Radio Shack MPA 31 User manual

Cat.No.32-2041
OWNER'SMANUAL
Pleasereadbeforeusingthisequipment
MPA31
20-WattAC/MobilePAAmplifier
RaAe/haek

FEATURES
YournewRatioShad< N/PA-31 20-
WattAC/McAJtePAAmpfififfl-givesyou
20WattsofsofidpowerforyourPA
system. ItswkJefrequencyresponse
easiyhandlesamplificationofvoiceand
music.Itopiatesoneither120volts
ACor12vdtsDCpowersoyoucan
useitinmeetinghallsandauditDriums,
atsportsevets,insdnools,intheoffice
forpagingsystems— ariywtTereyou
needtod^erspecialarvKXincemeits
withgreatsound.
ItinclLKJesVrsefolbwingfeatures:
N^a-opliotw\npuXJacks— letyoucon-
nectuptotwromiCTophones.
AuxifiarytnjaitJacks— letsyojcon^
necta varietyofaucfoinputsouroKfor
musicandspecialeffects.
70-VoHLireOutput— letsyou con-
nectlinetransformersfora multipte-
speakerPAsystem.
MasterVottmreContrDi— letsyouad-
justtheoverallsoundlevel.
TOhECkHttrd—letsyoucustomize
thetoneofthesoundtosuityourper-
sonal preferencesdependingonthe
acoustics,speakers,andsoundsource.
Readthismanualcarefully.Itdescrit)es
variousspeakercormectlorstohelpyou
setectthebestan-angementforyour
systemandatoexplainshowtouse
ttieampfflCT.
TheserialnuntierisontheampSfier's
backpanel.Foryourpenmaientrec-
ords, vwerecomrrffindyoirecorditbe-
low.
V«AflNW&TOREDUCETVCRS<OFFWEOfi
aECTFSC SHOCK, DONOT EXPOSE. THEfMM-
F€RTORAW cnhOSTUTC
CAUTION: TO PHXKETI-EHEKOFFffCOR
BJECTFBCSHOCK.IX)NOTPEMCMEThE
C0VB1(OHBACK).NOU^SfflVlCEABLE
FWnS INSCE. RffffiSBWK>tta TD OUAUFED
SB1VKSPBISOMCL
T Thissymbolisintamdedtoalertyouth^
dartyefcjusinnsutslBdvottagBwithnthe.
product'serdosLMOmightbeofsuffcivit
iTiaynitudetocorstitLdsa risK ofelectric
shock.Donotop^theproduct'scase.
TtiB'syrrtertisintendedto nformyouofim-
portantoperafingSFtimaintenance(servi-
ng)nstructionsinthepapersaccompany-
ingttiisproc&jct.
©1994TandyCorporation.
AN RightsReserved.
RacfioShockisaregisteredtractemiarkusedbyTandyCorporation.

PREPARATION
PLACINGTHEAMPUFCR
Beforeyouuseyouamftffier,beaireyouchoosea kxationwithadequateventila-
tion. Donotptebcetheampfifieronthickcarpeting(wtiichcanrestrictttisairfkiw)or
n^a heatsouce,ajchasa heatventorradiator(wtvchcancauseVheampfifierto
overheat).
PRESETTMGTHECONTROLS
SettingtheAmplifier
Warrang:Asuddenhighoutputfrom
theampliftercoukjdamageyotrhearing
orthespeakersconnectedtotheampH-
fia-'soutput.
Toavoid accidentaftyoverdrivinga chan-
nelorprenatur^annplifyinganaucfio
inpuL,setVneamfifier'scontrolstotfie
levels^lovmbetowbeforeyouconnect
ttreACpowercord ortumonpovwer.
Control
PO\NER
MIC1,MC2,PHOISHD/AUX.CX
TONE
MASTERVOLUME
Skiing
Off
MIN
MID
0
nilIIIIPIIIIIIl
1s
MikSTCBVnUiME
MXeXMtCaPMDHOL/AUX.CSTONE•.<''''l«
:()::
MU>MMIMXWMMKIMMM>LOUMXOM•'X.^./*^*
MP*~atP.A.AMm.TT'teB.t«av..^ev
SettingtheInputSource
Toavoidsuddenaudiofromtheinput
sources,setthercontrolstotfielevels
^tovimbetowbeforeyouconnecttheir
outputjackstotheannplifier'sinput
jad(S.
AucfioDevkw
Turntable
TapeDeck
CDPlayer
Amplifier/Receiver
Power
Power
Power
Power
Tone
Selling
Off
Off
Off
Off
Flat

