McIntosh C28 User manual

THEMCINTOSHC 28SOLIDSTATESTEREOPREAMPLIFIER
ReadingTime:37Minutes Price$1.25

TakeAdvantageof3 years
ofFREEService. . .
FillintheApplicationNOW.
ServiceContract..1
Installation..2,3
HowtoConnect..4,5,6,7
AdjustingTheTopPanelControls..8
UsingTheFrontPanelControls..9,10
UsingthePushbuttons..11
BalancingYourStereo..12,13
ListeningToYourStereo..13
PerformanceLimitsandRatings..14,15
PerformanceCharts..16
TechnicalDescription..18
BlockDiagram..20
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willbecancelled.Damagebyimproperuse
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ICECONTRACT.
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Copyright(c)1970ByMclntoshLaboratoryInc.
1
YourMC28StereoPreamplifier
willgiveyoumanyyearsofpleasant
andsatisfactoryperformance.Ifyou
haveanyquestions,pleasecontact:
CUSTOMERSERVICE
MclntoshLaboratoryInc.
2ChambersStreet
Binghamton,NewYork13903
Phone:[phone]
WARNING:TOPREVENTFIREORSHOCK
HAZARD,DONOTEXPOSETHISUNIT TORAIN
ORMOISTURE.
CONTENTS

HowToConnect
CONNECTINGA RECORDPLAYERTO PHONO1
Connectthecablefromthe"left"channelofthe
recordplayerintothe"L"PHONO1 inputjack.
Connectthecablefromthe"right"channelofthe
recordplayerintothe"R"PHONO1 jack.
PHONO2 isprovidedfortheuseofa secondrecord
player.
Connectthecablefromthe"left"channelofthe
recordplayerintothe"L"PHONO2 inputjack.
Connectthecablefromthe"right"channelofthe
recordplayerintothe"R"PHONO2 inputjack.
CONNECTINGA MICROPHONE
Connectthecablefromthe"left"microphoneto
the"L"MICinputjack.Connectthecablefromthe
"right"microphonetothe"R"MICinputjack.
CONNECTINGA TAPEDECKFORPLAYBACK
Connectthecablefromthe"left"taperecorderhead
onthetapedeck(onewithoutitsownelectronics)
tothe"L"TAPEHEADinput.Connectthecable
fromthe"right"taperecorderheadtothe"R"TAPE
HEADinput.
CONNECTINGA STEREOTUNER
Connectthecablefromthe"left"channeltunerout-
puttothe"L"tunerinputjack.
Connectthecablefromthe"right"channeltuner
outputtothe"R"TUNERjack.
AUX
Anyhighlevelprogramsourcesuchasanothertuner
ora TVsetcanbeconnectedtotheinputjacks
marked"AUX."
CONNECTINGA TAPERECORDER
ToRecord:
Connecta cablefromthe"L"TAPEOUTPUTjack
markedTAPE1 tothe"left"highlevelinputtape
recorder.
Connecta cablefromthe"R"TAPEOUTPUTjack
markedTAPE1 tothe"right"highlevelinputofthe
taperecorder.
Connecta secondtaperecorderinthesamefashion
totheTAPE2 outputs.
ToPlaybackMonitor:Connectthecablefromthe
"left"channeloutputofa taperecordertothehigh
levelinputs. . . "L"TAPE1.
Connectthecablefromthe"right"channeloutputof
ataperecordertothehighlevelinput. . . "R"TAPE.
Connecta secondtaperecorderinthesamefashion
totheTAPE2 inputjacks.
4

5

CONNECTINGTHEC 28TOPOWERAMPLIFIERS
ConnecttheMAINoutputjackstotheinputofa
stereopoweramplifier.The"L"jackisconnectedto
the"left"amplifierinputjack.The"R"jackiscon-
nectedtothe"right"amplifierinputjack.
TheoutputimpedanceattheMAINoutletis100
ohms.Longercablesthanaresuppliedcanbecon-
nectedbetweentheC 28andtheamplifiers.The
lengthofthecableislimitedbythecapacityofthe
cable.Thetotalcapacitymustnotexceed1000pF.
Forinstance:cableswitha capacityof25pFper
footmaybe40feetlong.13.5pFperfootcable
maybe75feetlong.Theinputimpedanceofthe
amplifiersshouldbe47,000ohmsorgreater.
CTROUTPUT
UsetheCTRoutputtofeedleftplusrightsignaltoa
separatepoweramplifierformonophonicback-
groundmusicorfora centerchannelspeaker.
SPEAKERCONTROLRELAY
Tocontrolmainandremoteloudspeakersfromthe
frontpaneloftheC 28anaccessoryMclntoshSCR
controlisneeded.PlugthecablefromtheSCRcon-
trolintheSPEAKERCONTROLRELAYreceptacle.
ThespeakersarethenconnectedtotheSCRcon-
trol.TheACoutletsontheSCRhavea totalcapa-
cityof2400watts.
ACOUTLETS
Thereare4 blackACoutlets,2 greenACoutlets,
andoneredACoutlet.ThepowertotheblackAC
outletsiscontrolledbythefrontpanelswitch.Use
theseoutletsfora tuner,taperecorder,etc.The
greenoutletsarecontrolledbythetoppanelswitch
marked"PowerAmplifier."Thetotalcapacityofall
ACpoweroutletsis600watts.Topreventburnout,
usetheACoutletsontheCRforpowerrequirements
exceeding600watts.Theredreceptacleisonatall
times.Usetheredoutletfora turntableorrecord
changer.Theturntableisprotectedbythisarrange-
ment.Itisnecessarytoturnofftheturntableorrec-
ordchangerwithitsownACswitch.
GROUNDCONNECTION
Asinglegroundpostisprovided.Groundsforturn-
tables,recordchangers,tapedecks,etc.shouldbe
connectedtothispost.Theleftandrightprogram
cablesandthegroundwirefromthatsourceshould
bewoundortwistedtogether.Toavoidhum,make
surethegroundwiredoesnotmakeanyconnec-
tionstotheshieldsoftheleftandrightprogram
cablesbetweentheprogramsourceandtheC 28.
6

