McIntosh MC275 User manual

MC275
STEREOPOWER
AMPLIFIER
MC275

MC275STEREOPOWERAMPLIFIER
INTRODUCTION
TheMC275Commemorativeissue
amplifierandowner'smanualarefaithful
reproductionsoftheoriginals,produced
fromMay,1961throughJuly,1973.A few
changesweremadetotheoriginalamplifier
designtoreflecttechnologyadvancesand
contemporarysystemrequirements.
GoldplatedBalancedAudioInputcon-
nectorsareaddedtotakeadvantageofthe
greaternoisereductioncapabilitiesof
BalancedCables.Highcurrentcapacity
GoldPlatedOutputterminalsarealsoin-
cluded.Closetolerancefilmresistors,
polypropelenecapacitorsandfiberglass
printedcircuitboardsareused.
Thisamplifierisdedicatedinremem-
branceofthelateGordonJ.Gow,coinven-
toroftheoriginalMclntoshUnityCoupled
circuit.Mr.GowwasVicePresidentofMcln-
toshLaboratoryInc.fromitsinceptionin
1949until1977whenhewaselectedPresi-
dent.HeheldthepositionofPresidentun-
til 1989.
GENERALDESCRIPTION
Overonemillionwattsofamplifieroutput
powercapacityhavebeenmanufacturedby
Mclntoshsince1949.Inthis1,000,000*watts
ofaudiopowertherearelessthan10watts
ofdistortioncapacityandlessthan1/100
wattofnoisecapacity!
Thedramaticdifferenceinthequalityof
musicreproductionwhenyoulistenthrough
Mclntoshinstrumentsisdirectlyduetolow
distortionperformance.Careful,devoted
researchisa wayoflifeatMclntosh.The
world'sfinestamplifieristhecreationof
persevering,resourcefulMclntoshengineers.
MclntoshLaboratoryistheonlymanufac-
turerintheentireindustrytoguaranteethe
lowestdistortionatallaudiofrequencies,at
fullpower.TheU.S.PatentOfficehas
recognizedtheadvancedtechnologyofthe
Mclntoshcircuitbygranting6 patents.
Longlife,flexibility,highestqualitycon-
structionarecharacteristicdesignsinevery
Mclntoshinstrument.Wideelectricaland
thermalmarginsofsafetyforallcom-
ponentsandtubes,advancedengineering,
andcooloperatingdesignaddtothelong
lifebuiltintoeveryMclntoshproduct.
Reliabilityprolongsyourinvestmentwithout
expensivemaintenancecosts.
TheMC275hasononechassistwo75
wattpoweramplifiers.Inadditiontoitsuse
asa stereoamplifier,theflexibilityofthe
MC275permitsittobeusedasa
monophonicamplifierthatdelivers150
watts,orastwoseparate75wattamplifiers
witheachchannelamplifyingcompletely
separateprograms,orastwoamplifiersfor
usewithanelectroniccrossovernetwork.
Suchflexibilitypermitsmaximumusefor
greatestreturnfromyourinvestment.
*1,000,000Wattswasthetotalpoweroutputproducedin
12yearsfrom1949through1961.In1992alone,Mclntosh
willproduceamplifierswitha totalpoweroutputofmore
than5,000,000wattswithlessthan1/100wattofdistor-
tioncapacity.
1U.S.PatentNos.:2,477,074;2.545,788;2,646,467;2,654.058;2,860,192;2.929,028.
1

