McIntosh MC 2500 User manual

Price:$2.00
ReadingTime:32Minutes
THEMclNTOSHMC2500STEREOPOWERAMPLIFIER

VARIOUSREGULATORYAGENCIESREQUIRETHATWEBRINGTHEFOLLOWING
INFORMATIONTOYOURATTENTION.PLEASEREADITCAREFULLY.
TheMclntoshMC2500isanex-
tremelyhighpoweramplifier.
Becauseofthehighpoweravailable
itisnecessarytoemphasizesome
prudentandsafeoperatingcondi-
tions.
1.Loudspeakerscanbedamagedor
destroyedbythehighpower
availablefromtheMC2500.Never
connectordisconnectinputsor
outputswhiletheamplifieristurn-
edon.
2.Neveroperatetheamplifierwith
thepowercordpluggedintoan
auxiliaryACpoweroutleton
sourceequipment.Theamplifier
draws15amperesandcandamage
thesourceequipment.
3.Donotoperatetheinstrumentplug-
gedintoanordinaryextension
cord.A heavydutyextensioncord
(14ga.orheavier)isrequired.
4. Be cautiouswhenliftingtheinstru-
ment.Itweighs129pounds.Make
certainthatwhatitisplacedon
cansupporttheweight.
5.Theamplifiercoolingfanscanbe
thermostaticallycontrolledwhen
thefanswitchontherearpanelis
placedinthethermallyswitched
positions.Whensooperated,the
fanswillnotrunwhentheamplifier
iscool,aswhentheamplifieris
firstturnedon.Asheatdevelops,
oneorbothfanswillautomatically
turnonasconditionsrequire.This
operatingbehaviorisnormal.
WARNING:TOPREVENTFIREORSHOCK
HAZARD,DONOTEXPOSETHISUNITTORAIN
ORMOISTURE.
TheMclntoshyouhavepurchasedisa Model
MC2500.Ithasa serialnumberlocatedontherearpanel
ofthechassis.Recordthatserialnumberhere:
Themodel,serialnumberandpurchasedateareimpor-
tanttoyouforanyfutureservice.Recordthepurchase
datehere:
Uponapplication,MclntoshLaboratoryprovidesa
Three-YearServiceContract.YourMclntoshauthorized
ServiceAgencycanexpediterepairswhenyouprovide
theServiceContractwiththeinstrumentforrepair.To
assist,recordyourServiceContractnumberhere:
SerialNumber
Purchasedate
ServiceContractNumber

YourMC2500StereoPowerAmplifier
cangiveyoumanyyearsof
satisfactoryperformance.Ifyou
haveanyquestions,pleasecontact:
CUSTOMERSERVICE
MclntoshLaboratoryInc.
2ChambersStreet
Binghamton,NewYork13903-9990
Phone:[phone]
Contents
SERVICECONTRACTINFORMATION1
INSTALLATION2
HOWTOCONNECT3
FRONTPANELINFORMATION8
REARPANELINFORMATION10
PERFORMANCELIMITS12
PERFORMANCECHARTS13,14
TECHNICALDESCRIPTION15
BLOCKDIAGRAM17
MclNTOSHTHREEYEARSERVICECONTRACT
AnapplicationforA THREEYEARSERVICECONTRACTisincludedwiththismanual.
Thetermsofthecontractare:
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.
Copyright1981© byMclntoshLaboratoryInc.
-1-
TakeAdvantageof3 years
ofContractService...
FillintheApplicationNOW.