7

AdjustingTheTopPanelControls
Youwillenjoythebestinstereoperformancemore
whenthestereosystemisproperlybalanced.For
properbalanceeachchannelmustbeequalinloud-
nessandsimilarinfrequencyresponse.Usethetop
panelLEVELcontrolstobalancesystemloudness
andthetoppanelBASSTRIMcontrolstobalancefor
frequencyresponsedifferencesatlowfrequencies.
DonotusethefrontpanelBALANCEcontrolto
correctforsystemunbalance.TheBALANCEcontrol
isusedtoadjustforanyunbalanceinthesource
material,therecord,etc.
BeforeattemptingtobalancetheoutputoftheC 28
makecertainthefrontpanelcontrolsaresetproperly.
TurntheVOLUMEcontroltotheOFF(totallycoun-
ter-clockwise)position.
SettheMODESELECTORtoSTEREOandthe
BALANCEcontrolatthecenteror12o'clockposition.
Turnthetonecontrolstothecenteror12o'clock
position.TheHFandLFpushbuttonsshouldbein
theOUTpositionandtheCOMPensationcontrolin
theFLATposition.TurntheINPUTSELECTORto
TUNER.
DepressthePANLOCbuttonsandpullthepre-
amplifierouttotheadjustposition.Thecontrolson
thetoppanelarenoweasilyadjusted.
OnthetoppanelturntheBASSTRIMcontrols
counter-clockwise.Turnallthelevelcontrolstothe
clockwiseposition.TurnthePOWERAMPLIFIER
switchon.
Withthetunerturnedon,turnuptheVOLUME
controlonthefrontpaneluntila comfortableroom
loudnessisreached.
ThenswitchtheINPUTSELECTORtoPHONO1,
theMODESELECTORtoL TOL & R,usea mono-
phonicrecordandturndownthetoppanelLEFT
PHONO1 LEVELcontroltomatchtheloudnessofthe
tuner.RepeattheprocedurewiththeRIGHTPHONO
1LEVELcontrol.Whenthechannelsareequallyloud
thesystemisbalancedforloudness.
RepeatthesamestepsforPHONO2.
BassTrim
Acontrolhasbeenprovidedineachchannelthat
adjuststhebassloudness.Thesecontrolsareused
tocompensateforthedifferencesinloudspeakerre-
sponseatlowfrequenciesorfortheroominwhich
theyareused.Theyarevariablecontrolsthatprovide
upto6 dBofboostbelow100Hz.Withthecontrol
inthecounter-clockwisepositiontheresponseisflat.
Clockwiserotationincreasesthebassloudness.Ad-
justonechannelata time.
HeadphoneLevel
Acontrolhasbeenprovidedineachchannelto
adjusttheloudnessoftheprogramwhenusinghead-
phones.Thecontrolsadjusttheoutputineachchan-
neltotheHEADPHONEjackonlyanddonotaffect
theprogramloudnessfromtheloudspeakers.Toad-
justforcomfortableheadphonelisteningfirstadjust
theVOLUMEcontroltothefrontpaneltonormal
roomloudness.Then,whilewearingtheheadphones,
adjusttheHEADPHONELEVELcontroltothedesired
loudnesslevel.
OutputLevel
Acontrolhasbeenprovidedineachchannelthat
adjuststheoutputofthepreamplifier.Usethesecon-
trolstocompensateforminordifferencesinloud-
speakerefficiencyoramplifiergain.Thesecontrols
adjusttheleveltotheMAINOUTPUTSonly.
WiththeMODESELECTORintheMONO(L+ R)
positionsettheBALANCEcontrolto12o'clock.Ad-
justthetoppanelOUTPUTLEVELcontrolsuntilthe
loudnessfromeachloudspeakerisequal.
CenterChannelLevel
Acontrolhasbeenprovidedthatadjuststhecom-
binedleftplusrightprogramleveltotheCTRoutput
jacks.
PowerAmplifierOn/Off
Aswitchhasbeenprovidedthatwillcontrolthe
ACpowertothegreenPOWERAMPLIFIERrecepta-
clesindependentlyoftheC 28frontpanelcontrol.
Usethisswitchtoturnofftheamplifierwhenlisten-
ingtotheheadphones,etc.
IMPORTANT:TOHEARTHEPROGRAMFROMTHE
LOUDSPEAKERSTHISSWITCHMUSTBEINTHE
ONPOSITION.
8

