Radio Shack 32-2001 Manuel utilisateur

Owner’s Manual

Please read before using this equipment.

MPA-40 20-Watt PA Amplifier

32-2001.fm Page 1 Friday, February 4, 2000 7:53 AM

Contents

Features ................................................................................................................... 3

Preparation .............................................................................................................. 4

Presetting the Controls ....................................................................................... 4

Choosing a Location ........................................................................................... 4

Making Connections ............................................................................................... 4

Speakers .................................................................................................................. 5

Speaker Phasing ................................................................................................ 5

Determining Total Speaker Impedance .............................................................. 6

Connecting Speakers ......................................................................................... 7

One Speaker ................................................................................................ 7

Two or More Speakers Without Transformers ............................................. 7

Multiple Speakers with Transformers ........................................................... 9

Controls and Operation ........................................................................................ 11

Troubleshooting .................................................................................................... 12

Care and Maintenance .......................................................................................... 13

Replacing Fuses ............................................................................................... 13

DC Cable ................................................................................................... 13

Amplifier ..................................................................................................... 13

Specifications ........................................................................................................ 14

Notes ...................................................................................................................... 15

RadioShack and RadioShack.com are registered trademarks used by Tandy Corporation.

32-2001.fm Page 2 Friday, February 4, 2000 7:53 AM

Features

Your RadioShack 20-Watt PA Amplifier

gives you 20 watts of solid power for

your PA system. Its wide frequency re-

sponse easily handles amplification of

voice and music. It operates on either

120 volts AC or 12 volts DC power so

you can use it anywhere you want to de-

liver special announcements or play mu-

sic with great sound.

Its features include:

Microphone Jacks — let you connect

up to two dynamic microphones.

Input Jacks — let you connect auxiliary

equipment, such as a CD player, tape

deck, or receiver, for music and special

effects.

Individual Microphone Controls — let

you separately adjust the volume of

each microphone.

Master Volume Control — lets you ad-

just the overall sound level.

Auxiliary Volume Control — lets you

adjust the volume of the connected

equipment.

Tone Control — lets you customize the

sound to suit your personal preferences,

depending on the acoustics, speakers,

and sound source.

Five Speaker Terminals — make

speaker connection easy.

Your amplifier is ETL listed to UL stan-

dards and meets all applicable FCC

standards.

WARNING: To reduce the risk of fire

or shock hazard, do not expose this

product to rain or moisture.

CAUTION

RISK OF ELECTRIC SHOCK.

DO NOT OPEN.

CAUTION: TO REDUCE THE RISK OF

ELECTRIC SHOCK, DO NOT REMOVE

COVER OR BACK. NO USER-SERVICE-

ABLE PARTS INSIDE. REFER SERVICING

TO QUALIFIED PERSONNEL.

This symbol is intended to

alert you to the presence of

uninsulated dangerous volt-

age within the product’s

enclosure that might be of suf-

ficient magnitude to consti-

tute a risk of electric shock.

Do not open the product’s

case.

This symbol is intended to

inform you that important

operating and maintenance

instructions are included in

the literature accompanying

this product.

32-2001.fm Page 3 Friday, February 4, 2000 7:53 AM

Preparation

PRESETTING THE

CONTROLS

Warning: A sudden high output from the

amplifier could damage your hearing or

the speakers connected to the amplifi-

er’s output.

To avoid accidentally overdriving a

channel or prematurely amplifying an

audio input, set all the amplifier’s front-

panel controls to their lowest settings.

To avoid sudden audio from the input

sources, be sure all audio devices are

turned off before you connect them to

the amplifier.

CHOOSING A LOCATION

Choose a location with adequate venti-

lation. Do not place the amplifier on thick

carpeting or cushions (which can restrict

the air flow) or near a heat source, such

as a heat vent or radiator (which can

cause the amplifier to overheat).

Making Connections

AUX/CD Input Jack — connect any

high-level sound source (such as a tape

deck, CD player, or receiver). To con-

nect a stereo audio source to this jack,

use a shielded Y-adapter (not supplied;

available at your local RadioShack

store).