Installation
InstallationoftheMC2500requirescareful
thoughtaboutthreeimportantfactors.Theyare:the
electricalpowertooperatetheunit,theweight,and
theheatgeneratedwhentheMC2500isoperating.
TheMC2500draws15ampereswhenoperatedat
fullpowerwhenamplifyingmusicorspeech.Donot
useordinaryextensioncords.Ifanextensionisused
thenitmustbeheavydutyand14ga.orheavier.
PlugtheACpowercorddirectlyintoa walloutlet.
MakecertainthattheACpoweroutlethasatleast
15amperescapacity.DonotplugtheMC2500into
anauxiliaryACpoweroutletona preampliferor
othersourceequipment.Ifremotepoweroperation
isrequired,anexternalrelayarrangementmustbe
made.
Theweightoftheinstrumentis129pounds.Make
certainthatthestructureonwhichitistobe
mountedcansupportthatweight.
Adequateventilationextendsthetrouble-freelife
ofelectronicinstruments.Itisgenerallyfoundthat
each10°centigrade(18°F)riseintemperature
reducesthelifeofelectricalinsulationbyonehalf.
Adequateventilationisaninexpensiveandeffective
meansofpreventinginsulationbreakdownthat
resultsfromunnecessarilyhighoperatingtempera-
tures.Thedirectbenefitofadequateventilationis
longer,trouble-freelife.Theheatgeneratedinthe
operationoftheMC2500isexhaustedfromtheunit
bytwotemperaturecontrolledlownoise,longlife
fans.
CoolinginputairisdrawnintotheMC2500
throughtheventilationholesonthesidesandbot-
tomoftheamplifier.Theairpassesoverthe
transformers,outputtransistorheatsinks;and,is
blownoutthebackoftheinstrumentbythefans.It
isrecommendedthatatleast2 inchesofclearspace
beprovidedoneachsideandbottom.
Topermitthefanstooperatebest,provideatleast
5inchesofspaceattherearoftheinstrument.A
sourceforinputairanda meanstoexhaustthe
heatedairisnecessarysothattheheatedairdoes
notrecirculatethroughtheMC2500.
RACKINSTALLATION
TheMC2500maybemountedina standard19"
rackbyremovingthealuminumsiderails.Whenrack
mounted,theMC2500requires10½inchesofpanel
space.Allowtwoinchesinfrontofthepanelforthe
knobs.A depthof17inchesplusventilationspaceis
required.
TheMC2500canbeslidemountedina rack.
Tappedweld-nutsandinternalstructurebracinghas
beenaddedtobothsidesoftheMC2500forusewith
ModelCTS-116sidemountedslideassemblies
manufacturedby
GeneralDevicesCo.
Chassis-TrakDiv.
525S.WebsterAve.
Indianapolis,IN46219
-2-

HowtoConnect
INPUT
Stereooperation:
Useshieldedcablestoconnectthesignalfrom
thepreamplifieroranyothersignalsourcetothepo-
weramplifier.Allconnectionsaremadeontheback
paneloftheMC2500.
Forstereooperation,theleftoutputofthepre-
amplifiershouldbeconnectedtotheLeftINPUTof
thepoweramplifier.Therightoutputofthepreamp-
lifiershouldbeconnectedtotheRight/MONOIN-
PUTofthepoweramplifier.TheMODESWITCH
mustbeplacedintheSTEREOposition.
Tominimizethepossibilityofhumtheshielded
leadsshouldberunparallelorlooselytwistedtoget-
her.LocatethecablesawayfromACpowercords.
Monophonicoperation:
Ashieldedcablefromthesignalsourceiscon-
nectedtotheRight/MONOinputoftheMC2500.The
MODEswitchonthebackpaneloftheamplifier
mustbeplacedinoneofthetwoMONOpositions.In
theseMONOpositionstheoutputoftherightchan-
nelinputamplifierisfedtobothleftandrightpower
amplifiers.TheLeftINPUTisdisconnected.Onlythe
signalfedintotheRight/MONOinputwillbeampli-
fied.
OUTPUT
Stereooperation:
Toconnecttheleftspeakerfirstchecktheimpe-
danceofthespeakerwhichisusuallyidentifiedon
thespeakeritselforintheowner'smanual.Connect
oneleadfromthecommonterminalofthespeaker
totheLEFTOUTPUTterminalstripscrewCOMmon.
Connecttheotherterminalofthespeakertothe
screwwiththenumbercorrespondingtothespeaker
impedanceontheLEFTOUTPUTterminalstrip.The
rightchannelspeakerisconnectedinthesame
mannerontheRIGHTOUTPUTterminalstrip.Figure
1showstheconnectionsforstereooperation.
STEREOLOAD
OHMSVOLTS
122.4
231.6
444.7
863.2
STEREOOPERATION
CONNECTLOW
SIDEOFLOADS
TOTERMINALS
LEFT
COM
COM
COM
COM
RIGHT
COM
COM
COM
COM
CONNECTHIGH
SIDEOFLOADS
TOTERMINALS
LEFT
1W
2W
4W
8W
RIGHT
1W
2W
4W
8W
Whenmultiplespeakersaretobeconnectedto
eitherorbothoutputs,thecombinedloadimpe-
dancemustbecalculatedandtheloadconnectedto
theappropriateimpedancetap.ThefollowingFigure
2willaidinselectingthecorrectimpedancematch.
Load
impedance
inohms
0.9to1.8
1.8to3.6
Connect
to
1W
2W
Load
impedance
inohms
3.6to7.2
7.2to14.4
Connect
to
4W
8W
Figure1
Figure2
Ifa loadimpedanceisusedthatislowerthanthe
outputimpedancetap,thenreducedpower,increas-
edheating,andpossibledistortionwillresult.Ifa
loadimpedanceisusedthatishigherthantheout-
putimpedancetap,thenthemaximumavailableout-
putislimited.
Makeallspeakerconnectionsattheamplifieron-
ly.Formultiplespeakeroperation,paralleltheleads
fromthespeakerstotheamplifier.
Becauseofthehighpoweravailablefromthe
MC2500,besuretouselargediameterspeaker
leads.Inallcases,theleadstoandfromthespeaker
shouldbetwinconductorortwistedtogether.
Lampcordorwirewithsimilartypeofinsulation
canbeusedtoconnectthespeakerstotheampli-
fier.Forthenormallyshortdistancesofunder20
feetbetweentheamplifierand8 ohmspeakers#18
wireorlargercanbeused.Fordistancesover20feet
betweentheamplifierandspeakeruselargerdiame-
terwire.Selectthecorrectsizewireforthewire
distancefromthechart(Figure5 page4).Itisrecom-
mendedthattheDCresistanceofthespeakerleads
notbeover5%oftheloadimpedance.Resistanceof
theleadsmustincludethelengthofwirebothtoand
fromthespeakerorspeakers.
-3-

