Radio Shack MPA 31 Owner's Manual

Cat. No. 32-2041

OWNER'S MANUAL

Please read before using this equipment

MPA 31

20-Watt AC/Mobile PA Amplifier

RaAe/haek

FEATURES

Your new Ratio Shad<

N/PA-31

20­Watt AC/McAJte PA Ampfififfl- gives you
20 Watts of sofid power for your PA
system.

Its wkJe frequency response
easiy handles amplification of voice and
music. It opiates on either 120 volts
AC or 12 vdts DC power so you can
use it in meeting halls and auditDriums,
at sports evets, in sdnools, in the office
for paging systems — ariywtTere you
need to d^er special arvKXincemeits
with great sound.

It inclLKJes Vrse folbwing features:

N^a-opliotw \npuX Jacks — let you con­nect up to twro miCTophones.

Auxifiary tnjait Jacks — lets yoj con^
nect a variety of aucfo input souroK for
music and special effects.

70-VoH Lire Output — lets you

con­nect line transformers for a multipte­speaker PA system.

Master Vottmre ContrDi — lets you ad­just the overall sound level.

TOhE CkHttrd —lets you customize
the tone of the sound to suit your per-

sonal preferences depending on the
acoustics, speakers, and sound source.

Read this manual carefully. It descrit)es
various speaker cormectlors to help you
setect the best an-angement for your
system and ato explains how to use
ttie ampfflCT.

The serial nuntier is on the ampSfier's
back panel. For your penmaient rec­ords,

vwe recomrrffind yoi record it be-

low.

V«AflNW&TO REDUCE TVC RS< OF FWE Ofi
aECTFSC

SHOCK,

DO NOT

EXPOSE.

THE fMM-

F€R TO RAW

cn hOSTUTC

CAUTION:

TO

PHXKE TI-E HEK OF FffC OR
BJECTFBC SHOCK. IX) NOT PEMCME ThE
C0VB1 (OH BACK). NO U^ SfflVlCEABLE
FWnS

INSCE.

Rffffi SBWK>tta

TD

OUAUFED

SB1VKS PBISOMCL

T

This symbol is intamded to alert you th^
dartyefcjus innsutslBd vottagB withn the.
product's erdosLMO might be of suffcivit
iTiaynitude to corstitLds a risK of electric
shock. Do not op^ the product's case.

TtiB'syrrtert is intended to nform you of im­portant operafing SFti maintenance (servi­ng) nstructions in the papers accompany­ing ttiis proc&jct.

© 1994 Tandy Corporation.

AN

Rights Reserved.

Racfio Shock is a registered tractemiark used by Tandy Corporation.

PREPARATION

PLACING THE AMPUFCR

Before you use you amftffier, be aire you choose a kxation with adequate ventila­tion.

Do not ptebce the ampfifier on thick carpeting (wtiich can restrict ttis air fkiw) or
n^ a heat souce, ajch as a heat vent or radiator (wtvch can cause Vhe ampfifier to
overheat).

PRESETTMG THE CONTROLS
Setting the Amplifier

Warrang:A sudden high output from
the amplifter coukj damage yotr hearing
or the speakers connected to the ampH-

fia-'s output.
To avoid

accidentafty overdriving a chan­nel or prenatur^ annplifying an aucfio
inpuL, set Vne amfifier's controls to tfie
levels ^lovm betow before you connect
ttre AC power cord or tum on povwer.

Control
PO\NER
MIC 1, MC 2, PHOISHD/AUX.CX
TONE
MASTER VOLUME

Skiing

Off
MIN
MID

0

nil IIIIPIIIIIIl

1 s

MikSTCB VnUiME

MXeX MtCa PMDHOL/AUX.CS TONE •.<''''l«

:( )::

MU> MMI MXW MM KIM MM> LOU MXOM •'X.^ ./*^*

MP*~at P.A. AMm.TT'teB. t«av..^ev

Setting the Input Source

To avoid sudden audio from the input
sources, set ther controls to tfie levels
^tovim betow before you connect their
output jacks to the annplifier's input

jad(S.

AucfioDevkw
Turntable
Tape Deck
CD Player
Amplifier/Receiver

Power
Power
Power
Power
Tone

Selling

Off
Off
Off
Off
Flat

CONNECTING AUDIO
COMPONENTS
TO THE AMPUFIER

You can connect optional comporents
(such as miCTophones, tuners, turnta­bles,

or CD players) to your amplifier to

expand your system. Your tocal Radio

Shack store canies a wide selection of
audio components, adapters, micro­phones, and speakers.

Cautton:

Make sure you make all ttie
connections to ttie anrtplifier and ^aeak­ers before connecting and tuming on

power.

