McIntosh MC 2120 User manual

THEMCINTOSHMC2120SOLIDSTATESTEREOPOWERAMPLIFIER
ReadingTime:29MinutesPrice$1.25
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YourMC2120StereoPowerAmplifier
willgiveyoumanyyearsofpleasant
andsatisfactoryperformance.Ifyou
haveanyquestions,pleasecontact:
CUSTOMERSERVICE
MclntoshLaboratoryInc.
2ChambersStreet
Binghamton,NewYork13903
Phone:[phone]
WARNING:TOPREVENTFIREORSHOCK
HAZARD,DONOTEXPOSETHISUNITTO
RAINORMOISTURE.
TakeAdvantageof3 years
ofFREEService...
FillintheApplicationNOW.
Contents
SERVICE. . . . 1
INSTALLATION. . . . 2
HOWTOCONNECT. . . . 4
FRONTPANELINFORMATION. . . . 8
REARPANELINFORMATION. . . . 10
PERFORMANCELIMITSANDRATINGS. . . . 11
PERFORMANCECHARTS. . . . 12
TECHNICALDESCRIPTION. . . . 14
BLOCKDIAGRAM. . . . 16
THREEYEARSERVICECONTRACT
Anapplicationfora FREETHREEYEARSERVICECONTRACTisincludedwiththismanual.
Thetermsofthecontractare:
1.Mclntoshwillprovideallparts,materialsand
laborneededtoreturnthemeasuredperform-
anceoftheinstrumenttotheoriginalper-
formancelimitsfreeofanycharge.The
SERVICECONTRACTdoesnotcoveranyship-
pingcoststoandfromtheauthorizedservice
agencyorthefactory.
2.AnyMclntoshauthorizedserviceagencywill
repairallMclntoshinstrumentsatnormal
servicerates.Toreceivethefreeserviceunder
thetermsoftheSERVICECONTRACT,the
SERVICECONTRACTCERTIFICATEmustac-
companytheinstrumentwhentakentothe
serviceagency.
3.Alwayshaveservicedonebya Mclntosh
authorizedserviceagency.Iftheinstrument
ismodifiedordamaged,asa resultofun-
authorizedrepairtheSERVICECONTRACT
willbecancelled.Damagebyimproperuse
ormishandlingisnotcoveredbytheSERV-
ICECONTRACT.
4.TheSERVICECONTRACTisissuedtoyouas
theoriginalpurchaser.Toprotectyoufrom
misrepresentationthiscontractcannotbe
transferredtoa secondowner.
5.ForyourprotectionMclntoshselectsonly
dealerswhohavetechnicalcompetenceto
guidepurchasersfairly,andprovideservice
whennecessary.ToreceivetheSERVICE
CONTRACTyourpurchasemustbemade
froma Mclntoshfranchiseddealer.
6.Yourcompletelyfilledinapplicationfora
SERVICECONTRACTmustbepostmarked
within30daysofthedateofpurchaseof
theinstrument.
7.ToreceivetheSERVICECONTRACTallin-
formationontheapplicationmustbefilled
in.TheSERVICECONTRACTwillbeissued
whenthecompletelyfilledinapplicationis
receivedatMclntoshLaboratoryIncorporated
inBinghamton,NewYork.
Copyright® 1976byMclntoshLaboratoryInc. 1
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INPUT
STEREOORTWINAMPLIFIEROPERATION
Useshieldedcablestoconnectthesignalfromthepre-
amplifierorsignalsourcetothepoweramplifierinput.To
minimizethepossibilityofhumtheshieldedcablesshould
berunparalleltoeachotherorlooselytwistedtogether.
LocatethecablesawayfromspeakerleadsandACpower
cords.Allconnectionsaremadeonthebackpanelofthe
MC 2120.
Forstereooperation,theleftoutputofthepreamplifier
shouldbepluggedintotheLeftinputjackofthepower
amplifier.Therightoutputofthepreamplifiershouldbe
pluggedintotheRight(MONO)inputjackofthepower
amplifier.
Fortwinamplifieroperationa separatesignalsource
canbeconnectedtoeachinput.Instereoortwinamplifier
operationtheMODESWITCHmustbeintheSTEREOposi-
tion.