UsingTheFrontPanelControls
Intheupperleftofthefrontpanelisa concentric
control.ThelargeouterknobistheBALANCEcon-
trol.ThesmallcenterknobistheCOMPENSATION
control.
BALANCE
TheBALANCEcontroladjustsforunequalloud-
nessineithertheleftorrightchannels.Theloud-
nessofthechannelscanbevariedrelativetoeach
otherwithoutaffectingtheircombinedloudness.
Left. . . turningthecontroltotheleftaccentsthe
leftchannelbyreducingtherightchanneloutput.
Right. . . turningthecontroltotherightaccents
therightchannelbyreducingtheleftchannelout-
put.
COMPENSATION
TheCOMPENSATIONcontrolisa threeposition
switch.
Pres:Intheleftofcenterpositiontheresponseof
theC28isshapedtoemphasizetheuppermid-
frequencies.
Flat:InthecenterpositiontheresponseoftheC 28
isunmodified.
Loud:TheLOUDnesscontrolautomaticallypro-
videsthecorrectamountofbassrequiredtocom-
pensateforthechangeinresponseofthehuman
earatlowloudnesslevels.
Whenthevolumeisreduced,themusicwill
seemtolosemuchofitsbassandsomeofits
treble.Thiseffectisduetothesensitivitycharac-
teristicofhumanhearing.Theresponseofthehu-
maneartobassandtreblepitchdecreasesmore
rapidlythanitsresponsetopitchcenteredinthe
mid-tonalrange.TheLOUDnessswitchconverts
thevolumecontroltoa loudnesscompensated
control.UseLOUDnesstolistenatlowvolumeand
stillhearfull-frequencyrange.
INPUTSELECTOR
Aux:Connectstheoutputfromanyhighlevelpro-
gramsourcerequiringflatamplificationtothehigh
levelinputstage.Sucha sourcecouldbea tele-
visionset.IntheAUXpositionthegainis20dBto
theMAINoutputs,0 dBtotheTAPEoutputs,and
17.5dBtotheLINEoutputs.Theinputimpedance
is250,000ohms.
Tuner:ConnectstheoutputfromanyAM,FMor
MPXFMtunertothehighlevelinputstage.Inthe
TUNERpositiontheC 28hasflatamplification.
Thereis20dBofgaintotheMAINoutputs,0 dB
totheTAPEoutputsand17.5dBtotheLINEout-
puts.Theinputimpedanceis250,000ohms.
Phono1:Connectstheoutputofanymagnetic
phonocartridgetothelowlevelinputstage.The
responsehasbeenshapedtocompensateforthe
characteristicsofthemagneticphonocartridge.
Thegainat1,000Hzis62dBtotheMAINoutputs,
42dBtotheTAPEoutputsand59.5dBtotheLINE
outputs.Theinputimpedanceis47,000ohms.
Phono2:SameasPHONO1.
Mic:Connectstheoutputofanyhighimpedance
microphonetothelowlevelinputstage.IntheMIC
positiontheC 28hasflatamplification.Thegain
is60dBtotheMAINoutputs,40dBtotheTAPE
outputsand57.5dBtotheLINEoutputs.Theinput
impedanceis500,000ohms.
TapeHd:Connectstheoutputofanytapehead(a
tapedeckwithoutitsownelectronics)tothelow
levelinputstages.Theresponsehasbeenshaped
tocompensateforthecharacteristicsofthetape
head.Thegainat500Hzis64dBtotheMAINout-
puts,44dBtotheTAPEoutputsand61.5dBtothe
LINEoutputs.Theinputimpedanceis500,000
ohms.
MODESELECTORConnectstheprogramtothe
loudspeakerinthefollowingsevenways:
LtoL & R:Connectsthe"left"inputtobothloud-
speakers.
RtoL & R:Connectsthe"right"inputtobothloud-
speakers.
StereoRev:Connectsthe"left"inputtothe"right"
loudspeakerandthe"right"inputtothe"left"
loudspeaker.
Stereo:Connectsthe"left"inputtothe"left"loud-
speakerandthe"right"inputtothe"right"loud-
speaker.
Mono(L+ R):addsthe"left"inputandthe"right"
inputandthenconnectstheL + R programtoboth
amplifiersandloudspeakers.
L+ R toL:Connectsthe"leftplusright"program
tothe"left"loudspeakeronly.
L+ R toR:Connectsthe"leftplusright"program
tothe"right"loudspeakeronly.
9