MONOLOAD
OHMSVOLTS
244.7
463.2
889.4
16126.5
MONOBRIDGEOPERATION
CONNECTLOW
SIDEOFLOAD
TOTERMINALS
L1W
L2W
L4W
L8W
CONNECTHIGH
SIDEOFLOAD
TOTERMINALS
R1W
R2W
R4W
R8W
INBRIDGEOPERATIONTHEOUTPUTISBALANCEDTO
GROUNDANDNEITHEROUTPUTLEADISATGROUND
POTENTIAL
MONOPARALLELOPERATION
CONNECTLOW
SIDEOFLOADTO
MONOLOADBOTHTERMINALS
OHMSVOLTS
1/222.4L COM& R COM
131.6LCOM&RCOM
244.7L COM& R COM
463.2L COM& R COM
CONNECTHIGH
SIDEOFLOADTO
BOTHTERMINALS
L1W & R 1W
L 2W & R 2W
L4W& R 4W
L8W& R 8W
DONOTOPERATEAMPLIFIERWITHMODESWITCHINTHE
STEREOORMONOBRIDGEPOSITIONWITHTHEOUTPUTS
PARALLELEDASABOVE
Figure3 Figure4
Monophonicoperation:
WhentheMC2500isusedasa monophonicor
singlechannelpoweramplifierthetwochannelsare
combinedtoproduceanoutputpowerof1000watts.
Theleftandrightoutputsmustbetiedtogetherat
theappropriateloadimpedancetaps.Figure3
showstheconnectionsforMONOBRIDGEopera-
tionandFigure4 showstheconnectionsforMONO
PARALLELoperation.
Makeallspeakerconnectionsattheamplifieron-
ly.Formultiplespeakeroperation,paralleltheleads
fromthespeakerstotheamplifier.
ACPOWER
TheMC2500isdesignedtooperateon120volts
50/60Hz.PlugtheACpowercorddirectlyintoa wall
outlet.MakecertainthattheACpoweroutlethasat
least15amperecapacitywithnothingelseconnec-
tedtothecircuit.DonotplugtheMC2500intoan
auxiliaryACpoweroutletona preamplifierorother
sourceequipmentunlessitisknownthereisade-
quatecurrentcapacity.Ifremotepoweroperationis
required,anexternalrelayarrangementmustbe
made.Ifanextensioncordisusedmakecertainthat
ithas15amperecurrentcapacity.
TheMC2500draws15amperesAClinecurrent
whenamplifyingmusicorspeechatratedoutputon
programpeaks.Theamplifierusesonly0.7ampere
whileidlingatnooutput.Iftheamplifierisdrivento
ratedoutputusingsinewavesignalsitwilldraw22
amperes.TheMC2500isdesignedforcontinuous
andreliableoperationatanypowerleveltoitsrated
500 wattscontinuousaveragepoweroutputper
channelor1000wattsasa monoamplifier.Forcon-
tinuoussinewaveoperationtheACsourcemustbe
capableofsupplyingtherequired22amperes.
-4-