MONOPHONICORSINGLECHANNELOPERATION
Ashieldedcablefromthesignalsourceispluggedinto
theRight(MONO)inputlackoftheMC2120only.The
MODESWITCHonthebackpaneloftheamplifiermustbe
placedintheMONOposition.IntheMONOpositionthe
rightchannelinputjackconnectstobothleftandright
poweramplifiers.TheLeftINPUTisdisconnected.Onlythe
signalfedintotheRight(MONO)inputwillbeamplified.
Monooutputisobtainedbystrappingbothoutputsin
parallel.ShouldtheMODESWITCHbeleftintheSTEREO
positionandtheoutputtransformersremainstrappedfor
amonophonicload,onechannelwillattempttodrivethe
otherwhichcauseshighcirculatingcurrentsandover-
heating.BecertainthattheMC2120isneveroperatedin
thestereomodewiththeoutputsconnectedformono-
phonicoperation.
OUTPUT
BecertaintheloudspeakersconnectedtotheMC2120
arecapableofhandlingthepoweroutputoftheampli-
fier.
Formultiplespeakeroperation,runseparateleadsfrom
eachspeakertotheamplifier.Becauseofthehighpower
availablefromtheMC2120,besuretouselargediameter
speakerleads.Inallcases,theleadstoandfromthe
speakershouldbetwinconductorortwistedtogether.Use
lampcord,bellwire,orwirewithsimilartypeofinsulation
toconnectthespeakerstotheamplifier.Whenusing8 ohm
speakersandforthenormallyshortdistancesofunder30
feetbetweentheamplifierandspeaker,#18wireor
largercanbeused.Fordistancesover30feetbetweenthe
amplifierandspeakeruselargerdiameterwire.Selectthe
correctsizewireforthewiredistancefromthechart.Itis
recommendedthattheDCresistanceofthespeakerleads
notbeover5%ofthespeakerimpedance.Upto10%can
betolerated.Resistanceoftheleadsshouldbecomputedfor
thelengthofwirebothtoandfromthespeakerorspeakers.
Wirelengthsaboverepresentthewireresistanceequalto
5%ofthespeakersimpedance.Useofsmallerwiresizes
willproducemoreresistancewhichcausesmorepower
lossandlessloudspeakerdamping.
STEREOORTWINAMPLIFIEROPERATION
Checktheimpedanceofthespeakerwhichisusually
identifiedonthespeakeritselforinthespeakerowner's
manual.Connecta leadfromthecommonterminalofthe
leftspeakertotheamplifierLEFTOUTPUTterminalstrip
COMmonscrewConnectanotherleadfromtheothertermi-
naloftheleftspeakertothescrewwiththenumbercor-
respondingtothespeakerimpedanceontheLEFTOUTPUT
terminalstrip.Therightchannelspeakerisconnectedin
thesamemannerontheRIGHTOUTPUTterminalstrip.For
stereoortwinchanneloperationitisnotnecessarytouse
thesameimpedanceloudspeakeroneachoutput.Connect
eachchannelfortheimpedancedesired.Fullpowerwill
bedeliveredtoeachproperlyconnectedspeaker.
4
Wire
Gauge
22
20
18
16
14
12
10
For4
Feet
6
10
15
25
40
60
100
MAXIMUM
OhmLoad
Meters
183
3.05
4.57
7.62
12.19
18.29
30.48
WIRE
For8
Feel
12
20
30
50
80
120
200
LENGTHS
OhmLoad
Meters
3.66
6.10
914
15.24
24.38
36.58
60.96
For16
Feel
24
40
60
100
160
240
400
OhmLoad
Meters
7.32
12.20
18.30
30.50
48.80
73.20
122.00
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Whenmultiplespeakersaretobeconnectedtoeitheror
bothoutputs,thecombinedloadimpedancemustbecalcu-
lated.Theloadmustbeconnectedtotheappropriateim-
pedancetap.Usethistabletoaidinselectingthecorrect
impedancematch:
LoadLoad
impedanceimpedance
inohmsConnecttorinohmsConnectfor
1.6to3.22 ohmoutput6.4to138 ohmoutput
3.2to6.44 ohmoutput13andup16ohmoutput
Ifa loadimpedanceisusedthatislowerthantheoutput
impedancetap,thenreducedpowerandpossibledistortion
willresult.Ifa loadimpedanceisusedthatishigherthan
theoutputimpedancetap,thenneitherthesignalnorthe
amplifierwillbeharmedbutthepoweravailableislimited.