VOLUMEON/OFF
TurningtheVOLUMEtotallycounter-clockwise
turnstheC28OFF.TheVOLUMEcontrolregulates
theloudnessinbothchannels.TheVOLUMEcon-
trolhasbeenprecisiontrackedthroughoutthelis-
teningrange(0to-65dB)foraccuratestereo
balance.
BASS
TheC 28hasseparate11positiontonecontrol
switchesineachchannelforbassandtreble.
Left:Adjuststhebassloudnessfromtheleftloud-
speaker.Clockwiserotationincreasesthebass
loudnesswhilecounter-clockwiserotationde-
creasesthebassloudness.Eachstepofthetone
controladjuststhebassloudness4 dB.
Right:Hasthesameeffectonthesoundfromthe
rightloudspeaker.
TREBLE
Left:Adjuststhetrebleloudnessfromtheleftloud-
speaker.Clockwiserotationincreasesthetreble
loudnesswhilecounterclockwiserotationde-
creasesthetrebleloudness.Eachstepofthetone
controladjuststhetrebleloudnessabout4 dB.
Right:Hasthesameeffectonthesoundfromthe
rightloudspeaker.
PANLOC
MclntoshdevelopedPANLOCmountingbringspro-
fessionalinstallationtechniquetostereo.Depress-
ingthePANLOCbuttonfaswitha ballpointpen)
willreleasetheinstrument.Itcanthenbepulled
towardyoutothe"adjustment"position.Inthis
positionthetoppanelcontrolscanbeadjusted.
10

UsingThePushbuttons
TheC 28isdesignedtobeusedwithone,twoor
threetaperecorders,forexample—tapeplayback
froma tapedeck(onewithoutitsownelectronics)
and,inaddition,twocompletetaperecorders.The
frontpanelpushbuttonscontrolthelatter.Thepush-
buttonspermitnormalplaybackofeitherrecorder,
monitorofeitherrecorderasrecordingsarebeing
made,orcopyingtapesfromonerecordertoanother
whilelisteningtoa separateprogram.
TAPE1
PUSHBUTTONOUT...Theprogramsourceisfed
tothepoweramplifiersandheardthroughtheloud-
speakers.
IN...Theprogramsourcebecomestherecorded
tapeonthetaperecorderconnectedtoTAPEINPUT
1.Therecordedprogramfromtaperecorder1 isfed
tothepoweramplifiersandheardfromtheloud-
speakers.
Withthepushbuttonsinthemonitorpositiona
rectangleislightedinREDabovethepushbutton.
WhentheREDlightisononlythetapecanbeheard.
Tolistentoothersourcesthepushbuttonmustbe
outandthelightoff.
TAPE2
Thesecondcompletetaperecordercanbeoper-
atedinthesamefashion.
TAPE2 pushbuttonalsocontrolstheprogramfrom
ataperecorderpluggedintothefrontpaneljacks
markedTAPEINPUTandTAPEOUTPUT.Whena
taperecorderispluggedintothefrontpaneljacksall
thefacilitiesforTAPE2 areautomaticallyswitched
tothefrontpanelpacks.A taperecorderplugged
intotheTAPE2 INPUTSandOUTPUTSontheback
panelisautomaticallydisconnected.
UsethefrontpanelpushbuttonTAPE2 torecord
onorplaybackfroma taperecorderpluggedinto
thefrontpaneljacks.
TAPECOPY1->2
Thispushbutton,whenpushedIN,connectsthe
TAPE1 OUTPUTjackstotheTAPE2 INPUTjacks
withoutaffectingtheprogrambeingheardfromthe
speakers.Inthispositiona copyoftheprogramon
taperecorder1 canbemadeontaperecorder2.
WiththepushbuttonINa rectangleislightedin
ORANGEabovethepushbutton.
TomonitorthetapecopyproceduresusetheTAPE
1MONITORpushbutton.
TAPECOPY2->1
Thispushbutton,whenpushedIN,connectsthe
TAPE2 OUTPUTjackstotheTAPE1 INPUTjacks
withoutaffectingtheprogramheardfromthe
speakers.Inthispositiona copyoftheprogramon
taperecorder2 canbemadeontaperecorder1.
WiththepushbuttonintheINpositiona rectangleis
lightedinORANGEabovethepushbutton.
TomonitorthetapecopyproceduresusetheTAPE
2MONITORpushbutton.
LF(LOWFREQUENCYFILTER)
UsetheLFfilterswitchtoreduceobjectionable
low-frequencynoisecreatedbya turntableorrecord
changeroracousticallycoupledfeedback.
OUT. . . filterdisconnected.
IN...low-frequencyrumbleandnoisebelow50
HzarereducedwhentheswitchispushedtotheIN
position.
WiththeLFpushbuttonintheINpositiona rec-
tangleislightedinGREENabovethepushbutton.
WhentheGREENlightisonthelowfrequencyfilter
isinoperation.
HF(HIGHFREQUENCYFILTER)
UsetheHFfilterswitchtoreduceobjectionable
high-frequencynoisesuchasrecordscratch.
OUT. . . filterdisconnect
IN...rollsOFFresponsesharplyabove7000Hz
WiththeHFpushbuttonintheINpositiona rec-
tangleislightedinGREENabovethepushbutton.
WhentheGREENlightisonthehighfrequencyfilter
isinoperation.
SPEAKERPUSHBUTTONS
Speakerscanbeturnedonandoffwhenproperly
connectedwiththeaccessoryMclntoshSCRcontrol.
(SeediagramPg.7)
Iftheprogramistobeheardfromthemainspeak-
ersonly,theREMOTEpushbuttonispushedIN.This
turnsofftheremotespeakersandlightsa rectangle
inBLUEabovethepushbutton.TheBLUElightindi-
catesthattheremotespeakersareoffandnopro-
gramcanbeheardfromthem.
Iftheprogramistobeheardfromtheremote
speakersonlytheMAINpushbuttonispushedIN.
Thisturnsoffthemainspeakersandlightsa rec-
11