Connecting Microphones

You can connect a microphone to one
or both of the amplifier's MIC 1 and
MIC 2 input jacks. The mcrophone(s)
can be

high-

or low-impedance and

must have a

V4

-inch

plug.

Connecting a Turntable

You can connect an audio input source,
such as a magnetic cartridge tumtatJle

to the amplifier's PHONO jack.

Cautton:

Do not connect a high level
audio input source, such as a ceramic
cartridge turntable, to the PHONO
input jack. Doing so could cause sound
distortion.

QROUNDWRE

Notes

• To avoid a low frequency hum,

con­nect your turntable's gpround wire
(usually blad< or green) to the ampli-

fier's GND screw.

» To prevent a tjeat or howing sairKJ

during monaural operation in the

PHONO mode, s^iarate the PHONO
input line from the ^Deaka- wire.

Connecting an Auxifrary
Audio Source

You can connect an auxiary audo
input source (such as a tape deck, CD
player, cerarrric cartridge turntable or
tuner) to the amp6fier's AUX.CO jack.

I

mr["mum'

^ ^

^JSSHJO

Note:

To connect a stereo audto
source to the AUX.CO jack, use a
shiekled Y-adapter, su* as Cat. No.
42-2438 (not suppled).

PUCING THE SPEAKERS

Sf»aker ptecanait depencte on your
room's size and anrangerr^it We re­commaxJ you piay a wide-range re­corcfing and experiment with speaker
placement until you firto the locations
ttrat result in the best

sound.

The foBow­ing drawings ^x)w recommerxjed
qjeaker placements for typical, nanow,
aixl wkJe sound coverage.

lypteal Covofsge

*^oo9-

•$oo9-

-Q009.
.QooQ-

Use the ^tortest latgth of wire possitde
to connect the qaeakers. After placing
the speakers, detemnine the vinre length
and choose tfie apfiv'opriate gauge size
as foUows:

Wire Length

25 feet or less

Over 25 feet

Wire Gauge

18-gauge
16-gauge

Sngia Spsaksr (Nanow Ccwsrsse)

Note:

If you connect speakers without
transfomners, the speaker wire shouto
be no longer than 50 feet

CONNECTING THE
SPEAKERS

You can connect one or more 4-, 8- or

16-ohm speakers, with or wittraut trans­formers, between the amplifier's output
and the speakers' input To ensure
equal volume from each speaker, all ttre
connected speakers should have the

same impedance rating.
Remove about 1 inch of insulation from

the end of each wire. Then twist tfie ex-

posed wire to secure all its strands.

I#l't\

b 0 0 0^

b 0 c_o3-
) 0 c_o5-

> 0 d 0^

>

0 j^oS-

IW0 Speakers (WMe Area Coverase)

•

•

W///C

Connect tfie speaker wire to the ampli­fier and the speaker(s) by pressing
down on the appropriate push terminal
lever and inserting the end of ttie twist­ed wire into Hie terminal's

hole.

Then re-

lease the lever to secure the vwre.

Phasing the Spe^ricers

Proper ptia^g is important when you
use more ttian one speaker in ttie same
room or area. Out-of-phase speakers
can lose up to one-half of ttieir potential
volume, and can have a significantty de­creased bass effect.

Most speaker terminals are cotor-coded
or have a mark that indicate ttie ter­minal's potenty. Usually, terminals with

positive polarity are red or have a plus
symbol (+), and terminate vwth negative
polarity are black or have a minus sym­bol (-).

Ptiasing is con-ect v^rtien you connect -F
to -F and - to -.

Detemiining Total Speaker
Impedance

Caution:

A total speaker impedance
that is higher than 16 ohms or tower
than 4 ohms can damage your amplifier

or speakers.

• Speakers are connected in series
when the first ^Deaker's poative ter­mrial is corBiected to the next speak­er's negative terminal. Determine the
total speaker impedance of speakers
connected in series by adcBng up the
individual impedances of afl the

con-

nected speakers.
For exampte, if you want to connect

two 8-ohm ^leakers in series, add 8
(ttie impedance of cme speaker) plus
8 (ttie impedance of the other speak­er) for a total speaker impedance of

16 dims.

COM 4 0 8CJ ISO 70V

• Sprakers ere comected in parallel
v^rtien afl ther negative taminals are
connected to^tfier and afl their

posi­tive terrrwiate are connected together.
Determine the total speaker imped­ance of speakers connected in paral­lel by dividing the inipedance of one
speaker by the number of speakers.

Before you connect speakers to the
amplifier, you must determine the total
speaker impedance. In determining the
total speaker impedance, you must first
detemnine if your speakers are connect­ed in series, paraUel, or a series/parallel
combinatton.