TECHNICALDESCRIPTION
ThepatentedMclntoshUnityCoupledcir-
cuitandoutputtransformerhaveestablish-
edMclntoshamplifiersastheunchalleng-
edleadersintheaudiofield.
Before1949,lowdistortionathighpower
andhighefficiencywasimpossible.A com-
pletelynewengineeringapproachresulted
inanamplifierthatforthefirsttimepermit-
tedhighpowerwithdistortionbelow1%.
ThatnewengineeringproducedtheMcln-
toshUnityCoupledcircuitandtheMclntosh
bifilarwoundoutputtransformer.Withthe
introductionoftheMclntoshamplifier,new
standardsfordistortion-freeperformance
wereestablished.
PRIMARYNO.1 PRIMARYNO.2
Fig.1—Representationofbifilarwinding.
TheMclntoshoutputtransformeris
unique.Ithastwoprimarywindingswhich
arewoundbifilarly.Inthebifilartechnique
bothprimarywiresarewoundsidebyside.
Eachturnofprimarynumberoneisnextto
thesameturnofprimarynumbertwo.There
isalmostcompletemagneticcouplingbe-
tweenthetwowires.Themagneticcoupl-
ingisreinforcedbythecapacitancebe-
tweenthetwowires.
INPUT
OUTPUT
Fig.2—RepresentationofMclntoshoutputcircuit.
IntheMclntoshUnityCoupledcircuitone
ofthebifilarprimarywindingsisconnected
throughthepowersupplytotheplateand
cathodeofoneoftheoutputtubes.The
otherbifilarprimarywindingissimilarlycon-
nectedtotheothertube.
Alllowdistortionhighpoweramplifiers
usepush-pulloutputcircuitsknownas
ClassAB1AB2orB.Twotubesarearranged
ina balancedcircuit.Thispermitseachtube
tooperatealternatelysomewhatoverhalf
thetime.Comparedtofulltimeoperationof
thetubes,thepush-pullmethodreduces
heatingandpermitsmorepowerfroma
giventypeoftube.Despitethisadvantage
oftheconventionalpush-pullcircuitonepro-
bleminparticularremainedtobesolved.
Whencurrentineachtubeiscutofftobegin
theidleperiod,distortionisproducedatthe
instantwhencurrentflows.Thisformof
distortionisknownasNotchDistortionas
waswellillustratedbyMr.PenTungSahin
the"ProceedingsoftheI.R.E."Volume24,
pp1522-1541in1936.
Imperfectcouplingbetweentheprimary
windingsfoundinallconventionaloutput
transformersproducestheconditionwhich
permitsnotchdistortion.Tryingtoimprove
couplingina conventionaltransformer
decreasesthepowerresponseatbothlow
andhighfrequencies,heatingtheoutput
tubesandloweringtheavailablepower
output.
TheMclntoshUnityCoupledoutputcir-
cuitandbifilartransformeristhefirstcom-
mercialbreakthroughthateliminatesnotch
distortionbycouplingbothoutputtubes
almosttoperfection.IntheMclntosh
transformer,theextremelyclosecouplingof
thebifilarwindingsremovesthecondition
whichpermitsnotchdistortion.Furthermore
thetwooutputtubesarearrangedaspar-
tialcathodefollowers.Halfoftheoutputcir-
cuitisinthecathodeandhalfintheplate
ofeachtube.Theoutputtubesnoware
operatingina localfeedbackloopwhich
reducestheirdistortion,reducestheirinter-
nalgeneratorresistance,andreducestheir
balancerequirements.TheMclntoshcircuit
inrealityperfectspush-pullhighefficiency
outputcircuits.
TheMC275usesanadvanceddesignof
theMclntoshoutputtransformer.Thenew
2

transformerdesignincorporatesallofthe
benefitsoftheoriginalMclntoshdesignand
evenfurtherimprovesthepowerbandwidth.
Leakageinductance(lackofcoupling)
betweentheprimaryandsecondarywind-
ingsoftheoutputtransformerlimitsthe
highfrequencyresponseofanamplifier. The
primaryandsecondarywindingsofthe
Mclntoshoutputtransformerareinterleav-
edfivetimestoimprovecoupling.Thein-
terleavingisaccomplishedbywinding5
groupsofsecondarywindingsspread
uniformlywiththeprimarywindingsand
parallelconnectingthesecondarywindings.
InterleavinghelpstoextendtheMclntosh
powerbandwidthtoover100,000cycles.
Since1949,thisnewtechnologyhasbeen
builtintoeveryMclntoshvacuumtube
poweramplifier.
Goodvoltageregulationinthepower
supplypermitsoverloadswithoutovershoot
orblocking,goodtransientresponse,and
completestability.Toimproveregulationa
siliconrectifierpowersupplyisusedinthe
MC275.Inadditiontobettervoltageregula-
tion,thesiliconrectifierallowsevenhigher
operatingefficiency,cooleroperation,and
longeramplifierlife.
Togreatlyextendtubeandcomponent
life,2 thermistorsintheMC275limitcurrent
surgesproducedwhentheequipmentis
turnedon.Thethermistorisa specialtype
ofresistor.Itsresistancedependsonits
temperature.Whentheamplifierisoff,the
thermistorhasa highresistancevalue
(about25ohms).Justaftertheamplifieris
turnedon,thecurrentwhichflowsthrough
thethermistorheatsitandcausesits
resistancetodecreasetoa lowvalue(less
than0.5ohms).Currentisthuslimitedwhen
theMC275isfirstturnedonbutisnot
limitedastheunitwarms.
3