FORSTEREOORTWINAMPLIFIERCONSTANTVOLTAGE
OPERATION:
Foroutput
voltageofConnectfor
25volts8 ohms
FORMONOPHONICCONSTANTVOLTAGELINE
OPERATION;
Foroutput
voltageofConnectedfor
25volts4 ohmoutput(mono)
Formultiplespeakeroperation,runseparateleadsfrom
eachspeakertotheamplifier.
ShouldtheMODESWITCHbeleftintheSTEREOposition
andtheoutputtransformersbestrappedfora monophonic
load,onechannelwillattempttodrivetheotherandcause
circulatingcurrentsandoverheating.Becertainthatthe
MC2120isneveroperatedinthestereomodewiththe
outputsconnectedformonophonicoperations.
ACPOWER
TheMC2120operateson120volts50/60Hz.Theaux-
iliaryACOUTLETontheMC2120isnotfusedorswitched.
MONOPHONICORSINGLECHANNELOPERATION
WhentheMC2120isusedasa monophonicorsingle
channelpoweramplifierthetwochannelsarecombinedto
produceoutputupto240watts.Theoutputsmustbetied
togetherattheappropriateloadimpedancetap.Inconnect-
inga loadtotheMC2120forsinglechanneloperation
connectthecommonsideoftheloadtotheLEFTCHANNEL
OUTPUTterminalmarkedCOM.,theotherleadiscon-
nected:
Ifthespeaker
orload
impedanceis
1ohm
2ohms
4ohms
8ohms
Thehotside
ofthelineis
connectedto
Left2
Left4
Left8
Left16
Connecta jumper
wirebetween
Left2 andRight2
Left4 andRight4
Left8 andRight8
Left16andRight16
Donotconnectunequalimpedancetapstogether
Iftheloadimpedanceisbetweenanyoftheabovefig-
ures,selectthebestimpedancematchfrom;
LoadLoad
impedanceimpedance
inohmsConnectforinohmsConnectfor
0.8to1.61 ohmoutput3.2to6 4 4 ohmoutput
1.6to3.22 ohmoutput6.4andup8 ohmoutput
5
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Stereophonic
Connections
MUSTBEINSTEREO
POSITIONFORSTEREO
PROGRAMS
6
STEREO
MODE
SWITCH
MONO
LEFT
SPEAKER
RIGHT
SPEAKER
PROGRAMSOURCE
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Monophonic
Connections
CONNECTFORMONOUSE
ONLYWHENTHEMODE
SWITCHISINMONOPOSITION
STEREO
MODE
SWITCH
MONO
7
PROGRAMSOURCE
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FrontPanel
Information
LEFTGAIN
UsetheLEFTGAINcontroltoadjustthevolumeinthe
leftchanneltothedesiredlisteninglevel.Turnthecontrol
clockwisetoincreasethevolume.
RIGHT/MONOGAIN
UsetheRIGHT/MONOGAINcontroltoadjustthevolume
intherightchanneltothedesiredlisteninglevel.Turnthe
controlclockwisetoincreasethevolume.
MONO
WiththeoutputofMC2120connectedformonophonic
operationandtherearpanelMODESWITCHinMONOthe
volumeiscontrolledbytheRIGHT/MONOGAINcontrol
only.
LIMIT/NORMAL
TheNORMAL(green)andLIMIT(red)indicatorsarea
newMclntoshdevelopment.Aslongastheamplifier
operateswithoutoverloadtheNORMALindicatorillu-
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minates.A waveformcomparatorintheMC2120con-
stantlycomparestheamplifierinputandoutputwave-
form.Waveformdifferenceoftheoutputwaveiscon-
vertedtoa voltagewhichisusedasa controlsignalto
"turn off" the NORMAL indicator andto "turn on"the
LIMITindicator.TheLIMITindicatorturnsonwheneverthe
waveformdifferenceexceeds0.5%.Generally,these
waveformdifferencesaretheresultofapplyingvoltages
attheinputinexcessofitsrating.Becausethelimitin-
dicatorcircuitcanshowanoverloadconditionasshortas
onecycleofa 20,000Hzsignala holdingcircuitkeepsthe
redindicatoronlongenoughfortheeyetosee.
