Radio Shack | 32-2001 | Owner`s manual | Radio Shack 32-2001 Owner`s manual

Please read before using this equipment.
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MPA-40 20-Watt PA Amplifier
Owner’s Manual
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ˆ Contents
Features ................................................................................................................... 3
Preparation .............................................................................................................. 4
Presetting the Controls ....................................................................................... 4
Choosing a Location ........................................................................................... 4
Making Connections ............................................................................................... 4
Speakers ..................................................................................................................
Speaker Phasing ................................................................................................
Determining Total Speaker Impedance ..............................................................
Connecting Speakers .........................................................................................
One Speaker ................................................................................................
Two or More Speakers Without Transformers .............................................
Multiple Speakers with Transformers ...........................................................
5
5
6
7
7
7
9
Controls and Operation ........................................................................................ 11
Troubleshooting .................................................................................................... 12
Care and Maintenance ..........................................................................................
Replacing Fuses ...............................................................................................
DC Cable ...................................................................................................
Amplifier .....................................................................................................
13
13
13
13
Specifications ........................................................................................................ 14
Notes ...................................................................................................................... 15
© 2000 Tandy Corporation.
All Rights Reserved.
RadioShack and RadioShack.com are registered trademarks used by Tandy Corporation.
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ˆ Features
Your RadioShack 20-Watt PA Amplifier
gives you 20 watts of solid power for
your PA system. Its wide frequency response 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 deliver special announcements or play music with great sound.
Your amplifier is ETL listed to UL standards 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
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.
RISK OF ELECTRIC SHOCK.
DO NOT OPEN.
CAUTION: TO REDUCE THE RISK OF
ELECTRIC SHOCK, DO NOT REMOVE
COVER OR BACK. NO USER-SERVICEABLE PARTS INSIDE. REFER SERVICING
TO QUALIFIED PERSONNEL.
This symbol is intended to
alert you to the presence of
uninsulated dangerous voltage within the product’s
enclosure that might be of sufficient magnitude to constitute a risk of electric shock.
Do not open the product’s
case.
Individual Microphone Controls — let
you separately adjust the volume of
each microphone.
Master Volume Control — lets you adjust 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.
!
!
This symbol is intended to
inform you that important
operating and maintenance
instructions are included in
the literature accompanying
this product.
Five Speaker Terminals — make
speaker connection easy.
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ˆ Preparation
PRESETTING THE
CONTROLS
Warning: A sudden high output from the
amplifier could damage your hearing or
the speakers connected to the amplifier’s output.
To avoid accidentally overdriving a
channel or prematurely amplifying an
audio input, set all the amplifier’s frontpanel 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 ventilation. 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).
ˆ Making Connections
Be sure you connect all auxiliary equipment
and speakers before connecting and turning on
power.
AUX/CD Input Jack — connect any
high-level sound source (such as a tape
deck, CD player, or receiver). To connect 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 turntable with a moving magnetic cartridge. To
4
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-frequency hum. You can also use this
screw to ground any other system connection.
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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. Insert 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 negativeground 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.
ˆ Speakers
SPEAKER PHASING
Phasing is the direction the speaker
cone moves with reference to the polarity of the connection wires. Proper phasing 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 polarity (+ to + and – to –) when connecting
the speakers.
Most speaker terminals are color-coded
or have a mark that indicates the terminal’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 symbol (–). If the speaker terminals are unmarked, 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.
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3. Touch the positive end of a 1.5-volt
flashlight battery to the speaker terminal that does not have the wire
attached to it.
4. Touch the negative end of the battery with the lose end of the wire
attached to the other speaker terminal. 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 battery 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 amplifier, you must determine the total
speaker impedance. In determining the
total speaker impedance, you must first
determine if your speakers are connect-
6
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 series or parallel connections.
• Speakers are connected in series
when the first speaker’s positive terminal is connected to the next
speaker’s negative terminal. Determine the total impedance of speakers 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 impedance 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 speakers) for a total speaker impedance
of 4 ohms.
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• If you connect more than two speakers using only series or only parallel
connections, the total impedance
might exceed the amplifier’s maximum impedance (16 ohms) or fall
below its minimum impedance (4
ohms).
You can connect one or more 4-, 8-, or
16-ohm speakers, with or without transformers, 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.
For example, if you connect four 8ohm speakers:
One Speaker
–In series, the total impedance is 32
ohms (8 + 8 + 8 + 8 = 32). This
exceeds the maximum rating.
Connect the speaker’s negative (–) terminal to the amplifier’s COM (common)
terminal. Then connect the speaker’s
positive (+) terminal to the amplifier’s
terminal that matches the speaker’s impedance. For example, if you have an 8ohm speaker, connect the speaker’s
positive terminal to the amplifier’s 8-ohm
terminal.
–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 complete 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 available 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 amplifier 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.
8Ω Speaker
Two or More Speakers Without
Transformers
We recommend connections without
transformers if the length of the connecting 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 ensure 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
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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 impedance of 16 ohms.
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 speakers) for a total speaker impedance
of 4 ohms.