MC275TubePowerAmplifier
SPECIFICATIONS
PerformanceLimits
POWEROUTPUT,STEREO
75wattsinto16,8 or4 ohmloadsisthe
minimumsinewavecontinuousaverage
poweroutputperchannelfrom20Hzto
20,000Hz.
TheoutputRMSvoltageis:
34.6voltsacross16ohms
24.5voltsacross8 ohms
17.3voltsacross4 ohms
POWEROUTPUT,MONOPARALLEL
150wattsinto8,4 or2 ohmloadsisthe
minimumsinewavecontinuousaverage
poweroutputfrom20Hzto20,000Hz.
TheoutputRMSvoltageis:
34.6voltsacross8 ohms
24.5voltsacross4 ohms
17.3voltsacross2 ohms
OUTPUTLOADIMPEDANCE
16,8 or4 ohmsstereo
8,4 or2 ohmsmonoparallel
RATEDPOWERBAND
20Hzto20,000Hz
TOTALHARMONICDISTORTION
.5%maximumharmonicdistortionatany
powerlevelfrom250milliwattstorated
powerfrom20Hzto20,000Hz.
INTERMODULATIONDISTORTION
.5%maximumifinstantaneouspeak
poweroutputdoesnotexceedtwicetheout-
putratingforanycombinationoffrequen-
ciesfrom20Hzto20,000Hz.
FREQUENCYRESPONSE(at1 wattoutput)
20Hzto20,000Hz+0-0.2dB
10Hzto100,000Hz+0-3dB
NOISEANDHUM(A-Weighted)
100dBbelowratedoutput
Ratings
IHFDYNAMICHEADROOM
1.1dB
DAMPINGFACTOR
Greaterthan10
INPUTIMPEDANCE
100,000ohmsunbalanced
180,000ohmsbalanced
INPUTSENSITIVITY
Unbalanced,1.0voltto30voltsthrough
gaincontrol
Balanced,2.0voltsfixed
GeneralInformation
POWERREQUIREMENTS
120volts,50/60Hz
240wattsatzerosignaloutput
400wattsatratedoutput
Theamplifiermaybeconnectedfor100,120,
220or240volt50/60Hzoperation.Itisship-
pedconnectedfor120V.
MechanicalInformation
SIZE
16"wide(40.6cm)by7-1/2"high(19.0cm)
by12"deep(30.5cm)
WEIGHT
67pounds(30.5kg)net,75pounds(34.1kg)
inshippingcarton
4

PANELFACILITIES
Fig.3—TheendpaneloftheMC275showingallinputandoutputfacilities.
INPUT
TheMC275hasa two-positionMODE
Slideswitchtopermittheamplifiertobeus-
edasa 75wattperchannelstereoamplifier
ora 150wattmonoamplifier.
Theamplifieralsohasa twopositionin-
putslideswitchtopermitselectionofun-
balancedorbalancedinputconnections.
Theunbalancedinputsensitivityis1 volt.
Gaincontrolsareprovidedforhigherinput
levels.Thecontrolsaremarkedfor2.5volt
sensitivityforusewithMclntoshpreamp
equipment.Theunbalancedinputsensitivity
is2 voltsfixed.Formonooperation,thein-
putsignalistobefedtotheR (mono)input
connectors.
OUTPUT
Thetwobarrierterminalstripsmarked
OUTPUTprovidestereoconnectionsforthe
normalspeakerimpedancesof4 ohms,8
ohms,and16ohms.Formonophonicopera-
tion,connectionsfor2 ohms,4 ohms,and
8ohmsareprovided.Theterminalstripsmay
alsobeconnectedfora constantvoltageout-
putof25voltsineitherstereoormono.
TheCommonconnectionsfromtheout-
puttransformersecondarywindingsare
grounded.
FUSE
TheMC275usesa 5.0ampereslo-blotype
fuse.TheauxiliaryACsocketisnotfused.
ACOUTLET
TheauxiliaryACoutletcanbeusedto
supplypowertootherequipmentinthe
system.Theoutletwillprovidea maximum
of500wattsofpower.TheACoutletisnot
fused.
LINEVOLTAGE
TheMC275operateson120volts,
50/60Hz.Thepowertransformerhastwotap-
pedprimarywindingsandcanbeconnected
for100,120,220,or240volts.
INSTALLATION
Adequateventilationextendsthetrouble-
freelifeofelectronicinstruments.Itis
generallyfoundthateach10°centigrade
(18°F)riseintemperaturereducesthelifeof
electricalinsulationbyonehalf.Adequate
ventilationisaninexpensiveandeffective
meansofpreventinginsulationbreakdown
thatresultsfromunnecessarilyhigh
operatingtemperatures.Thedirectbenefit
ofadequateventilationislonger,trouble-
freelife.
Thesuggestedminimumspacefor
mountingtheMC275is20"longx 14"wide
x10"high.Alwaysallowforairfloweither
byventilationholesorspacenexttothebot-
tomoftheamplifieranda meansforthe
warmairtoescapeatthetop.
TheMC275canbemountedinanyposi-
tionexceptupsidedown.Iftheamplifieris
tobeinstalledona verticalsurfaceitis
recommendedthatthetransformersbeon
thedownside.Theadvantageofthisposi-
tionisthattheflowofheatfromthetubes
risesverticallyanddoesnottendtoheatthe
transformers.
5