tangleinBLUEabovethepushbutton.TheBLUElight
indicatesthatthemainspeakersareoffandnopro-
gramcanbeheardfromthem.
Tohearprogramfrombothmainandremote
speakers,boththeMAINandREMOTEpushbuttons
mustbeoutandtheBLUElightsmustbeoff.
FRONTPANELJACKS
TAPEINPUT- TAPEOUTPUT
Whena taperecorderispluggedintothefront
paneljacksallthefacilitiesforTAPE2 areautomatic-
allyswitchedtothefrontpaneljacks.A taperecorder
pluggedintotheTAPE2 INPUTSandOUTPUTSon
thebackpanelisautomaticallydisconnected.
UsethefrontpanelpushbuttonTAPE2 torecord
orplaybackfroma taperecorderpluggedintothe
frontpaneljacks.
HEADPHONES
ThefrontpanelHEADPHONEjackhasbeende-
signedtofeedlowimpedancedynamicheadphones.
Electrostaticheadphonesgenerallyrequirehigher
powerthandynamicheadphones.Theymustbecon-
nectedtotheoutputconnectorsonthepoweram-
plifier.
PlugheadphonesintothefrontpanelHEADPHONE
jack.AdjustthefrontpanelVOLUMEcontrolforcom-
fortableheadphonelistening.(SeePg.8}
AnamplifierintheC 28providesthepowerthat
feedsboththeHEADPHONEjackonthefrontpanel
andtheLINEOUTPUTjacksontherearpanel.Ifyou
choosetolistentoheadphonesonlyitisnotneces-
sarytooperatethepoweramplifiersthatfeedthe
loudspeakers.
Toturnthepoweramplifiersoffusetheswitchon
thetoppaneloftheC 28markedPOWERAMPLIFIER.
ThisswitchcontrolstheACtothegreenPOWER
AMPreceptaclesonthebackpanel.Withthisswitch
intheOFFpositionnothingwillbeheardfromthe
speakers.
BalancingyourStereo
Theperformanceandenjoymentofa stereosys-
temisgreatlyincreasedwhenthesoundisproperly
balanced.Thebalanceofthestereosystemisaf-
fectedbymanythingsincludingroomacoustics,
furnitureplacement,roomshape,smalldifferences
inloudspeakers,etc.Factorsthataffectproperstereo
balancearecorrectphaseforbothchannelsand
equalprogramloudness.
TOBALANCELOUDNESS
1.SettheMODEtoMONO.
2.Playa familiarrecording.
3.TurntheBALANCEcontroltothe12o'clockposi-
tion.
4.Whiletheprogramisplaying,standbetweenthe
twoloudspeakers.Listenfora differenceinloud-
nessbetweenspeakers.Balancethesystemby
adjustingthecontrolsonthepoweramplifiers.
Next,settheMODEselectortoSTEREO.Ifthere
isthena differenceinloudnessturntheBAL-
ANCEcontroltowardthespeakerthatisnotas
loud.AdjusttheBALANCEcontroluntilthesound
isequallyloudfrombothspeakers.
12

ListeningtoyourStereo
LISTENINGTOSTEREORECORDS
TurntheINPUTSELECTORtoPHONO1,or
PHONO2,whicheverisconnectedtotherecord
playeryouwishtohear.
SettheMODESELECTORtoSTEREO.
AdjusttheVOLUMEcontroltothedesiredvolume.
LISTENINGTOMONOPHONICRECORDS
TurntheINPUTSELECTORtoPHONO1,or
PHONO2,whicheverisconnectedtotherecord
playeryouwishtohear.
TurntheMODESELECTORtoMONO.
AdjusttheVOLUMEcontroltothedesiredvolume.
LISTENINGTOA TUNER
TurntheINPUTSELECTORtoTUNER.
Adjustthevolumetoa comfortablelevel.
LISTENINGTOA TAPERECORDER
TwotaperecorderscanbeusedwiththeC 28.
Connecta threeheadtaperecordertoTAPE1 inputs.
Connecta secondthreeheadtaperecordertoTAPE
2.Recordingandmonitoringcanbedoneusing
eithertaperecorders.
PresstheTAPE1 orTAPE2 pushbuttonIN,which
everisconnectedtothetaperecorderyouwishto
hear.
TurntheMODESELECTORswitchtoSTEREOor
MONO,dependingontheprogramonthetape.
AdjusttheVOLUMEcontroltothedesiredvolume.
Tomonitorwhilerecordingyourtaperecorder
musthaveseparaterecordandplaybackheadsand
separateelectronics.Thepushbuttonswitchletsyou
monitorthequalityoftaperecordingsmadeduring
therecordingprocess.WhentheTAPEpushbutton
isintheINpositionitwillplaythesoundfromthe
tapeasitpassestheplaybackhead,a momentafter
itisrecorded.Therecordingprocesscontinuesas
usual.WhentheswitchisintheOUTpositionpro-
gramthatisbeingrecordedisheard.
HOWTOCOPYTAPE
Putthetapetobecopiedontherecordercon-
nectedtoTAPE1 input.
PressintheTAPECOPY1->2pushbutton.
ThesignalavailableattheOUTPUTTAPE2 jacks
istheplaybackofTAPE1.
RecordontherecorderconnectedtoOUTPUT
TAPE2.Therecordingcanbemonitoredbypress-
ingintheTAPE2 pushbutton.Instantaneouscom-
parisonontherecordedprogramwiththeoriginal
canbeheard.
LISTENINGTOTAPEDECKS
Tolistentotapefroma tapedeck,proceedasfol-
lows:
TurntheINPUTSELECTORtoTAPEHEAD.
TurntheMODESELECTORtoMONO(L+ R)or
STEREO,dependingontheprogramonthetape.
AdjusttheVOLUMEcontroltothedesiredvolume.
USINGMICROPHONES
MicrophonesinstereomaybeusedwiththeC28.
ThetwoMICchannelsamplifymicrophonesignals.
Theyhavea sensitivityof2.5millivoltsandaninput
impedanceof0.5megohm.Forloweroutputmicro-
phonesinputtransformersshouldbeusedtoraise
thevoltageoutputtothe2.5millivoltrange.
13