Two 8Ω Speakers
2. Connect the speakers’ negative (–)
terminals together.
3. Connect the speakers’ positive (+)
terminals together.
Two 8Ω Speakers
2. Connect the first speaker’s positive
(+) terminal to the second speaker’s
negative (–) terminal.
3. Connect the first speaker’s negative (–) terminal to the amplifier’s
COM terminal.
4. Connect the second speaker’s positive (+) terminal to the amplifier’s terminal that matches the total speaker
impedance (16Ω, in this example).
8
4. Connect the speakers’ negative (–)
terminals to the amplifier’s COM terminal.
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 combination of series and parallel connections to get a total impedance that
matches one of the amplifier’s terminals.
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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 amplifier’s COM terminal.
Four 8Ω Speakers
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 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.
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).
6. Connect the positive (+) terminals of
all pairs of speakers to the amplifier’s terminal that matches the total
speaker impedance (8Ω, in this
example).
Multiple Speakers with
Transformers
For complex multiple-speaker arrangements that require many speakers and
long runs of connecting wire, we recommend you use a line transformer (not
supplied), available at your local RadioShack 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.
3. Connect the pairs of speakers in
parallel.
• You can connect speakers with different impedances without causing
differences in output between the
speakers.
4. To determine the total impedance of
speakers connected in serial and
parallel combined, divide the impedance of one pair of speakers by the
number of pairs.
• You can add or remove a speaker
without having to recalculate the
entire system’s impedance.
For example, if you want to connect
four 8-ohm speakers in serial and
parallel combined, divide 16 (the
• You can reduce signal loss when
you use speaker wire over 50 feet
long.
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Cautions:
• Before you connect the speakers,
be sure the total wattage of the primary taps you intend to use does
not exceed the amplifier’s maximum
20-watt output power rating.
Taps
• Avoid multiple connections to the
amplifier’s 70V and COM terminals.
You need a separate transformer for
each speaker.
Line transformers have several connectors called taps.
The primary taps (on one side of the
transformer) are the inputs and are rated
in watts. The secondary taps (on the opposite side of the transformer) are the
outputs and are rated in ohms.
Primary
Taps
10
Secondary
Taps
Follow these steps to make the connections.
1. Connect a wire from the amplifier’s
70V terminal to the transformer’s primary tap that matches your
speaker’s wattage rating.
Note: Usually, the wattage connections 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.
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3. Connect a wire from the speaker’s
positive (+) terminal to the transformer’s
secondary
tap
that
matches the speaker’s total impedance (4, 8, or 16 ohms).
4. Connect a wire from the speaker’s
negative (–) terminal to the COM tap
on the transformer’s secondary
side.
ˆ Controls and Operation
MICROPHONE 1 and 2 Inputs — connect any standard dynamic microphones.
POWER Switch and Indicator — press
the bottom half of the switch to turn on
the amplifier. The indicator lights. Press
the top half of the switch to turn off the
amplifier. The indicator goes off.
MASTER VOLUME Control — controls
the overall loudness level of all sources
equally. With the amplifier turned on and
the audio input source turned on (and
set to play, if needed), set this control to
its mid-range setting (5). Adjust this control again after adjusting the source mixing controls.
Caution: Be careful not to raise the volume level too high. Doing so might overload the system.
the two microphones to get the proper
volume and balance.
PHONO/AUX Source Mixing Control
— adjusts the input level of the PHONO
or AUX/CD sound source for the best
volume and balance.
TONE Control — adjusts the sound
quality according to the speakers, sound
source, and room acoustics. Turn TONE
toward LOW to adjust the sound toward
a low-frequency tone, or toward HIGH to
adjust it toward a high-frequency tone.
Notes:
• Leaving TONE set to HIGH produces
a flat, unadjusted tone.
• If feedback occurs, turn TONE
toward LOW or turn down the volume.
MIC 1 and MIC 2 Source Mixing Controls — adjust the input level for each of
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ˆ Troubleshooting
If you should run into difficulties, check
the wiring of the system. Are there any
short circuits in the speaker wiring?
Have you provided adequate ventilation? Did you calculate speaker impedance correctly?
Be sure microphones and connecting
cables are not defective.
Be sure you are using large enough
speaker wire. You should always use
16-gauge (or larger) wire. The longer the
run of speaker wire, the heavier the
gauge should be.
If you have feedback problems, reposition your microphones and speakers, or
adjust TONE or MASTER VOLUME to a
lower setting.
If the amplifier does not work at all,
check the fuse on the rear panel. If it is
blown, replace it with one of the same
size and type (see “Replacing Fuses” on
Page 13).
If you are operating the unit from a 12volt DC power source, check the fuse in
the supplied cable that connects to your
vehicle’s 12-volt accessory socket. If the
fuse is blown, replace it with a 4-amp
125-volt fuse (see “Replacing Fuses” on
Page 13).
If you cannot solve the problem, contact
your local RadioShack store for assistance.
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ˆ Care and Maintenance
To enjoy your amplifier for a long time:
• Keep the amplifier dry. If it gets wet,
wipe it dry immediately.