Stereophonic
Connections
-5-

Mono-Bridge
Connections
-6-

Mono-Parallel
Connections
-7-

FrontPanelInformation
METERS
Outputpowermonitormetersindicatetheoutput
powerofeachchannel.Eachmeterhastwoprimary
scales:WATTSandDECIBELS.WhentheMETER
RANGEswitchinoneofthedecibel(dB)positions,
peaksignalreadingsareindicatedonthelowerDE-
CIBELscale.WhentheMETERRANGEswitchinthe
WATTSposition,directpowerinwattsisreadfrom
theupperWATTSscale.Themetersarecalibratedto
readaveragewatts.Theintermediatemarkingsbet-
weenthecalibrationsrepresent,beginningwith500
watts,200watts,100watts,theindicated50watts,
20watts,10watts,theindicated5 watts,2 watts,1
watt,theindicated0.5watt,0.2watt,0.1watt,thein-
dicated0.05watt,0.02watt,0.01watt,theindicated
0.005watt,0.002wattand0.001watt.Althoughthe
metercalibrationsareinaveragewattsfora sine
wavesignal,themeterselectricallyrespondtosig-
nalpeaks.Themetersarevoltageactuatedandindi-
catepoweraccuratelywhentheamplifierisoper-
atedintoratedoutputloadimpedances.
Themetersrespondtothepeakoutputofeach
channel.Ordinarymeterslackthecapabilityofindi-
catingtheshortintervalinformationina sound
wave.Themassofthemetermovementistoogreat
torespondtothenearlyinstantaneouschangesin
musicprogrammaterial.Shortintervalinformation
canhavea durationasshortashalfa thousandthof
asecond.Ordinarily,a meterpointermovingoverits
scaleinsucha shorttimewouldnotbeseen.
Mclntoshhasdevelopedcircuitsthatdrivetheme-
terstorespondtotheshortintervalinformationina
soundwavetoanaccuracyof90%.Theelectrical
pulsethatdrivesthemeterpointeristimestretched
longenoughsothatthepeakpositionofthepointer
canregisterinthepersistenceofvisioncharacteris-
ticoftheretinaofthehumaneye.
LEFTGAIN
TheLEFTGAINcontroladjuststhevolumeinthe
leftchanneltothedesiredlisteninglevel.Turnthe
controlclockwisetoincreasethevolume.
RIGHT/MONOGAIN
TheRIGHT/MONOGAINcontroladjuststhevol-
umeintherightchanneltothedesiredlisteninglev-
el.Turnthecontrolclockwisetoincreasethevol-
ume.WhentheoutputoftheMC2500isconnected
formonophonicoperationandtherearpanelMODE
SWITCHintheMONOposition,thevolumeiscon-
trolledbytheRIGHT/MONOGAINcontrolonly.
-8-

METERRANGE
TheMETERRANGEswitchhasfivepositions.
WATTS
IntheWATTSpositioneachmeter'sprimary
calibrationisfrom.005watt(fivemilliwatts),upto
500 watts,theratedpoweroutputoftheMC2500.
Themeteriscalibratedfor1000wattsattheright
handendofthemeterscale.WhiletheMC2500can-
notreachthispowerlevelcontinuously,itispossi-
bleforshortintervalpeakstoexceed,considerably,
the500 wattcontinuousrating.
HOLD
IntheHOLDposition,eachmeterindicates
WATTSandlockstothehighestpowerpeakina se-
quenceofpeaks.Themeterwillbedriventomax-
imumpowerandelectronicallyheldthereuntila
higherpeakpassesthroughtheamplifier.Ifnofur-
therpeaksarereachedthemeterneedlewillvery
slowlyreturntoitsrestpositionata decayrateof6
dBperminute.
DECIBELS(dB)
IntheotherthreepositionsoftheMETERRANGE
switchthemeterswillindicatetheoutputofeach
channelinDECIBELSrelativeto500wattsorany
otherchosenreference.
0dBInthispositionoftheswitch,iftheampli-
fierdelivers500averagewatts,themeterin-
dicates0 dB;at250averagewattsthemeter
indicates- 3 dB.Iftheamplifierisoverdriv-
ento+ 2 dBtheindicatedoutputwouldbe
792watts.
-10dBInthispositionoftheswitch,iftheampli-
fierdelivers50averagewatts,themeterin-
dicates0 dB;at25averagewattsthemeter
indicates-3dB.
-20dBInthispositionoftheswitch,iftheampli-
fierdelivers5 averagewatts,themeterin-
dicates0 dB;at2.5averagewattsthemeter
indicates-3dB.
POWERGUARD
POWERGUARDassuresthattheMC2500cannot
beoverdriven,thusamplifieroutputclippingis
eliminated.Clippingiscausedwhenanamplifieris
askedtoproducemorepoweroutputthanitcande-
liverwithlowdistortion.Amplifiersarecapableof
deliveringlargequantitiesofhighlydistortedpower
whentheyaredriventoclipping.Theextraenergy
contentoftheclippedsignalwilldamagemost
speakers.Mcintosh'sPowerGuardcircuitprotects
yourspeakersfromthiskindofdamage.TheMC
2500hasa builtin"waveformcomparator"thatcom-
paresthewaveshapeoftheoutputsignaltothein-
putsignal.Ifthedisparitybetweenthetwosignals,
duetooverdrive,exceeds0.5%(equivalentto0.5%
totalharmonicdistortion)a redLIMITindicatoril-
luminates.Withanyfurtherincreaseindistortion
thePowerGuardcircuitoperatestolimitthe
amplifierinputdynamicallysothattheamplifier
cannotbeoverdriven.PowerGuardeliminates
amplifieroutputclipping.Aslongastheamplifier
operateswithoutoverloadtheNORMALindicatoril-
luminates.
HEADPHONES
TheoutputofthefrontpanelHEADPHONESjack
hasbeendesignedtofeedlowimpedancedynamic
stereoheadphones.
TheHEADPHONESoutputisnotaffectedbythe
SPEAKERSswitch.
SPEAKERS
OFF:Theloudspeakersareturnedoffwhenthe
SPEAKERswitchisintheOFFpositionandthered
speakerindicatorison.
ON:Musicwillbeheardthroughtheloudspeak-
ers.Usethisasthenormallisteningposition.A
greenspeakerlightindicateswhenthespeakersare
ON.
THESPEAKERSWITCHMUSTBEINTHE"ON"POSI-
TIONTOHEARMUSICFROMTHELOUDSPEAKERS.
POWER
ThepowerswitchturnstheMC2500ONorOFF.The
powerswitchisalsoa powerinputcircuitbreaker.If
thepowerinputtotheamplifierexceedsa safelimit
thecircuitbreakerwilltripoff.Torestorepowersim-
plyplacetheswitchintheOFFpositionandthento
theONposition.
-9-