HEADPHONES
TheoutputofthefrontpanelHEADPHONEjackhasbeen
designedtofeedlowimpedancedynamicstereohead-
phones.Electrostaticheadphonesgenerallyrequirehigher
powerthandynamicheadphones.ConnectthemtotheLEFT
andRIGHTOUTPUTcontactsonthebackoftheMC2120.
PlugdynamicheadphonesintothefrontpanelHEADPHONE
jack.AdjustthefrontpanelLEFTGAINandRIGHT/MONO
GAINcontrolforcomfortableheadphonelistening.
POWER
ThepowerswitchturnstheMC2120ONorOFF.The
switchdoesnotcontrolthepoweroutletonthebackpanel.
IfyouwishtocontroltheACpowerfroma preamplifier
controlcenterleavetheswitchintheONposition.Besure
theACcordoftheMC2120ispluggedintothecontrolled
outletsontherearofthepreamplifiercontrolcenter.
OFF:IntheOFFpositiontheACpowertotheamplifier
isturnedoff.
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RearPanelInformation
LEFTandRIGHTOUTPUT
Forstereooperation,outputimpedancesof2,4,8 and
16ohmshavebeenprovidedona secure,screwtypebar-
rierstrip.Formonophonicoperationproperinterconnection
provides,1,2,4 and8 ohmsfromthesamebarrierstrips.
POWERGUARD
Amplifiersarecapableofdeliveringlargequantitiesof
powerwhentheyaredriventoclipping.Clippingis
causedwhentheamplifierisaskedtoproducemorepower
outputthanitcandeliverwithlowdistortion.A clipped
amplifiercanhavemorethan40%harmonicdistortion.
Theextraenergycontentoftheclippedsignalwilldam-
agemostspeakers.A newMclntoshadvancementhelps
toprotectyourspeakerfromthiskindofdamage.The
MC2120hasa builtin"waveformcomparator"thatcom-
paresthewaveshapeoftheinputsignalwiththeoutput
signal.Ifthenon-linearitybetweenthetwosignalsexceeds
0.5%a frontpanelsignalilluminatesinred(LIMIT).With
thePOWERGUARDswitchintheNORMALposition,1.0%
distortionwillcausethePOWERGUARDcircuittooperate.
ThePOWERGUARDcircuitlimitstheinputdynamicallyso
thattheamplifiercannotbeoverdriven.Thiseliminates
amplifieroutputclipping.Recommendedoperationsiswith
thePOWERGUARDswitchintheNORMALpositionatall
times.
MODESWITCH
TheMC2120canbeusedinthesedifferentmanners:
stereo(orasa twinamplifier)andmono.Becauseofthe
excellentchannelseparationoftheMC2120,itcan
allofthesefunctions.Whentheinstrumentisproperlycon-
nectedtheMODEswitchisusedtoselectthemethodof
operationdesired.
INPUT
Inthestereoortwinamplifiermodeofoperationboth
inputjacksacceptsignal.Inthemonomodeofoperation
theLeftchannelinputjackisdisconnectedandonlythe
Right(MONO)channelinputjackacceptssignal.
INPUTLEVEL
TheinputsensitivityoftheMC2120is0.75voltsor
2.5V dependingonthepositionoftheINPUTLEVELswitch.
WithindicatedvoltageappliedandtheGAINcontrolclock-
wise,theamplifierwilldeliveritsratedpower.AllMclntosh
preamplifiershavebeendesignedtodeliver2.5voltsout-
putwithratedinput.Forthebestsignaltonoiseratiowhen
usingMclntoshsourceequipment,alwayshavetheINPUT
LEVELswitchinthe2.5V positionandthefrontpanelLEFT
andRIGHT/MONOGAINcontrolsinthefullyclockwiseposi-
tion.Ifmoregainisdesiredthe0.75V positionmaybe
used.ForsourceequipmentotherthanMclntoshsetthe
switchinthepositionnearesttothestatedoutputratingof
thesourceequipment.
ACPOWER
TheMC2120isratedfor120volts,50/60hertz.Ituses
50wattswhenthereisnosignaloutputand460watts
withbothchannelsdeliveringratedpower.An8 ampere
fuseprotectstheMC2120electrically.AnauxiliaryAC
poweroutlethasbeenprovided.Theauxiliaryoutletis
neitherfusednorswitched.