CONNECTINGAUDIO
COMPONENTS
TOTHEAMPUFIER
Youcanconnectoptionalcomporents
(suchasmiCTophones,tuners,turnta-
bles, orCDplayers)toyouramplifierto
expandyoursystem.Yourtocal Radio
Shackstorecaniesa wideselectionof
audiocomponents,adapters,micro-
phones,andspeakers.
Cautton: Makesureyoumakeallttie
connectionstottieanrtplifierand^aeak-
ersbeforeconnectingandtumingon
power.
ConnectingMicrophones
Youcanconnecta microphonetoone
orbothoftheamplifier'sMIC1 and
MIC2 inputjacks.Themcrophone(s)
canbe high- orlow-impedanceand
musthavea V4 -inch plug.
Connectinga Turntable
Youcanconnectanaudioinputsource,
suchasa magneticcartridgetumtatJle
totheamplifier'sPHONOjack.
Cautton: Donotconnecta highlevel
audioinputsource,suchasa ceramic
cartridgeturntable,tothePHONO
inputjack.Doingsocouldcausesound
distortion.
QROUNDWRE
Notes
•Toavoida lowfrequencyhum, con-
nectyourturntable'sgproundwire
(usuallyblad<orgreen)totheampli-
fier'sGNDscrew.
»Topreventa tjeatorhowingsairKJ
duringmonauraloperationinthe
PHONOmode,s^iaratethePHONO
inputlinefromthe^Deaka-wire.
ConnectinganAuxifrary
AudioSource
Youcanconnectanauxiaryaudo
inputsource(suchasa tapedeck,CD
player,cerarrriccartridgeturntableor
tuner)totheamp6fier'sAUX.COjack.
I mr["mum'
^ ^
^JSSHJO
Note: Toconnecta stereoaudto
sourcetotheAUX.COjack,usea
shiekledY-adapter,su*asCat.No.
42-2438(notsuppled).

PUCINGTHESPEAKERS
Sf»akerptecanaitdepencteonyour
room'ssizeandanrangerr^itWere-
commaxJyoupiaya wide-rangere-
corcfingandexperimentwithspeaker
placementuntilyoufirto thelocations
ttratresultinthebest sound. ThefoBow-
ingdrawings^x)wrecommerxjed
qjeakerplacementsfortypical,nanow,
aixlwkJesoundcoverage.
lyptealCovofsge
*^oo9-
•$oo9-
-Q009.
.QooQ-
Usethe^tortestlatgthofwirepossitde
toconnecttheqaeakers.Afterplacing
thespeakers,detemninethevinrelength
andchoosetfieapfiv'opriategaugesize
asfoUows:
WireLength
25feetorless
Over25feet
WireGauge
18-gauge
16-gauge
SngiaSpsaksr(NanowCcwsrsse)
Note: Ifyouconnectspeakerswithout
transfomners,thespeakerwireshouto
benolongerthan50feet
CONNECTINGTHE
SPEAKERS
Youcanconnectoneormore4-,8-or
16-ohmspeakers,withorwittrauttrans-
formers,betweentheamplifier'soutput
andthespeakers'inputToensure
equalvolumefromeachspeaker,allttre
connectedspeakersshouldhavethe
sameimpedancerating.
Removeabout1 inchofinsulationfrom
theendofeachwire.Thentwisttfieex-
posedwiretosecureallitsstrands.
I#l't\
b0 0 0^
b0 c_o3-
)0 c_o5-
>0 d 0^
> 0 j^oS-
IW0Speakers(WMeAreaCoverase)
•
•
W///C
Connecttfiespeakerwiretotheampli-
fierandthespeaker(s)bypressing
downontheappropriatepushterminal
leverandinsertingtheendofttietwist-
edwireintoHieterminal's hole. Thenre-
leasethelevertosecurethevwre.