RearPanelInformation
INPUTJACKS
Inthestereomodeofoperation,bothinputjacks
acceptsignal.Inthemonomodeofoperationonly
theRight(MONO)channelinputjackacceptssignal
andtheLeftchannelinputjackisdisconnected.
INPUTLEVEL
TheinputsensitivityoftheMC2500is0.75voltsor
2.5V dependingonthepositionoftheINPUTLEVEL
switch.Withindicatedvoltageapplied,theamplifier
willdeliveritsratedpower.AllMclntoshpreampli-
fiershavebeendesignedtodeliver2.5voltsoutput
withratedinput.Forthebestsignaltonoiseratio
whenusingMclntoshsourceequipment,always
havetheINPUTLEVELswitchinthe2.5V position
andthefrontpanelLEFTandRIGHT/MONOGAIN
controlsinthefullyclockwiseposition.Ifmoregain
isdesiredthe0.75V positionmaybeused.For
sourceequipmentotherthanMclntosh,setthe
switchinthepositionnearesttothestatedoutput
ratingofthesourceequipment.
MODESWITCH
TheMC2500willoperateinthreemodes,Stereo,
MonoBridge,andMonoParallel.TheMonomodes
differintheloadstheywilldriveandtheconnection
sequencetothespeakerterminals.A connection
chartappearsontherearpanel.
LEFTandRIGHTOUTPUTTERMINALS
Forstereooperation,outputimpedancesof1,2,4
and8 ohmshavebeenprovidedona secure,screw
typebarrierstrip.Formonophonicoperation,proper
interconnectionprovides0.5,1,2,4,8,and16ohms
fromthesamebarrierstrips.Seepage3 forconnec-
tinginstructions.
FANSSWITCH
Amplifiercoolingisaccomplishedbymeansof
twolownoisefansattherearoftheamplifier.Fan
operationiscontrolledbythethreepositionFANS
switch.IntheTHERMALLYSWITCHEDLOposition
thefansdonotoperatewhentheamplifieriscool,
aswhentheamplifierisfirstturnedon.Asheat
develops,oneorbothfanswillautomaticallyturnon
tolowspeedoperationasconditionsrequire.Opera-
tioninthispositionoftheswitchproducesthe
lowestfannoise.Adequatecoolingwillbeprovided
forallnormaloperatingconditions.
TheTHERMALLYSWITCHEDHIswitchposition
functionsliketheLOpositionbutthefansoperateat
-10-

ahighspeedunderthermalcontrol.TheCON-
TINUOUSHIpositionmakesthefansoperatecon-
tinuouslyathighspeedwhenevertheamplifieris
turnedon.Thesetwoswitchpositionscanbeused
forindustrialapplicationsorwheresevereoperating
conditionsarepresent.Amplifierreliabilityis
enhancedbylowoperatingtemperatures.
Shouldthecoolingairbeblockedorshouldthe
amplifieroperatingtemperaturebecometoohigh,
thermalcutoutswithintheamplifierwillturnoffthe
powertotheamplifier.Thefanswillcontinueto
operate.Whentheamplifierhascooleditwill
automaticallyturnonagain.
ACPOWER
TheMC2500isdesignedtooperateon120volts
50/60Hz.Theamplifierdraws15ampereslinecur-
rentwhenamplifyingmusicorspeechatratedout-
putonprogrampeaks.Theamplifierusesonly0.7
amperewhileidlingatnooutput.Iftheamplifieris
driventoratedoutputusingsinewavesignalsitwill
draw22amperes.TheMC2500isdesignedforcon-
tinuousandreliableoperationatanypowerlevelto
itsrated500 wattscontinuousaveragepoweroutput
perchannelor1000wattsasa monoamplifier.For
continuoussinewaveoperationtheACsourcemust
becapableofsupplyingtherequired22amperes.
-11-