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PerformanceLimitsandRatings
PERFORMANCEGUARANTEE
PerformanceLimitsarethemaximumdeviationfromper-
fectionpermittedfora Mclntoshinstrument.Wepromise
youthattheMC2120youbuymustbecapableofper-
formanceatorexceedingtheselimitsoryougetyour
moneyback.Mclntoshistheonlymanufacturerthatmakes
thisguarantee.
PERFORMANCE
Mclntoshaudiopowerratingsareinaccordancewiththe
FederalTradeCommissionRegulationofNovember4,1974
concerningpoweroutputclaimsforamplifiersusedinhome
entertainmentproducts.
POWEROUTPUT
STEREO
120wattsminimumsinewavecontinuous
averagepoweroutputperchannel,both
channelsoperatinginto2 ohms,4 ohms,8
ohms,or16ohmsloadimpedance,whichis:
15.5voltsRMSacross2 ohms
21.9voltsRMSacross4 ohms
31.0voltsRMSacross8 ohms
43.8voltsRMSacross16ohms
MONO
240wattsminimumsinewavecontinuous
averagepoweroutputinto1 ohm,2
ohms,4 ohms,or8 ohmsloadimpedance,
whichis:
15.5voltsRMSacross1 ohm
21.9voltsRMSacross2 ohms
31.0voltsRMSacross4 ohms
43.8voltsRMSacross8 ohms
OUTPUTLOADIMPEDANCE
STEREO
2ohms,4 ohms,8 ohms,and16ohms;
separateterminalsareprovidedforeach
output
MONO
1ohm,2 ohms,4 ohms,and8 ohms;ob-
tainedbyconnectingtogethertheappro-
priateterminalsofbothchannels
RATEDPOWERBAND
20Hzto20,000Hz
TOTALHARMONICDISTORTION
STEREO
0.1%maximumharmonicdistortionatany
powerlevelfrom250milliwattsto120
wattsperchannelfrom20Hzto20,000
Hz,bothchannelsoperating
MONO
0.1%maximumharmonicdistortionatany
powerlevelfrom250milliwattsto240
wattsfrom20Hzto20,000Hz
INTERMODULATIONDISTORTION
STEREO
0.1%maximumifinstantaneouspeakpoweroutputis240
wattsorlessperchannelwithbothchannelsoperatingfor
anycombinationoffrequencies,20Hzto20,000Hz
MONO
0.1%maximumifinstantaneouspeakpoweroutputis480
wattsorlessforanycombinationoffrequencies,20Hzto
20,000Hz
FREQUENCYRESPONSE(atonewattoutput)
20Hzto20,000Hz,+0-0.25dB
10Hzto100,000Hz+0-3.0dB
NOISEANDHUM
95dBbelowratedoutput
RATINGS
OUTPUTVOLTAGES
25voltsfordistributionlines
DAMPINGFACTOR
STEREO
16at2 ohmsoutput,50at4 ohmsoutput,20at8 ohms
output,14at16ohmsoutput
MONO
16at1 ohm,50at2 ohms,20at4 ohms,and14at8
ohmsoutput
INPUTIMPEDANCE
100,000ohms
INPUTSENSITIVITY
Switchable:0.75voltor2.5volts—Levelcontrolprovided
forhigherinputvoltages.
GENERALINFORMATION
POWERREQUIREMENTS
120volts50/60Hz,50wattsatzerosignaloutput,460
wattsatratedoutput
SEMICONDUCTORCOMPLEMENT
39silicontransistors
24siliconrectifiersanddiodes
2integratedcircuits
MECHANICALINFORMATION
SIZE
Frontpanelmeasures16incheswide(40.7cm)by5 7/32
incheshigh(13.2cm).Chassismeasures145/8inches
wide(37.2cm)by5 incheshigh(12.7cm)by13inches
deep(33cm),includingconnectors.Knobclearancere-
quiredis1 1/2inches(3.8cm)infrontofmountingpanel
FINISH
Frontpanelisanodizedgold.Chassisischromeandblack
WEIGHT
57pounds(25.9kg)net,70pounds(31.8kg)inshipping
carton
11
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PerformanceCharts
PerChannelBothChannelsOperating
12
HARMONICDISTORTIONATRATEDOUTPUT
FREQUENCYRESPONSEAT1 WATTOUTPUT
POWERBANDWIDTHAT0.1%THD
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OUTPUTSIGNALWAVEFORMSHOWINGACTIONOFPOWER
GUARDTOELIMINATEOUTPUTSIGNALCLIPPING.AMPLIFIER
INPUTISOVERDRIVENBY20dBFORBOTHOSCILLOGRAM
TRACES.