toTapeOutput40dB
toHeadphone/LineOutput70dB
TapeHeadat500Hz
toMainOutput64dB
toTapeOutput44dB
toHeadphone/LineOutput74dB
TRANSISTORCOMPLEMENT
26silicon-planartransistors,4 silicondiodes,2
siliconbridgerectifiers
POWERREQUIREMENT
120volts,50/60Hz,45watts
FACILITIESANDFEATURES
BASSCONTROLS
Separate11positionrotaryswitchesforeachchan-
nel, +20dBto-20dBat20Hz
TREBLECONTROLS
Separate11positionrotaryswitchesforeachchan-
nel,+18dBto-18dBat20,000Hz
COMPENSATIONSWITCH
ThreepositionswitchforFlat,Loudness,orPres-
ence.Loudnesspositionboostslowfrequenciesfor
lowlevellistening.Presencepositionboostsmid
frequencies4 dBtoincrease"presence"effect.
Thiscontroloperatesasa functionofvolumecon-
trolpositionsofullcompensationisobtainedat
lowervolumelevelsandflatresponseisobtained
atfullvolume.
BALANCECONTROL
Naturalbalanceatcenterposition,attenuationof
leftorrightchannelbyrotatingcontrol
VOLUMECONTROL
Precision"tracked"atalllisteninglevels.(0to
-65dB).Doesnotchangestereobalanceasloud-
nessischanged.ThepowerON/OFFswitchis
coupledwiththiscontrol.
INPUTSELECTOR
Sixpositions:Auxiliary,Tuner,Phono1,Phono
2,Microphone,andTapeHead
MODESELECTOR
Sevenpositions:Leftchannelonlytoboth
speakers,Rightchannelonlytobothspeakers.
StereoReverse,Stereo,Mono,L + R toleftspeaker
only,andL - R torightspeakeronly
TAPEMONITORSWITCHES
Twopushbuttonswitches.Eitheroftwotapere-
corderscanbemonitoredbyselectingtheTAPE1
pushbuttonorTAPE2 pushbutton.Theyareme-
chanicallyinterlockedtoacceptonlyonepush-
buttonattheINpositionatonetime.
TAPECOPYSWITCH
Twopushbuttonswitches.Eitheroftwotapere-
corderscanbeconnectedtocopyfromtapema-
14
PerformanceLimits
andRatings
PerformanceLimitsarethemaximumdeviation
fromperfectionpermittedfora Mclntoshinstrument.
WepromiseyouthatyourC 28mustbecapableof
performanceatorexceedingtheselimitsoryouget
yourmoneyback.Mclntoshistheonlymanufacturer
thatmakesthisguarantee.
FREQUENCYRESPONSE
+0-0.5dB20Hzto20,000Hz
DISTORTION
0.1%atratedoutputlevel,20Hzto20,000Hz
INPUTSENSITIVITYANDIMPEDANCE
Auxiliary,Tuner,Tape1,Tape2,0.25voltsat250,-
000ohms.Phono1 andPhono2,2 millivoltsat
47,000ohms(1,000Hz).Microphone,2.5millivolts
at500,000ohms
TapeHead,2 millivoltsat500,000ohms(500Hz)
HUMANDNOISE
Auxiliary,Tuner,Tape1,Tape2,90dBbelow
ratedoutput
Phono1,Phone2 andTapeHead,78dBbelow
10millivoltsinput,equivalenttolessthan1.2
microvoltsattheinputterminals
Microphone,1.5microvoltsattheinputterminals
OUTPUTLEVELANDIMPEDANCE
MainOutput2.5voltswithratedinput,lessthan
100ohmssourceimpedance,tooperateinto47,000
ohmsorgreater
TapeOutput0.25voltswithratedinput,lessthan
150ohmssourceimpedance,tooperateinto47,000
ohmsorgreater
Headphone/LineOutput0.75voltsinto8 ohm
loador2.5voltsinto600ohmline,0.2ohmsource
impedance,levelcontrolsprovided
CenterChannelOutput1.25voltswithratedin-
puttobothchannels,tooperateinto47,000ohms
orgreater,levelcontrolprovided
VOLTAGEAMPLIFICATIONINDECIBELS
Auxiliary,Tuner,Tape1 andTape2
toMainOutput20dB
toTapeOutput0 dB
toHeadphone/LineOutput30dB
Phono1 andPhono2 at1 kHz
toMainOutput62dB
toTapeOutput42dB
toHeadphone/LineOutput72dB
Microphone
toMainOutput60dB