PHONO Input Jack — connect a turnta-

ble with a moving magnetic cartridge. To

avoid sound distortion, do not connect a

high-level audio input source to this jack.

GROUND Screw — connect the ground

wire (usually black or green) from your

turntable to this screw to avoid a low-fre-

quency hum. You can also use this

screw to ground any other system con-

nection.

Be sure you connect all auxiliary equipment

and speakers before connecting and turning on

power.

32-2001.fm Page 4 Friday, February 4, 2000 7:53 AM

PHONO/AUX/CD Switch — select the

input source. Even if you connected

auxiliary equipment to both the PHONO

and AUX/CD input jacks, you can use

only one source at a time.

Speaker Push Terminals — press the

appropriate tab to open a terminal. In-

sert the end of a speaker wire into the

opening, then release the tab to close

the terminal and secure the wire (see

“Connecting Speakers” on Page 7).

AC Power Cord — to use AC power,

simply plug this attached cord into a

standard AC outlet.

4A/12V (4-Amp/12-Volt DC Power)

Jack — connect the supplied DC power

cable’s barrel plug to this jack if you

want to temporarily power the amplifier

from your vehicle’s 12-volt battery. Then

connect the cable’s other end to your

vehicle’s 12-volt accessory socket (such

as a cigarette-lighter socket).

Cautions:

• Your vehicle must have a negative-

ground electrical system. If you are

not sure it does, check with your

vehicle dealer.

• Unplug the AC power cord before

you connect the DC power cable.

Likewise, disconnect the DC power

cable before you plug in the AC

power cord.

Speakers

SPEAKER PHASING

Phasing is the direction the speaker

cone moves with reference to the polari-

ty of the connection wires. Proper phas-

ing is important when you use more than

one speaker in the same room or area.

Out-of-phase speakers can lose up to

one-half of their potential volume, and

can have a significantly decreased bass

effect.

Speakers are in phase if all the speaker

cones move in the same direction when

an equal signal is applied. Phasing is

correct if you observe the correct polari-

ty (+ to + and – to –) when connecting

the speakers.

Most speaker terminals are color-coded

or have a mark that indicates the termi-

nal’s polarity. Usually, terminals with

positive polarity are red or have a plus

symbol (+), and terminals with negative

polarity are black or have a minus sym-

bol (–). If the speaker terminals are un-

marked, follow these steps to determine

their polarity.

1. Remove about 1 inch of insulation

from both ends of a short piece of

wire (not supplied; available at your

local RadioShack store). Then twist

the exposed wire to secure all its

strands.

2. Connect one end of the wire to one

of the speaker terminals.

32-2001.fm Page 5 Friday, February 4, 2000 7:53 AM

3. Touch the positive end of a 1.5-volt

flashlight battery to the speaker ter-

minal that does not have the wire

attached to it.

4. Touch the negative end of the bat-

tery with the lose end of the wire

attached to the other speaker termi-

nal. Note the direction of the cone

movement — inward or outward.

5. If the speaker cone moves outward,

the speaker terminal where you

touched the positive end of the bat-

tery is positive. Mark that terminal

with a + and mark the other terminal

with a –.

If the speaker cone moves inward,

the speaker terminal with the wire

attached that touched the negative

end of the battery is positive. Mark

that terminal with a + and mark the

other terminal with a –.

6. Remove the wire.

Repeat Steps 2–6 for each speaker with

unmarked terminals you plan to connect

to the amplifier.

DETERMINING TOTAL

SPEAKER IMPEDANCE

Caution: A total speaker impedance

that is higher than 16 ohms (Ω) or lower

than 4 ohms can damage your amplifier

or speakers. Be sure to make the proper

connections.

Before you connect speakers to the am-

plifier, you must determine the total

speaker impedance. In determining the

total speaker impedance, you must first

determine if your speakers are connect-

ed in series, parallel, or series/parallel

combined.