CONNECTINGTHEMC275
INPUT-STEREO
UNBALANCEDCONNECTIONS
ConnectshieldedcablesfromtheLeft
andRightUnbalancedOutputsofa preamp-
lifiertotheLandR UNBALANCEDINPUTS.
BALANCEDCONNECTIONS
ConnectBalancedcableswithXLRcon-
nectorsfromtheLeftandRightBalanced
Outputsofa preamplifiertotheL andR
BALANCEDINPUTS.
BalancedJackPinconfiguration
Pin1.SystemGround
Pin2.+ Input
Pin3.- Input
INPUT-MONO
Connecta shieldedcablefromeitheran
UnbalancedorBalancedMONOoutputof
apreamplifiertothecorrespondingR
(MONO)UNBALANCEDorBALANCED
INPUT.
Be sure to set the INPUTS switch to the
BALanced or UNBALanced position to cor-
respond to the Inputs being used.
OUTPUT-STEREO
Forstereooperation,itisnotnecessary
tousethesameimpedanceloadoneach
output.Simplyconnecteachoutputforthe
impedancedesired.
Warning:Donotparalleltheamplifierout-
putswhenusingtheamplifierasa stereo
amplifier.Damagetotheoutputtubesmay
resultifparalleloperationisattempted.
Speakersareconnectedatthebarrier
stripsmarkedOUTPUTontheleftendofthe
panel.
IncompliancewiththeNationalElectrical
Code,ClassIIwiringcanbeusedbetween
thespeakerandtheamplifieratthe4 ohm,
8ohm,or16ohmconnection.ClassIIwir-
ingislampcord,bellwire,orotherwirewith
thistypeofinsulation.Forthenormally
shortdistancesofunder100feetbetween
theamplifierandspeaker,#18wireorlarger
canbeused.Fordistancesover100feetbe-
tweentheamplifierandspeakeruselarger
wire.
Theloudspeakerimpedanceisusually
identifiedontheloudspeakeritself.Connect
oneoftheleadsfromtheleftloudspeaker
tothescrewmarkedCOMontheleftbar-
rierstrip.Connecttheotherleadfromthe
leftloudspeakertothescrewmarkedwith
thenumbercorrespondingtothespeaker
impedanceontheLEFTbarrierstrip.
Ifthespeaker
impedanceis:
4ohms
8ohms
16ohms
Connectoneleftspeaker
leadtoscrewLEFT-COM
andtheotherto:
LEFT-4
LEFT-8
LEFT-16
Connectonerightspeaker
leadtothescrewmarked
RIGHT-COMandtheotherto:
RIGHT-4
RIGHT-8
RIGHT-16
Theonlyadverseeffectontheoperation
ofa Mclntoshamplifierwhenitisimproperly
matchedisa reductionintheamountof
distortion-freepoweravailabletothe
loudspeaker.Closeimpedancematchingis
desirableformaximumdistortion-free
power.
Usethistabletodetermineproper
speakerconnections:
OUTPUT-MONOPHONIC
WhentheMC275istooperateasa 150
wattmonophonicamplifier,theoutputsof
thetwochannelscombinetoproducea
single150wattoutput.Thischartliststhe
properconnectionsandinterconnections
formonophonicoperation.
Ifthe
speaker
impedance
is:
2ohms
4ohms
8ohms
Connectonespeaker
leadtothescrew
markedLEFT-COM
andtheotherto:
LEFT-4
LEFT-8
LEFT-16
Connecta
wirebetween:
LEFT-COMand
RIGHT-COM
LEFT-COMand
RIGHT-COM
LEFT-COMand
RIGHT-COM
Connectanother
wirebetween:
LEFT-4and
RIGHT-4
LEFT-8and
RIGHT-8
LEFT-16and
RIGHT-16
Whenconnectedasoutlined,theMC275
operatesasa 150wattmonophonic
amplifier.
LINEOUTPUT-STEREO
For25voltlineoperation,connectoneof
theleftleadstothescrewmarkedCOMon
6
Ifthespeakerimpedance
isbetween:
3.2to6.5ohms
6.5to13ohms
13to26ohms
Connectthe
speakerleads
betweenCOMand:
4ohms
8ohms
16ohms

