit
operates.OperationisindicatedwhentheredLIMIT
indicatorsturnon.Aslongastheamplifieroperates
withoutoverloadtheNORMALindicatorilluminates.
7

RearPanelInformation
LEFTandRIGHTOUTPUT
Forstereooperation,outputconnectionsforim-
pedancesof2.7to8 ohmsareprovidedona secure,
screwtypebarrierstrip.Formonophonicoperation
properinterconnectionprovides8 ohmsfromthe
samebarrierstrip.
MODESWITCH
TheMC502canbeusedforstereoormono(single
channel)operation.TheMODEswitchisusedto
selectthemethodofoperationdesired.
ACPOWER
TheMC502isratedfor120volts,50/60hertz.It
uses0.2amperes(20watts)whenthereisnosignal
outputandupto4 amperes(400watts)withboth
channelsdeliveringratedpower.A 4 amperefuse
protectstheMC502electrically.TheACpower
outletprovidedforauxiliaryequipmentisneither
fusednorswitched.
INPUT
Inthestereomodeofoperationbothinputjacks
acceptsignal.Inthemonomodeofoperationonly
theRight(MONO)channelinputjackacceptssignal
andtheLeftchannelinputjackisdisconnected.
INPUTLEVEL
TheinputsensitivityoftheMC502is0.75voltsor
2.5V dependingonthepositionoftheINPUTLEVEL
switch.WithindicatedvoltageappliedandtheGAIN
controlclockwise,theamplifierwilldeliveritsrated
power.AllMclntoshpreamplifiershavebeendesign-
edtodeliver2.5voltsoutputwithratedinput.Forthe
bestsignaltonoiseratiowhenusingMclntosh
sourceequipment,placetheINPUTLEVELswitchin
the2.5V positionandthefrontpanelLEFTand
RIGHT/MONOGAINcontrolsinthefullyclockwise
position.Ifmoregainisdesiredthe0.75V position
maybeused.ForsourceequipmentotherthanMcln-
toshsettheswitchinthepositionnearesttothe
statedoutputratingofthesourceequipment.
8

PerformanceLimits
WepromiseyouthattheMC502youbuymustbe
capableofperformanceatorexceedingtheselimits
atthetimeofpurchaseoryougetyourmoneyback.
MclntoshPERFORMANCELIMITSarethemaximum
departurefromperfectionpermittedfora Mclntosh
instrument.
PERFORMANCE
Mclntoshaudiopowerratingsareinaccordance
withtheFederalTradeCommissionRegulationof
November4,1974concerningpoweroutputclaims
foramplifiersusedinhomeentertainmentproducts.
POWEROUTPUT
STEREO:
75wattsinto2.7to4 ohmloads,50wattsinto8
ohmsloads,minimumsinewavecontinuous
averagepoweroutputperchannelfrom20Hzto
20,000Hz,bothchannelsoperating.
Whichis:14.2voltsRMSacross2.7ohms
17.3voltsRMSacross4 ohms
20.0voltsRMSacross8 ohms
MONO(Bridged):
150wattsintoan8 ohmload,minimumsinewave
continuousaveragepoweroutputfrom20Hzto
20,000Hz.
Whichis:34.6voltsRMSacross8 ohms.
OUTPUTLOADIMPEDANCE
Stereo:2.7ohmsto8 ohms.
Mono:8 ohmsobtainedbyconnectingacrossthe
outputterminalsofbothchannels.
RATEDPOWERBAND
20Hzto20kHz
TOTALHARMONICDISTORTION
STEREO:
.02%maximumharmonicdistortionatanypower
levelfrom250milliwattstoratedpowerperchannel
from20Hzto20,000Hz,bothchannelsoperating.
MONO:
.02%maximumharmonicdistortionatanypower
levelfrom250milliwattstoratedpowerfrom20Hz
to20,000Hz.
INTERMODULATIONDISTORTION
STEREO:
.02%maximumperchannelwithbothchannels
operatingforanycombinationoffrequencies,20Hz
to20,000Hz.
MONO:
.02%maximumatanypowerlevelfrom250
milliwattstoratedpowerforanycombinationoffre-
quencies,20Hzto20,000Hz.
FREQUENCYRESPONSE(atonewattoutput)
20Hzto20,000Hz+0,-0.25dB
10Hzto100,000Hz+0,-3.0dB
HUMANDNOISE
95dBbelowratedoutput
RATINGS
OUTPUTVOLTAGES:
25voltsfordistributionlines
DAMPINGFACTOR
Greaterthan50
INPUTIMPEDANCE
75,000ohms
INPUTSENSITIVITY
Switchable:0.75voltor2.5volts-Levelcontrolpro-
videdforhigherinputvoltages.
POWERREQUIREMENT
120Volts,50/60Hz,0.2to4 amperes(20to400
watts).
MECHANICALINFORMATION
SIZE:FrontPanelmeasures16incheswide(40.6
cm)by3 5/8incheshigh(9.2cm).Chassismeasures
143/4incheswide(37.5cm)by2 3/8incheshigh(6.0
cm)by141/2inchesdeep(36.8cm),includingcon-
nectors.Knobclearancerequiredis1 1/4inches(3.2
cm)infrontofthemountingpanel.
FINISH:Frontpanelisanodizedgoldandblack
withspecialgold/tealnomenclatureillumination.
Chassisisblack.
WEIGHT:27pounds(12.2kg)net.38pounds(17.2
kg)inshippingcarton
9