Note: We recommend that you use

speakers that are rated the same (for

example, all 8-ohm speakers) for all se-

ries or parallel connections.

• Speakers are connected in series

when the first speaker’s positive ter-

minal is connected to the next

speaker’s negative terminal. Deter-

mine the total impedance of speak-

ers connected in series by adding

up the individual impedances of all

the connected speakers.

For example, if you want to connect

two 8-ohm speakers in series, add 8

(the impedance of one speaker)

plus 8 (the impedance of the other

speaker) for a total speaker imped-

ance of 16 ohms.

• Speakers are connected in parallel

when all their negative terminals are

connected together and all their

positive terminals are connected

together. For speakers rated the

same, find the total impedance of

speakers connected in parallel by

dividing the impedance of one

speaker by the number of speakers.

For example, if you want to connect

two 8-ohm speakers in parallel,

divide 8 (the impedance of one

speaker) by 2 (the number of speak-

ers) for a total speaker impedance

of 4 ohms.

32-2001.fm Page 6 Friday, February 4, 2000 7:53 AM

• If you connect more than two speak-

ers using only series or only parallel

connections, the total impedance

might exceed the amplifier’s maxi-

mum impedance (16 ohms) or fall

below its minimum impedance (4

ohms).

For example, if you connect four 8-

ohm speakers:

–In series, the total impedance is 32

ohms (8 + 8 + 8 + 8 = 32). This

exceeds the maximum rating.

–In parallel, the total impedance is 2

ohms (8 ÷4 = 2). This falls below

the minimum rating.

You can arrive at a proper total

impedance by combining series and

parallel connections.

CONNECTING SPEAKERS

You need speakers and wire to com-

plete the connections to your amplifier.

Use 16-gauge wire or larger. For parallel

connections or those involving more

than two speakers, you also need wire

connectors. All of these items are avail-

able at your local RadioShack store.

Remove about 1 inch of insulation from

both ends of each wire. Then twist the

exposed wire to secure all its strands.

Connect the speaker wire to the amplifi-

er by pressing down on the appropriate

push terminal tab and inserting the end

of the twisted wire into the terminal’s

hole. Then release the tab to secure the

wire.

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

16-ohm speakers, with or without trans-

formers, between the amplifier’s output

and the speakers’ input. To ensure

equal volume from each speaker, all the

connected speakers should have the

same impedance rating.

One Speaker

Connect the speaker’s negative (–) ter-

minal to the amplifier’s COM (common)

terminal. Then connect the speaker’s

positive (+) terminal to the amplifier’s

terminal that matches the speaker’s im-

pedance. For example, if you have an 8-

ohm speaker, connect the speaker’s

positive terminal to the amplifier’s 8-ohm

terminal.

Two or More Speakers Without

Transformers

We recommend connections without

transformers if the length of the connect-

ing wires between the amplifier and the

speakers is 50 feet or less.

When making connections without

transformers, all the speakers must

have the same impedance rating to en-

sure equal volume from each speaker.

When there is more than one speaker in

a sound system, you must determine the

total impedance of the speakers before

8ΩSpeaker

32-2001.fm Page 7 Friday, February 4, 2000 7:53 AM

you can make the correct connection.

The total impedance of all the speakers

must match one of the amplifier’s output

terminals (4Ω, 8Ω, or 16Ω).

Speakers in Series

1. To determine the total impedance of

speakers connected in series, add

the impedance of all the speakers.

For example, if you want to connect

two 8-ohm speakers in series, add 8

(the impedance of one speaker)

plus 8 (the impedance of the other

speaker) for a total speaker imped-

ance of 16 ohms.

2. Connect the first speaker’s positive

(+) terminal to the second speaker’s

negative (–) terminal.

3. Connect the first speaker’s nega-

tive (–) terminal to the amplifier’s

COM terminal.

4. Connect the second speaker’s posi-

tive (+) terminal to the amplifier’s ter-

minal that matches the total speaker

impedance (16Ω, in this example).