PerformanceLimits
PERFORMANCEGUARANTEE
PerformanceLimitsarethemaximumdeviation
fromperfectionpermittedfora Mclntoshinstru-
ment.WepromiseyoutheMC2500youbuymustbe
capableofperformanceatorexceedingtheselimits
oryougetyourmoneyback.Mclntoshistheonly
manufacturerthatmakesthisguarantee.
PERFORMANCE
Mclntoshaudiopowerratingsareinaccordance
withtheFederalTradeCommissionRegulationof
November4,1974concerningpoweroutputclaims
foramplifiersusedinhomeentertainmentproducts.
POWEROUTPUT
STEREO
500wattsminimumsinewavecontinuousaverage
poweroutput,perchannel,bothchannelsoperating
into1 ohm,2 ohms,4 ohms,or8 ohmsloadim-
pedance,whichis:
22.4voltsRMSacross1 ohm
31.6voltsRMSacross2 ohms
44.7voltsRMSacross4 ohms
63.2voltsRMSacross8 ohms
MONO
1000wattsminimumsinewavecontinuousaverage
poweroutputinto0.5ohm,1 ohm,2 ohms,4 ohms,8
ohms,or16ohmsloadimpedance,whichis:
22.4voltsRMSacross0.5ohm
31.6voltsRMSacross1 ohm
44.7voltsRMSacross2 ohms
63.2voltsRMSacross4 ohms
89.4voltsRMSacross8 ohms
126.5voltsRMSacross16ohms
OUTPUTLOADIMPEDANCE
STEREO
1ohm,2 ohms,4 ohms,and8 ohms;separateter-
minalsareprovidedforeachoutput.
MONO-PARALLEL
0.5ohm,1 ohm,2 ohms,and4 ohms;obtainedby
connectinginparalleltheappropriateterminalsof
bothchannels.
MONO-BRIDGED
2ohms,4 ohms,8 ohms,or16ohms;obtainedby
connectingtotheoutputterminalsofbothchannels.
Thebridgedoutputisbalancedtoground.Neither
sideisgrounded.
RATEDPOWERBAND
20Hzto20,000Hz
TOTALHARMONICDISTORTION
STEREO
0.02%maximumharmonicdistortionatanypower
levelfrom250 milliwattsto500 wattsperchannel
from20Hzto20,000Hz,bothchannelsoperating.
MONO
0.02%maximumharmonicdistortionatanypower
levelfrom250 milliwattsto1000wattsfrom20Hzto
20,000Hz.
INTERMODULATIONDISTORTION
STEREO
0.02%maximumifinstantaneouspeakpowerout-
putis1000wattsorlessperchannelwithbothchan-
nelsoperatingforanycombinationoffrequencies,
20Hzto20,000Hz.
MONO
0.02%maximumifinstantaneouspeakpowerout-
putis2000wattsorlessforanycombinationoffre-
quencies,20Hzto20,000Hz.
FREQUENCYRESPONSE(atonewattoutput)
20Hzto20,000Hz+0-0.25dB.
10Hzto100,000Hz+ 0.25- 1 dB.
NOISEANDHUM
95dBbelowratedoutput.
RATINGS
DAMPINGFACTOR
Greaterthan30
INPUTIMPEDANCE
50,000ohms.
INPUTSENSITIVITY
Switchable:0.75voltor2.5volts—Levelcontrolpro-
videdforhigherinputvoltages.
POWERGUARD
THDnottoexceed2%withupto20dBoverdriveat1
kHz.
GENERALINFORMATION
POWERREQUIREMENTS
120Volts50/60Hz0.7to22amps.,15ampsUL/CSA
SEMICONDUCTORCOMPLEMENT
91silicontransistors
35siliconrectifiersanddiodes
6integratedcircuits
MECHANICALINFORMATION
SIZE
Frontpanelmeasures19incheswide(48.26cm)by
10½incheshigh(26.67cm).Chassismeasures17in-
cheswide(43.18cm)by10incheshigh(25.4cm)by
17inchesdeep(43.18cm),includingconnectors.
Clearanceinfrontofmountingpanelincluding
knobs2 inches(5.08cm)
FINISH
Frontpanelisanodizedgoldandblack.Chassisis
blackbakedenamel.
MOUNTING
Standard19"(48.26cm)rackmounting
WEIGHT
129pounds(58.5kg)net,144pounds(65.3kg)in
shippingcarton
-12-