PhasingtheSpe^ricers
Properptia^gisimportantwhenyou
usemorettianonespeakerinttiesame
roomorarea.Out-of-phasespeakers
canloseuptoone-halfofttieirpotential
volume,andcanhavea significanttyde-
creasedbasseffect.
Mostspeakerterminalsarecotor-coded
orhavea markthatindicatettieter-
minal'spotenty.Usually,terminalswith
positivepolarityareredorhavea plus
symbol(+),andterminatevwthnegative
polarityareblackorhavea minussym-
bol(-).
Ptiasingiscon-ectv^rtienyouconnect-F
to-Fand- to-.
DetemiiningTotalSpeaker
Impedance
Caution: A totalspeakerimpedance
thatishigherthan16ohmsortower
than4 ohmscandamageyouramplifier
orspeakers.
•Speakersareconnectedinseries
whenthefirst^Deaker'spoativeter-
mrialiscorBiectedtothenextspeak-
er'snegativeterminal.Determinethe
totalspeakerimpedanceofspeakers
connectedinseriesbyadcBngupthe
individualimpedancesofaflthe con-
nectedspeakers.
Forexampte,ifyouwanttoconnect
two8-ohm^leakersinseries,add8
(ttieimpedanceofcmespeaker)plus
8(ttieimpedanceoftheotherspeak-
er)fora totalspeakerimpedanceof
16dims.
COM4 0 8CJISO70V
•Sprakerserecomectedinparallel
v^rtienaflthernegativetaminalsare
connectedto^tfierandafltheir posi-
tiveterrrwiateareconnectedtogether.
Determinethetotalspeakerimped-
anceofspeakersconnectedinparal-
lelbydividingtheinipedanceofone
speakerbythenumberofspeakers.
Beforeyouconnectspeakerstothe
amplifier,youmustdeterminethetotal
speakerimpedance.Indeterminingthe
totalspeakerimpedance,youmustfirst
detemnineifyourspeakersareconnect-
edinseries,paraUel,ora series/parallel
combinatton.

Forexam|:te,ifyouplantoconnect
two8-ohmspeakersinparaUel,di-
vide8 (theimpedanceofonespeak-
er)by2 (thersjmberofspeakers)fa
atotalspeakerimpec^ceof4
ohms.
COM4 0 8016070V
8Q^
an
• Ifyouconnectmorethantwospeak-
asusngonly seri^ oronlyparaBel
comecttons,thetotalimpedance
mightexceedtheamplifier'smaximum
impedance(16ohnrs)orfaflbelowits
mininrHjnimpedance(4ohms).
Forexample,ifyouconnectfour8-
ofvnspeakers:
-Inseries,ttietotalimpedanceis
32ohms(8-F8 -f8 -F8 = 32).
Thisexceedsthemaximumrating.
-Inparaflel,thetotalimpedance
is2 ohms(8dividedby4 = 2).
Thisfaflsbdowtheminimumrating.
Youcananiveata propertotalim-
pedancebycombiningseriesand
parallelccmriections.
ConnectingOrriyOne
Speaker
Connectthespeaker'snegative(-)ter-
minaltotheamplifier'sCOMtenrrwial.
Thenconnectthe^leaker'spositive
(-F)terminaltotheterminalthatmatch-
esthespeaker'simpedance(4Q,8Q,
or16Q).
isn70V
ConnectingSg;»aker8in
Series
Followthesestepstoconnectspeakers
inseries. COM4 0 8016070V
8U
1. CannecAthefirstspeaker'spositive
(-F)terminal tothesecondspeak-
er'snegative(-)termirral.
2. Connectthefirstspeaker'snegative
(-)terminaltotheamplifier'sCOM
tenninal.
3.Connectthesecondspeaker's posi-
tive(-F)terminaltotheamplifier's
4Q, 8Q,or16Q temninalthat
matchesthetoalspeakerimpectence.

ConnectingSpeakersin
Parallel
Fdtowthesestepstoconnectspeakers
inparallel. CDM408016070V
1. Connectthespeakers'negative(-)
tenninalstogether.
2. Connectthespeakers'positive(-F)
tenninalstogether.
3.Connectthespeakers'negative(-)
terminalstottieamplifier'sCOMter-
minal.
4. Connectthespeakers'positive(-F)
terminalstotheamplifier's4£2,82,
or16Qtenninalthatmatchesthe
totalspeakerimpedance.
ConnectingFourSpeakers
inSeries/ParallelCombination
Followthesestepstoconnectfour
speakersinseries/parallelcombrtation.
COM4(1snisnTOV
KI<IIQK3l>Za)E<|
LjiSllSIJiill:?^
^^y^^
160
1. Groupthefourspeakersintotwo
pairs.
2. Connecteadipairofspeakersin
series.Thetotalimpedanceofeach
pairis16ohms(8+ 8 = 16).
3.Connectthetwopairsofspeakersin
parallel. Thetotalimpedanceofboth
pairsis8 ohms(16/ 2 = 8).
4. Connectbothspeakers'negative(-)
terminatetotheampfifier'sCOMter-
minal.
5.(Connectboth^jeakers'positive(-F)
terminatetotheampfifier's4Q,8Q,
or16Qterminalthatmatctwsthe
totalspeakerimpedance.
ConnectingSpeakerswith
Transformers
Forthebestresultswhenyouconnect
twoormorespeakerstoyoursystem,
werecommendyouusea linetrans-for-
mer(suchas(^tNo. 32-1031, not
suppfied)tareachspeaker.
Transfomiersofferttieseadvantages:
•Youcanconnectspeakerswithdiffer-
entimpedancewithoutcausrig dif-
ferences in outputbetweenthespeak-
ers.
•Youcanaddorremovea speaker
fromtfiesystemwittxxithavingtore-
cateulatettieentiresystem'simped-
ance.
•Youcanreducesignallosswfienyou
usespeakerwireover50feettong.
8