13
HARMONICDISTORTIONVS.POWEROUTPUT
INTERMODULATIONDISTORTIONVS.POWEROUTPUT
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Technical
Description
INPUTAMPLIFIER
Separateinputamplifiersareusedfortherightand
leftchannels.Eachinputamplifierisa twotransistorfeed-
backamplifierwhichhasunitygain.Theyareusedtopro-
videhighinputimpedanceandlowoutputimpedancefor
drivingthepoweramplifiersections.Inaddition,impe-
dancematchingisprovidedforthePOWERGUARDsystem
inputattenuator.
WhentheMODEselectorisswitchedtoMONO,onlythe
rightchannelinput,GAINcontrol,POWERGUARDattenua-
tor,andinputamplifierisusedtodrivebothpowerampli-
fiers.
POWERAMPLIFIER
Therearetwoidenticalpoweramplifiersections.Atthe
inputtoeachpoweramplifierisa monolithicdifferential
transistorselectedforlownoise.Themonolithicdesign
yieldscloselymatchedtransistorsforlowdistortion.The
inputandthefeedbacksignalsareappliedtothisdifferen-
tialamplifier.Thesecondstageisa classA voltageampli-
fierusinga singletransistorwithfeedback.
Thepredriver,driver,andoutputsectionsfollowandare
fullycomplimentarywithhighefficiencyandnegligibledis-
tortion.Thepredriverisa complimentarytransistorpair
biasedtonearlyclassA operationforlowcrossoverdistor-
tion.Biasissuppliedbya transistorwhichservesasa tem-
peraturetrackingcircuitsothepredriverisproperlybiased
regardlessoftemperature.Thepredriverhasa generous
amountofemitterdegenerationforlowdistortion.
Anothercomplimentarypairofpowertransistorsbiased
forclassABoperationmakeuptheoutputdriverstages.
Theirbiasisdevelopedacrossa thermistorwhichholdsthe
biascurrentstableregardlessoftemperature.
Theoutputstageconsistsof4 ruggedcomplimentary
powertransistorsconnectedinsingleendedpush-pull
parallel.Thisstageisconnectedasa balancedemitterfol-
lowerwhichallowsgreatstabilityandlowdistortion.Due
toa uniquearrangementoftheoutputbiasnetwork,the
outputtransistorsareoperatedclassB freeofcrossover
distortion.Heatisnotproducedbythesetransistorswhen
thereisnooutput.Thebiasnetworkfortheoutputstage
istemperaturecompensatedassuringclassB operation
atanytemperatureorpowerlevel.
Theoutputtransistorsanddriversaremountedongener-
oussizedblackanodizedaluminumheatsinks.Thefree
flowofroomtemperatureairthatpassesthroughtheover-
sizedheatsinksprovidesthecoolingnecessaryfurthe
longlifeofcomponents.
TheMC2120hastransientfreeturnonandturnoffchar-
acteristics.Thisisbecausetheoutputoftheamplifieris
switchedbya heavydutyrelaytotheoutputautotrans-
former.Therelayisdrivenbya transistorswitch.Thecon-
trolsignaltothistransistorisderivedfroma longtimecon-
stantcapacitorchargingnetwork.Theswitchturnsonthe
relayapproximatelytwosecondsaftertheMC2120is
turnedon.Thesamecircuithasa shortturn-offtimecon-
stantwhichturnsofftherelaybeforetheamplifier'smain
powersupplyhashada chancetodrop.Theoutputsignal
isfedthrougha matchingnetworktothefrontpanel
HEADPHONEjack.TheHEADPHONEoutputisdesignedto
feedlowimpedancedynamicstereoheadphones.