PerformanceCharts
-13-

-14-

TechnicalDescription
INPUTAMPLIFIER
Eachchannelinputcontainsa completeseven
transistorlowpoweramplifier.A differentialtran-
sistorpairprovideshighinputimpedanceandlow
noise.Thedifferentialsignalsarecombinedina cur-
rentmirrorcircuitwhichdrivesa classA amplifier
stage.Thefollowingoutputstageisa complemen-
tarypairoftransistorswithclassABbiasing.The
outputsignaldrivesthemeteringcircuit,headphone
jack,andthehighpoweroutputamplifier.This
discretetransistoramplifierdesignwasselectedfor
lownoise,lowdistortion,adequatepoweroutput
capabilitytodriveheadphones,andfreedomfrom
turnonandturnofftransients.
TheINPUTLEVELselectorandGAINcontrolsare
passiveattenuatorswhichprecedetheinput
amplifiercircuitry.Therefore,theinputsystemtothe
amplifiercannotbeoverloadedwhenthecontrols
arecorrectlyset.
IntheMONOmodeofoperationtheinputsignal
feedsonlytherightinputamplifierviathe
RIGHT/MONOGAINcontrol.Theoutputoftheright
inputamplifierfeedsbothoutputpoweramplifier
sections.WhenMONOBRIDGEMODEisselected
theleftchannelinputamplifierisusedasa phasein-
verterbeforetheleftoutputamplifier.Theoutput
channelsaretherefore180°outofphasewith
respecttoeachotherwhichisthecorrectarrange-
mentforbridgeoutputconnections.WhenMONO
PARALLELMODEisselectedthechannelsoperate
inphase.Thechannels,ofcourse,alsooperatein
phasefortheSTEREOMODE.
OUTPUTPOWERAMPLIFIER
Thepoweramplifierinputsarecoupledtothein-
putamplifieroutputsthroughanelectronicswitch.
Theswitcheliminatesturn-onandturn-offtransients
andisusedforspeakerswitching.A junctionFET
andLED/lightdependentresistornetworkmakeup
theswitch.Thiscombinationallowsthelowest
possibledistortionwhentheamplifierisonandhigh
isolationwhentheoutputpoweramplifierisoff.The
controlsignaltotheswitchisheldoffduringthe
turn-ondelayperiod.Therefore,transientsthatmay
comeintotheamplifierfromsourceequipmentwill
notbeamplifiedorreachtheloudspeaker.Sincethe
HEADPHONEoutputandmetersarepoweredbythe
inputamplifier,theiroperationisnotaffectedbythe
SPEAKERswitchorturnondelaysystem.
Thefirststageoftheoutputpoweramplifierisa
differentialtransistorpairbiasedforbestlinearity.
Theoffsettothedifferentialpairisadjustable.Cor-
rectadjustmentallowsthelowestpossibledistor-
tionatlowfrequencies.A currentmirrorcircuitcom-
binesthedifferentialoutputsintoonesignalwhich
isthenamplifiedbya followingclassA voltage
amplifier.Boththedifferentialtransitorsandthe
voltageamplifieraresuppliedbyactivecurrent
sources.Theresultsarelowerdistortionandcleaner
turnoncharacteristics.
Thedriverstageconsistingofa complimentary
pairofpowertransitorsbiasedclassABfollowthe
voltageamplifier.Next,sixcomplimentarypairsof
ruggedpowertransistorsmakeupthepoweroutput
stage.Allpowertransistorsaremountedonconser-
vativelysizedanodizedaluminumheatsinks.
Becauseofa uniqueconnectionofthebiasnetwork,
theoutputtransistorsoperateclassB andexhibitno
crossoverdistortionoftenassociatedwithclassB
operation.Theheatsinks,thereforeremaincool
whenthereisnooutput.Duringheavydemands,
temperaturecontrolledfanskeeptheheatsinks
cool.
Theamplifieroutputsignalisfedtotheoutputter-
minalsthroughtheoutputautotransformer.The
Mclntoshdesignedinterleavedmultifilarwound
autotransformerisusedtoproperlymatchthe
amplifiertostereooutputloadtapsfor1,2,4 and8
ohms.TheMC2500willdeliverfullpoweroverthe
entireaudiofrequencyrangeatanyoftheseim-
pedances.Theautotransformeralsoprotects
speakersfromdamageintheeventofamplifier
failure.Shoulda directcurrentcomponentappearin
theoutputitisshuntedbytheautotransformerand
DCcannotdamagethespeaker.
AMclntoshpatentedSentryMonitoringcircuit
constantlymonitorstheoutputsignalandinstantly
reactstopreventoverloadoftheoutputtransistors.
Atsignallevelsuptoratedoutputthiscircuithas
highimpedanceandhasnoeffectupontheoutput.
Ifthepoweroutputexceedsdesignmaximum,the
SentryMonitoringcircuitoperatestolimitthesignal
totheoutputtransistors.Intheeventofa shortcir-
cuitacrosstheamplifieroutputorsevereim-
pedancemismatchtheSentryMonitoringcircuitwill
protecttheoutputtransistorsfromfailure.Both
positiveandnegativehalvesoftheoutputsignalare
monitoredandprotectedindependently.
POWERGUARDPROTECTIONCIRCUIT
TheMclntoshpatentedPowerGuardcircuit
eliminatesamplifierclippingduetooverdrive.The
circuitalsoilluminatesredLIMITindicatorlamps
whentheamplifierisdrivenbeyonditsmaximum
outputcapacity.PowerGuardpreventsloudspeaker
damageandeliminatesharshoutputdistortion
causedbyamplifierclipping.
-15-