PerformanceCharts
RMS
WATTS
10
10k 100k
FREQUENCYINHz
10
100
10k
100k
FREQUENCYINHz
1k 100
240
220
200
180
160
140
120
100
80
60
40
20
0
140
130
120
110
100
90
80
70
60
50
40
30
20
10
0
RMS
WATTS
10
100
1k 10k
100k
dB
-1
0
-1
-2
-3
FREQUENCYINHz
10
1k

%
DISTORTION
20 100
FREQUENCYINHz
10k20k
.05
.04
.03
.02
.01
0
RMSWATTS
100 1000
100 1000
100
1000
11
%
DISTORTION
1 10
%
DISTORTION
.05
.04
.03
.02
.01
0
.1 1 10
RMSWATTS
%
DISTORTION
.05
.04
.03
.02
.01
0
.1
110
RMSWATTS
.05
.04
.03
.02
.01
0
1k

TechnicalDescription
WhiletheMC502performsa ratherelementary
functionasa basicstereopoweramplifier,itdoes
containannumberofrathersophisticatedcircuits.
Eachchannelconsistsofaninputpreamplifier,a
poweramplifiersection,threeseparateprotecting
controlcircuits,anda phaseinverterwhichisin-
tegralwiththeleftchannel.Thepowersupplyin-
cludespositiveandnegativeelectronicregulators.A
descriptionofeachcircuitfollows:
Preamplifiers
Eachchannel'spreamplifierisa highperfor-
mancelownoiseoperationalamplifierarrangedfor
astagegainof4.5times.Theinputsignalstothe
MC502firstpassthroughtheinputlevelswitchat-
tenuator,thentotheleftorright/monogaincontrols,
nexttoelectronicattenuatorswhicharepartofthe
PowerGuardcircuittobedescribedlaterandthen
tothepreamplifiers.Theleftchannelinputpathalso
includesthemodeswitchwhich,instereo,connects
tothenormalstereosignalpathor,ifswitchedto
mono,directstheoutputfromtherightchannel
preamplifiertotheinvertinginputoftheleftchannel
preamplifier.Thismonoarrangementdrivesthe
followingpoweramplifierstagesinphaseopposi-
tionsothatmonooutputpowerisderivedasthe
sumoftheleftandrightpowerchannels.
Poweramplifiersections
Eachoftheidenticalpoweramplifiersections
consistof4 stagesofamplification.Attheinputisa
differentialtransistorpairoptimisedforlownoise.
Inputandfeedbacksignalsareappliedtothisdif-
ferentialamplifier.Thetwooutputsofthedifferen-
tialamplifierarecombinedina currentmirrorand
theresultingsignaldrivesthesecondstage,a class
Avoltageamplifier.Thesecondstageoutputfeeds
complementaryemitterfollowerswhichinturndrive
theoutputtransistorswhicharealsocomplemen-
taryemitterfollowers.Biasingforthedriverandout-
puttransistorsisaccomplishedinthebasecircuitof
thedrivertransistorsusinga temperaturesensitive
transistorthermallycoupledtotheoutputtransistor
heatsinks.Coolefficientoperationistherebyob-
tained.
Protectioncontrolcircuits
Threecontrolcircuitsareused.Thefirstcontrol
circuitistheSentryMonitorwhichsensesthecur-
rentflowintheoutputstagetransistors.Thesense
characteristicisshapedtoconfinecurrentand
voltageconditionstothesafeoperating
characteristicsoftheoutputstagepowertran-
sistors.Thisprotectiongreatlycontributestothe
reliable,longlife,performanceoftheMC502.
PowerGuardisthesecondcontrolcircuit.
Amplifiersarecapableofdeliveringlargequantities
ofpowerwhentheyaredriventoclipping.Clipping