theLEFTbarrierstrip.Theotherleftleadis
connectedtothescrewmarked8 onthe
LEFTbarrierstrip.Connecttherightleads
inthesamemannerontheRIGHTbarrrier
strip.
LINEOUTPUT-MONOPHONIC
Tofeeda 25voltline,connectonesideto
thescrewmarkedCOMontheLEFTbarrier
strip.Connecttheothersidetothescrew
marked8 ontheLEFTbarrierstrip.Then
connectwitha wirethescrewmarkedCOM
ontheLEFTbarrierstriptothescrewmark-
edCOMontheRIGHTbarrierstrip.Connect,
withanotherwire,thescrewmarked8 onthe
LEFTbarrierstriptothescrewmarked8 on
theRIGHTbarrierstrip.
ACPOWER
TheMC275operateson120volt,50to60
cyclepower.Theamplifierwillbeturnedon
andoffifitspowercordispluggedinone
oftheauxiliaryACoutletsofanaccessory
component.
OPERATINGTHEMC275
BeforeturningtheMC275on,checkall
connectionsandplugstoseethattheyare
firmlyandcorrectlyconnected.Checkto
makesurethatthetubesarefirmlyseated
inthepropersockets.Afterthefollowingad-
justmentshavebeencompleted,theMC275
operatewithoutanyfurtherattention.
ADJUSTMENTS
MODESWITCH
WiththeMODEswitchintheStereoposi-
tion,theinputsignalsaretobefedtoeither
theL andR unbalancedjacksortothe
balancedXLR,L andR connectors.Withthe
switchintheMonoposition,theinputsignal
istobefedtoeithertheR (MONO)unbalanc-
edjackortothebalancedXLR,R (MONO)
connector.
INPUTSSWITCH
PlacethisswitchintheUnbalancedposi-
tiontousetheUnbalancedinputsorinthe
BalancedpositiontousetheBalanced
inputs.
UNBALANCEDGAIN
Forstereooperation,positiontheL and
Rgaincontrolsfordeciredsensitivity.When
usingMclntoshpreamplifiers,wesuggest
settingthegaincontrolstothe2.5Vposi-
tion.Formonooperation,usetheR MONO
control.
IMPORTANT
Theexcellentperformancethatisin-
herentinallMclntoshamplifiersdoesnot
dependonthecriticaladjustmentofbiasor
balancecontrolsintheoutputcircuit.The
patentedMclntoshcircuitdeliversitsadver-
tisedspecificationswithoutanyneedfor
thesecontrolsandisnotdependenton
carefullybalancedtubesforitsperform-
ance.WithMclntoshyoucaninstallthe
amplifierandforgetit.
TheMC275youhavepurchasedwillgive
youyearsofpleasantandsatisfactoryper-
formance.Ifyouhaveanyquestionscon-
cerningtheoperationormaintenanceofthis
amplifier,pleasecontact:
MclntoshLaboratoryInc.
2ChambersStreet
Binghamton,NY13903-2699
Ourtelephonenumberis607-723-3512
Designsubjecttochangewithoutnotice.
7

LABORATORYINC.
2CHAMBERSSTREET,BINGHAMTON,NY13903-2699
MadeinU.S.A.
Phone[phone]
039952
BE032003