Speakers in Parallel

1. To determine the total impedance of

speakers connected in parallel,

divide the impedance of one

speaker by the number of speakers.

For example, if you want to connect

two 8-ohm speakers in parallel,

divide 8 (the impedance of one

speaker) by 2 (the number of speak-

ers) for a total speaker impedance

of 4 ohms.

2. Connect the speakers’ negative (–)

terminals together.

3. Connect the speakers’ positive (+)

terminals together.

4. Connect the speakers’ negative (–)

terminals to the amplifier’s COM ter-

minal.

5. Connect the speakers’ positive (+)

terminals to the amplifier’s terminal

that matches the total speaker

impedance (4Ω, in this example).

Series and Parallel Combined

If you must hook up more than two

speakers, you might have to use a com-

bination of series and parallel connec-

tions to get a total impedance that

matches one of the amplifier’s terminals.

Two 8ΩSpeakers

32-2001.fm Page 8 Friday, February 4, 2000 7:53 AM

If you connect four 8-ohm speakers in

series, the total impedance is 32 ohms

(8 + 8 + 8 + 8 = 32). This exceeds the

maximum rating.

If you connect these speakers in paral-

lel, the total impedance is 2 ohms (8 ÷4

= 2). This falls below the minimum rat-

ing.

You can arrive at a proper total imped-

ance by combining series and parallel

connections.

1. Group speakers into pairs.

2. Connect each pair of speakers in

series.

For example, the total impedance of

each pair of 8-ohm speakers is 16

ohms (8 + 8 = 16).

3. Connect the pairs of speakers in

parallel.

4. To determine the total impedance of

speakers connected in serial and

parallel combined, divide the imped-

ance of one pair of speakers by the

number of pairs.

For example, if you want to connect

four 8-ohm speakers in serial and

parallel combined, divide 16 (the

impedance of one pair of speakers)

by 2 (the number of speaker pairs)

for a total speaker impedance of 8

ohms.

5. Connect the negative (–) terminals

of all pairs of speakers to the ampli-

fier’s COM terminal.

6. Connect the positive (+) terminals of

all pairs of speakers to the ampli-

fier’s terminal that matches the total

speaker impedance (8Ω, in this

example).

Multiple Speakers with

Transformers

For complex multiple-speaker arrange-

ments that require many speakers and

long runs of connecting wire, we recom-

mend you use a line transformer (not

supplied), available at your local Ra-

dioShack store.

Note: When running lengths of wire

longer than 50 feet, use wire that is

heavier than 16 gauge.

There are several advantages to using

transformers.

• You can connect speakers with dif-

ferent impedances without causing

differences in output between the

speakers.

• You can add or remove a speaker

without having to recalculate the

entire system’s impedance.

• You can reduce signal loss when

you use speaker wire over 50 feet

long.

Four 8ΩSpeakers

32-2001.fm Page 9 Friday, February 4, 2000 7:53 AM

You need a separate transformer for

each speaker.

Line transformers have several connec-

tors called taps.

The primary taps (on one side of the

transformer) are the inputs and are rated

in watts. The secondary taps (on the op-

posite side of the transformer) are the

outputs and are rated in ohms.

Cautions:

• Before you connect the speakers,

be sure the total wattage of the pri-

mary taps you intend to use does

not exceed the amplifier’s maximum

20-watt output power rating.

• Avoid multiple connections to the

amplifier’s

70V and COM terminals.

Follow these steps to make the connec-

tions.

1. Connect a wire from the amplifier’s

70V terminal to the transformer’s pri-

mary tap that matches your

speaker’s wattage rating.

Note: Usually, the wattage connec-

tions on the primary side of the

transformer are the same for all

speakers. If you want a particular

speaker to have a higher volume

level, use a higher wattage tap on

the primary side of its transformer.

2. Connect a wire from the amplifier’s

COM terminal to the COM taps on

the transformer’s primary side.

Taps

Primary

Taps Secondary

Taps

32-2001.fm Page 10 Friday, February 4, 2000 7:53 AM

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