iscausedwhentheamplifierisaskedtoproduce
morepoweroutputthanitcandeliverwithlow
distortion.A clippedamplifiercanhavemorethan
40%harmonicdistortion.Theextraenergycontent
oftheclippedsignalwilldamagemostloud-
speakers,particularlydelicatehighfrequency
tweeters.A newMclntoshadvancementhelpspro-
tectyourspeakersfromthiskindofdamage.TheMC
502hasa built-inwaveformcomparatorwhichcom-
paresthewaveshapeoftheoutputsignaltothein-
putsignal.Ifthedisparitybetweenthetwosignals
exceeds0.5%(equivalentto0.5%totalharmonic
distortion)a redlimitindicatorilluminates.Withany
furtherincreaseindistortionthePowerGuardcir-
cuitwilloperate.Thiscircuitlimitstheinput
dynamicallysothattheamplifiercannotbeover-
driven.PowerGuardeliminatesamplifieroutput
clipping.
PowerGuarddoesnotlimitthedynamicrangeor
thepoweroutputofthepoweramplifier.Clippingoc-
curswhenanamplifierisaskedtoexceeditsdesign
limitsandthecapacityofthepowersupply.Since
PowerGuarddoesnotbegintoworkuntilthispoint
isreached,thepowercapabilityoftheamplifieris
neveraffected.
Thethirdcontrolcircuitisthespeakerprotection
andturnondelayarrangement.Inmostdirectcoupl-
edcircuitsa failureofanytransistorinthepower
amplifierwillcausea DCpotentialtoappearinthe
output.Toassurethatnodamagingorinterfering
DCappearsacrosstheoutputterminals,a special
veryfastactingprotectorcircuitconstantly
monitorstheoutputforDC.If,atanytime,a cons-
tantDClevelappears,thespeakersarediscon-
nected.Theprotectivecircuitreactsinmilliseconds.
Speakersremaindisconnecteduntilthecausehas
beenfixed.Integralwiththiscircuitisthetransient
freeturnonandturnoffcontroller.Thepower
amplifieroutputsignalspassthrougha heavyduty
relaybeforereachingtheoutputterminals.Therelay
iscontrolledbyanelectroniccircuitwhichcloses
therelayapproximatelytwosecondsafterthepower
switchisturnedonandreleasestherelayalmostin-
stantlywhenthepoweristurnedoff.
TheSentryMonitor,PowerGuard,andspeaker
protectorcircuitsweredevelopedbyMclntosh
engineersandareeachcoveredbyU.S.patents
assignedtoMclntoshLaboratory.
12

PowerSupply
Twohighcurrentpowersupplies- a positive40
voltDCanda negative40voltDC- areusedtodrive
theoutputpoweramplifier.Twoverylargefilter
capacitors,10,000microfaradseach,areusedto
storea largeamountofenergytoprovidegoodfilter-
ingandexcellentvoltageregulation.Goodlowfre-
quencyresponseandnegligiblelowfrequency
distortioninthepoweramplifierstagesdependson
theregulationofthepowersupply.Twoadditional
electronicallyregulatedpowersuppliesareusedin
theMC502.A + 15voltanda - 15voltlinesupply
alloperationalamplifiers.
13

Block
Diagram
MclNTOSH
MC502
STEREOPOWERAMPLIFIER
14

MclNTOSHLABORATORYINC.
2CHAMBERSST.,BINGHAMTON,N.Y.13903-2699
607-723-3512
Thecontinuousimprovementofitsproductsisthepolicyof
MclntoshLaboratoryIncorporatedwhoreservetherightto
improvedesignwithoutnotice.
PrintedinU.S.A.
039266
BE032003