ThePowerGuardcircuitconsistsofa waveform
comparatorwhichmonitorsthewaveshapeofthe
amplifierinputandoutputsignals.Normallythereis
nodisparitybetweenthesesignalsandthecom-
paratorproducesnooutput.Whentheamplifieris
drivenbeyonditsmaximumpowercapacitya dif-
ferencewilldevelope.Ifthedisparityexceeds0.5%
(equivalentto0.5%totalharmonicdistortion)the
comparatoroutputcausestheredLIMITindicators
tolight.Ifthereisa furtherincreaseinthedisparity
thecomparatoroutputcontrolsanelectronicat-
tenuatorattheamplifierinputtoreducethe
amplifiergain,thusholdingtheamplifieroutputto
itsmaximumundistortedvalueregardlessofthe
degreeofoverdrivetotheamplifier.Theamplifier
maybeoverdrivenby20dBbeforetheoutputdistor-
tionexceeds2%.
Thecomparatorisanespeciallycompensated
operationalamplifierintegratedcircuit.Itsoutputis
detectedbya fullwavebridgethatfeedssignalsto
thecontrolcircuitryfortheLIMITandNORMALin-
dicatorsandtotheelectronicattenuatoratthe
amplifierinput.Theattenuatorisa lightemitting
diode/lightdependentresistornetworkselected
especiallyforitslowdistortionandtimeconstant
characteristics.
METERCIRCUIT
Themetercircuithasthreebasicsections:a loga-
rithmicamplifier,a fullwaverectifier,anda DCamp-
lifier.IntheWATTSranges,thelogarithmicamplifier
isused.IntheDECIBELranges,thesignalbypasses
thisamplifierandgoesdirectlytothefullwaverec-
tifiersthroughanattenuatorwhichiscontrolledby
theMETERRANGEswitch.
Thelogarithmicamplifierconsistsofa highgain
operationalamplifierwitha biopolarconnectedsili-
condiodepairasfeedbackelements.Thesediodes
havea uniformlogarithmiccharacteristicoveran80
dBrange.Only60dBofthislogarithmicrangeisus-
edintheMC2500.
Thefullwaverectifiercircuitusesanoperational
amplifierwithsilicondiodefeedbacknetworks.This
amplifieddiodecircuithasnearlyperfectrectifica-
tioncharacteristics.Onerectifierdetectsonlyposi-
tivesignals.Theotherrespondsonlytonegative
signalsandproducesa positiveoutput.Theoutputs
oftherectifiersarecombinedattheoperational
amplifieroutput,sothehighestsignal,either
positiveornegative,istheonethatisindicatedby
themeters.Gatediodesareusedtochargea low
leakagecapacitorwhichattainsandholdsa charge
duringsignalpeaks.Theoperationalamplifierpro-
videsa largeamountofcurrentsothecapacitorcan
chargesuddenly.Thechargeonthepeakholding
capacitorisamplifiedina twotransistorDC
amplifierwhichisusedtodrivethemeter.Fromthe
outputofthisamplifierthereisa DCfeedbacknet-
workthatconnectsbacktothedetectortoassure
excellentoveralllinearityandfrequencyresponse.
Thecurrentdrivetothemetershasa peaking
capacitortoacceleratetheupscaleresponseofthe
meterneedle.Themetersalsohavea parallelshunt
resistortocorrectlydamptheiraction.IntheWATTS
modethedischargeofthepeakholdingcapacitoris
controlledbya resistorcurrentsource.InWATTS
HOLD,theresistorisdisconnectedsothepeak
readingisretained.Therateofdecayisabout6 dB
perminute.
POWERSUPPLY
Thepowersupplyisa conventionalfullwave
bridgerectifierarrangementprovidingplusand
minus50voltsDC.Electronicregulatorsstepdown
andregulateplusandminus15voltsourcesforlow
levelcircuits.Thermistorsareusedinthepower
transformerprimarycircuittolimittheturn-oncur-
rent.
-16-

MclNTOSHLABORATORYINC.
2CHAMBERSST.,BINGHAMTON,N.Y.13903-9990
607-723-3512
Thecontinuousimprovementofitsproductsisthepolicyof
MclntoshLaboratoryIncorporatedwhoreservetherightto
improvedesignwithoutnotice.
PrintedinU.S.A.
039292