Linetransformershaveseveralconnec-
torscafledtaps.Theprimarytaps(on
onesideofthetransfonnner)arethein-
puts and arerated in watts. Thesecond-
arytaps(cff)theoppositesicteofthe
transformer)aretheoutputsandare
ratedinohms.
{SECONDARY)
Cauttons:
•Beforeyouconnectthespeakers,be
ajrethetotalwattageoftheprimary
tapsyouintendtousedoesnotex-
ceedtheampfifier'smaximum20-
wattoutputpowerrating.
•Avoidmultipleconnectionstotheamp-
lifier's70VandCOMterminals.
COM 4 0 an16070V
CDM4 0 8 0 16070V
(PRIMARY)(SECONDARY)
1. Connecta wirefromtheampliftor's
TOVtenninaltothetransfomner's pri-
marytapthatmatchesyourspeak-
er'swattegeratFig.
Note: Usually,eac*speaker in asys-
temusesthesamewattagetap.If
youwanta particularspeakerto
havea highervolumelevel,usea
higherwattagetaponitstransformer.
2. Connecta wirefromtheampfifier's
COM(common)tenninaltotheC
(common)tapsonthetransfomner's
primaryside.
3.Connecta wirefromthespeaker's
positive(-F)terminaltothetrans-
former'ssecondarytapthatmatches
thespeaker'stotalimpedance(4
ohms, 8 ohms,or16ohms).
4. Connecta wirefromthespeaker's
negative(-)tenninaltotheC (com-
mon)taponthetransformer'ssec-
ondaryside.
CONNECTINGPOWER
Youcanpoweryouramplifierfrom stan-
dardACpowerorfroma 12-vdtDC
powersource(suchasyourvehide's
battery).
UsingACPower
TousestandardACpower,simplyplug
thesuppfiedACcorclintoanystandard
ACoutiet.
UsingVehicle
BatteryPower
Youcanpowertheamplifierfromyour
vehicle's12-voltbattery.

Cautinn:
•Yourvehidemusthavea negative-
grounddectricalsystem.Ifyouare
unsureaboutwhattypeofelectrical
systemyourvehide has, askyourve-
hidedealer.
•DisconnecttheACconjbeforeyou
connecttiie DCpowercable.
Touse12-vottDCpower,connectthe
suppliedDCpowercable'ssmallendto
theampfifier's12VNEGATIVEGND
jack.Thenconnectthecable'sother
endtoyourvehide'scigarette-lighter
socket.
ReplacingtheDCCable's
Fuse
Thecable's4A,125-voltin-linefuse
protectsyourampfifierfromvoltage
surges.Iftfieampfifier'spowerindicator
doesnotlightwhenyoupressPOWER,
checkthefuse.Ifitisbtown,replaceit
witha 4-amp,125-voltfuse(suchas
Cat.No.270-1010,notsupplied).
Toreplacettie fuse,unscrewttie tip of
tiie dgarette-lighter plug. Thenremove
theoldfuseandinserttfienewfuse.
Securetiie fusebyreattedwigthe
plug'stip.
ReplacingtheAmplifier's
Fuse
Thefuse(locatedonttie backofttie
ampfifier)protectsyourampfifierfrom
power(vottageorcunrent)surges.Ifttie
POWERHidicatordoesnotlightwfien
youpressPOWER,tiie fusemightbe
btovm.
CautfanuUseonlyanidentical5 x 20
mm, fast-acting,2-amp,250-voltfuse
(suchasCat.No.270-1052).
Foflowthesestepstoreplacetiie ampfi-
fier's fuse.
1. Unpfijgtheampfifier'spower cord.
2. Usanga tiatislade saewdriver,push
andtijm tiie fuseholder'scapcoun-
terdockwiseuntfltfiefijse holder
popsout.
3.Removettie fuseholderandrepbce
ttie fuse.
4. Usriga flatt}lade saewdriver,push
andtijm ttiefuseholder'scapfuOy
dockwisetoreplaceit.
10