chine1 totapemachine2 orviceversa.Theyare
mechanicallyinterlockedtoacceptonlyonepush-
buttonattheINpositionatonetime.
LFFILTERSWITCH(RumbleFilter)
Flatorroll-off12dBperoctavebelow50Hz,down
to18dBat20Hz
HFFILTERSWITCH(ScratchFilter)
Flatorroll-off12dBperoctaveabove7000Hz,
downto18dBat20,000Hz
SPEAKERSWITCHES(Operateswithaccessory
speakercontrolrelay)
Main:SwitchtheMAINloudspeakersystemON
orOFFwithoutaffectingtheperformanceofRE-
MOTEspeakers.
Remote:SwitchtheREMOTEloudspeakersys-
temONorOFFwithoutaffectingtheperformance
ofMAINspeakers.
FRONTPANELTAPEJACKS
Allowsconnectiontoinputandoutputofa tape
recorderfromthefrontpaneloftheC 28.Inserting
plugsintotheirjacksdisconnectstheTAPE2 cir-
cuitsfromtherearpaneloftheC 28andusesthe
TAPE2 facilitiesforthefrontpaneljacks.
HEADPHONEJACK
Forlisteningwitheitherloworhighimpedance
dynamicstereoheadphones.Powertothisjackis
suppliedbyanamplifierprovidedintheC 28.
Headphonelisteningcanbeaccomplishedwithout
theuseofanexternalpoweramplifier.
SECONDARYCONTROLS
Thefollowingcontrolsarelocatedbehindthefront
panelontopoftheC 28chassis.Thesecontrols
arereadilyaccessiblebydepressingthePANLOC
buttonsandslidingtheC 28forwardfromthe
mountingpanel.
LOWFREQUENCYTRIMCONTROLS
Permitstheincreaseinoutputoffrequenciesbe-
low100Hzofupto6 dBtocompensateforunequal
speakerresponseortheunequalinfluenceofroom
acoustics.
PHONO1 ANDPHONO2 LEVELCONTROLS
Permitsvariationsinthephonoinputsensitivityof
upto10dB.Providesforoptimumsignaltonoise
ratioandproperbalanceofthechannelsofthe
phonocartridge.
OUTPUTLEVELCONTROLS
Permitsthebalanceoftheentiresystemtobe
convenientlypreset.
HEADPHONELEVELCONTROLS
Adjuststhelevelandbalanceoftheheadphone/
lineoutput.
POWERAMPLIFIERON-OFFSWITCH
Controlspowertothegreen120voltPowerAmpli-
fierreceptacles.Allowsthepoweramplifierstobe
turnedoffwhennotneededforheadphonelisten-
ing,etc.
MECHANICALINFORMATION
SIZE:Frontpanelmeasures16incheswide(40.64
cm)by5-7/16inchesdeep(13.81cm).Chassis
measures15incheswide(38.1cm)by5 inches
high(12.7cm)by13inchesdeep(33.02cm),in-
cludingPANLOCshelfandbackpanelconnectors.
Knobclearancerequiredis1½inches(3.81cm)in
frontofthemountingpanel.
FINISH:Frontpanelisanodizedgoldandblackwith
specialgold/tealnomenclatureillumination.Chas-
sisisblack.
MOUNTING:ExclusiveMclntoshdevelopedprofes-
sionalPANLOC
WEIGHT:25pounds(11.34kg)net,37pounds(16.78
kg)inshippingcarton
15