Theamplifieroutputsignalisfedtotheoutputterminals
throughtheautotransformer.TheMclntoshdesignedinter-
leavedmultifilarwoundautotransformerisusedtoproperly
matchtheamplifiertoanyoutputimpedancetap.TheMC
2120willdeliverfullpowerovertheentireaudiofrequency
rangeatanyoftheseimpedances.TheMC2120willnot
overheatwhendrivinglowimpedanceloads(2ohmsor
4ohms).Theautotransformeralsoprotectsspeakersfrom
damageintheeventofamplifierfailure.Shoulda direct
currentcomponentappearintheoutputitisshuntedbythe
autotransformerandcannotdamagethespeaker.
TheMclntoshpatentedSentryMonitoringcircuitcon-
stantlymonitorstheoutputsignalandinstantlyreactsto
preventoverloadoftheoutputtransistors.Atsignallevels
uptoratedoutputthiscircuithashighimpedanceandhas
noeffectupontheoutput.Ifthepoweroutputexceeds
designmaximum,theSentryMonitoringcircuitoperates
tolimitthesignaltotheoutputtransistors.Intheevent
ofa shortcircuitacrosstheamplifieroutputorsevereim-
pedancemismatchtheSentryMonitoringcircuitwill
protecttheoutputtransistorsfromfailure.Thepositiveand
negativehalvesoftheoutputsignalaremonitoredinde-
pendently.
LIMITINDICATORANDPOWERGUARD
ThefrontpanelNORMAL(green)andLIMIT(red)indi-
catorsareactivatedbya newlydesignedMclntoshcircuit.
TheMC2120hasa builtin"waveformcomparator."The
waveformcomparatorelectricallycomparestheamplifier's
outputwaveformwiththeinputwaveform.Shouldthe
waveformdifferencesreach0.5%,theredLIMITindicator
onthefrontpanelisturnedonandtheNORMALindicator
isturnedoff.Ifthedifferencesincreaseto1%thePOWER
GUARDcircuitisactivated.POWERGUARDprovidesan
unusualmarginofsafetyforloudspeakersbythepreven-
tionofamplifierclippingyetpermitstheamplifiertode-
liverdesignedmaximumpower.
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Theinputwaveformandtheoutputwaveformarecom-
paredinanintegratedcircuitdifferentialamplifier.Any
differences,duetodistortionoftheoutputwaveform,are
convertedtoa controlvoltage.Thiscontrolvoltageisapplied
differentiallytoa highgamoperationalamplifierwhereit
isamplifiedabout100times.Theamplifiedcontrolvolt-
ageisdetectedbya fullwavebridgerectifierthenentersa
Schmidttriggerwhichisprogrammedtosequencethe
frontpanelindicatorsfromNORMAL(green)toLIMIT(red]
ata presetdistortionlevel.Alsobuiltintothetriggercircuit
isa timingcapacitorwhichholdstheLIMITindicatoron
longenoughtobevisibleevenwhenthedurationofthe
overloadismuchshorter.TheLIMITindicatorwillbeillumi-
natedforanoverloadconditionasshortasonecycleofa
signalat20,000Hz.
ThesamecontrolvoltageusedtoactivatetheLIMIT
indicatoralsoactuatesthePOWERGUARDsystem.Ifthe
controlvoltageisgreaterthanisneededtoilluminatethe
LIMITindicationthenthePOWERGUARDbeginsoperating.
Thecontrolvoltageisusedtooperatea lightemitting
diode/lightdependentresistor(LDR)network.TheLDRis
partofanattenuatorbetweenthevolumecontrolandthe
inputamplifier.Theinputtothepoweramplifiercanbe
attenuatedtoaslittleasonetenth,ifnecessary,inorder
tomaintainlowdistortionintheeventofexcessiveinput
signal.
ThePOWERGUARDcircuitisactivatedwhentherear
panelswitchisintheNORMALposition.POWERGUARD
protectionisdisabledm theOUTposition.TheLIMITindi-
catoroperatesregardlessofthepositionofthePOWER
GUARDswitch.
15
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5/10/2017
Digitized in Heiloo Netherland

16
www.freeservicemanuals.info
5/10/2017
Digitized in Heiloo Netherland

www.freeservicemanuals.info
5/10/2017
Digitized in Heiloo Netherland

MCINTOSHLABORATORYINC.
2CHAMBERSST.,BINGHAMTON,N.Y.13903
607-723-3512
Designsubjecttochangewithoutnotice.
PrintedinU.S.A.
038-921
www.freeservicemanuals.info
5/10/2017
Digitized in Heiloo Netherland