PerformanceCharts
16

17

TechnicalDescription
Basically,eachchannelofyourC 28consistsof
fourmainparts:lowlevelamplifier,highlevelampli-
fier,activefiltersandheadphoneamplifier.Common
regulatedpowersuppliesfurnishpowertobothchan-
nels.
LOWLEVELAMPLIFIER
Eachchannelofthelowlevelamplifierisa four
transistordifferentialamplifierinputcircuit.Pre-
ciseequalizationismaintainedbytheuseofnegative
feedbackandappropriatecomponentsspeciallyde-
signedforlownoise.Particularattentionhasbeen
exercisedintheuseoftransistorshavingexceptional
lownoisecharacteristics.Thedifferentialamplifier
stageisthebasisforthehighopen-loopgainofthis
portionofthecircuit.Witha higheropen-loopgain
thereisamplenegativefeedbackavailableunderall
equalizationconditionstoensureanextremelylow
levelofharmonicdistortion.Becauseequalization
hasbeendesignedforminimumerrortheresponse
frombothtapeandrecordsisaccurate.
Oneoftheimportantfeaturesofthissectionisthe
useofsetlevelpotentiometersinthephonocircuit.
Thegamofthephonopreamplifierstagescanbe
adjustedfrom32dBto42dBtocorrespondtothe
sensitivityofyourphonocartridge.
ThephonosetlevelcontrolsinyourC 28arede-
signedintothenegativefeedbackloop.Theyarenot
potentiometersattheinputoroutputofthelowlevel
amplifier.Thisimportantdesignconsiderationallows
thegamtobereducedfrom42dBto32dBwithout
adverselyaffectingthenoiselevel.Becausethecon-
trolisinthefeedbackloopthelevelofnoiseand
distortionisreduced10dBbutthemaximuminput
voltagethatthissectioncanhandleisincreasedby
10dB.With42dBofgaininthephonosectionthe
maximuminputvoltagethatthelowlevelamplifier
canacceptwithoutoverloadingis150mVat1000Hz.
Whenthegamisreducedto32dBthissectioncan
acceptover0.5voltwithoutoverloadingat1000Hz.
18

HIGHLEVELAMPLIFIER
Thelinearhighlevelamplifierhasa gainof20dB.
Thesamecarefuldesignconsiderationsapplywith
respecttonoiseandthereductioninnon-lineardis-
tortion.Eachchannelofthehighlevelamplifiercon-
sistsof3 transistorsselectedforlownoisecharacter-
istics.Inthefeedbackloop,two11positionswitches
allowupto20dBofboostorcutat20Hzandupto
18dBofboostorcutat20,000Hz.Attheinputofthe
highlevelamplifierarethebasstrimcontrolandthe
switchwhichselectsloudnessorpresencecompen-
sation.
FILTERAMPLIFIER
Thefilteramplifierservesthreefunctions:(1)a
highfrequencyfilter,(2)a lowfreqencyfilter,and
(3)a 20.000Hzlowpassfilter.
Eachchannelofthefilteramplifierhastwotran-
sistorsconnectedincompoundemitterfollowercon-
figuration.Theresistiveandcapacativeelements
forma 12dBperoctaveactiveRCfilter.Thehigh
frequencyfilterisa 7000Hzactive,lowpass,filter.
Thelowfrequencyfilterisa 50Hzactive,highpass,
filter.Thefilteramplifieralsoformsa 20,000Hzactive
lowpassfiltertoreducenoiseoutsidetheuseful
soundspectrumwhileleavingthepreamplifierre-
sponseflatto20000Hz.A 20,000Hzactive,lowpass,
filterisconnectedatalltimestoreducewideband
noiseyetmaintainingflatfrequencyresponseupto
20,000Hz.
Thenon-lineardistortionoftheseactivefiltersis
verylowevenatcutofffrequency.Thisisnotthe
caseinpassivefilterswherea verylowimpedance
atthecutofffrequencyispresentedtothedriving
source.Thislowimpedancewillcausehighnon-
lineardistortionatandnearthecutofffrequency.
ThedesignoftheC 28eliminatesthisproblem.
HEADPHONEAMPLIFIER
Theheadphoneamplifieriscapableofproducing
0.75voltsofoutputinto8 ohmheadphoneswithless
than0.1%harmonicdistortion.Thisisadequate
powertodriveanydynamictypeheadphone.
Themaximumoutputoftheheadphoneamplifier
is2.5voltsinto50ohmsorgreaterattheLINEOUT-
PUTterminalsontherearpanel.Theinternalim-
pedanceoftheheadphoneamplifierislessthan0.2
ohms.Longshieldedcablescanbeusedwithoutad-
verselyaffectingthefrequencyresponse.Thehead-
phonelevelcontrolsadjusttheoutputleveland
balanceoftheheadphoneamplifier.
POWERSUPPLY
Thepowersupplyconsistsofa lowimpedance75
voltpowersupplyanda 14voltpowersupplyforthe
headphoneamplifier.The75voltsupplyisstabilized
bya seriesregulatortransistoranda zenerdiode.
Anaccessoryfilterchainandvoltagedividingnet-
workprovides18voltstothelowlevelcircuits.
Theheadphoneamplifierispoweredbya separate
14voltsupplywhichusesa transistorasaneffective
ripplefilter.
Aprincipledesignconsiderationwasexceptionally
lowhumlevel.FirstthepowertransformeroftheC 28
usesa magneticcorewithverylowfluxdensity.
Doubleshieldinghasbeenaccomplishedbysur-
roundingthetransformerfirstwithstripsofcopper,
andthenwithstripsoftreatedsoftiron.Theshielded
transformerispottedina heavygaugesoftsteelcan.
Topreventunbalanceofthemagneticfluxofthe
transformer,thesecondaryvoltageisrectifiedby
silicondiodesina bridgeconfiguration.
19

Block
Diagram
20