• Use and store the amplifier only in
normal temperature environments.
• Handle the amplifier gently and
carefully. Don’t drop it.
• Keep the amplifier away from dust
and dirt.
• Wipe the amplifier with a damp cloth
occasionally to keep it looking new.
Modifying or tampering with the amplifier’s internal components can cause a
malfunction and invalidate its warranty.
If your amplifier is not performing as it
should, take it to your local RadioShack
store for assistance.
REPLACING FUSES
DC Cable
Amplifier
The power fuse protects your amplifier
from power (voltage or current) surges
when operating it from an AC power
source. If the POWER indicator does not
light when you press POWER, check the
fuse and replace it with one of the same
size and type. (A spare fuse is taped to
the back of the amplifier.)
Follow these steps to replace the amplifier’s fuse.
1. Unplug the amplifier’s power cord.
2. Using a flat-blade screwdriver, push
and turn the fuse holder’s cap counterclockwise until the fuse holder
pops out.
3. Remove the fuse holder and replace
the fuse.
4. Slide the fuse holder back in place
then, using a flat-blade screwdriver,
push and turn the fuse holder’s cap
fully clockwise to secure it.
The supplied cable is equipped with a
4A 125V fuse (in line) to protect your
amplifier from voltage surges and abnormal conditions. If the power indicator
does not light when you press POWER,
check the fuse and, if necessary, replace it with one of the same size and
type.
To replace the fuse, unscrew the tip of
the cigarette-lighter plug. Then remove
the old fuse and insert the new fuse. Secure the fuse by reattaching the plug’s
tip.
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ˆ Specifications
Output Power at 1 kHz, 10% THD ............................................................................... 20 Watts
THD at 1W, 1 kHz Low-Pass Filter
MIC1 ..............................................................................................................................
MIC2 ..............................................................................................................................
AUX CD .........................................................................................................................
PHONO .........................................................................................................................
1%
1%
1%
1%
Input Sensitivity at 10% THD, 1 kHz
MIC1 ........................................................................................................................ 2.5 mV
MIC2 ........................................................................................................................ 2.5 mV
AUX CD .................................................................................................................. 150 mV
PHONO ................................................................................................................... 3.5 mV
Signal-to-Noise Ratio (Input Shorted) with 30 kHz Low-Pass Filter
MIC1 ..........................................................................................................................
MIC2 ..........................................................................................................................
AUX CD .....................................................................................................................
PHONO .....................................................................................................................
60 dB
60 dB
65 dB
65 dB
Frequency Response at 8 ohms, 1W
MIC1 ........................................................................................................... 130 Hz–20 kHz
MIC2 ........................................................................................................... 130 Hz–20 kHz
AUX CD ...................................................................................................... 130 Hz–20 kHz
PHONO ................................................................................. 100 Hz 12 dB 10 kHz–13 dB
Hum and Noise at 8 ohms
at Master Volume Minimum ........................................................................................ 1 mV
at Master Volume Maximum ..................................................................................... 10 mV
Tone Control (High Cut at 10 kHz) .................................................................................. –15 dB
Power Requirement ........................................................................................... 120V AC 60 Hz
Dimensions (HWD) ......................................................................... 315/16 × 123/4 × 105/8 Inches
(99.5 × 324 × 270 mm)
Weight ................................................................................................................................... 9 lb
(4.08 kg)
Specifications are typical; individual units might vary. Specifications are subject to change and
improvement without notice.
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ˆ Notes
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Limited One-Year Warranty
This product is warranted by RadioShack against manufacturing defects in material and workmanship under normal use for one (1) year from the date of purchase from RadioShack company-owned
stores and authorized RadioShack franchisees and dealers. EXCEPT AS PROVIDED HEREIN, RadioShack MAKES NO EXPRESS WARRANTIES AND ANY IMPLIED WARRANTIES, INCLUDING
THOSE OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, ARE LIMITED
IN DURATION TO THE DURATION OF THE WRITTEN LIMITED WARRANTIES CONTAINED
HEREIN. EXCEPT AS PROVIDED HEREIN, RadioShack SHALL HAVE NO LIABILITY OR RESPONSIBILITY TO CUSTOMER OR ANY OTHER PERSON OR ENTITY WITH RESPECT TO ANY
LIABILITY, LOSS OR DAMAGE CAUSED DIRECTLY OR INDIRECTLY BY USE OR PERFORMANCE OF THE PRODUCT OR ARISING OUT OF ANY BREACH OF THIS WARRANTY, INCLUDING, BUT NOT LIMITED TO, ANY DAMAGES RESULTING FROM INCONVENIENCE, LOSS
OF TIME, DATA, PROPERTY, REVENUE, OR PROFIT OR ANY INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, EVEN IF RadioShack HAS BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.
Some states do not allow limitations on how long an implied warranty lasts or the exclusion or limitation of incidental or consequential damages, so the above limitations or exclusions may not apply to
you.
In the event of a product defect during the warranty period, take the product and the RadioShack
sales receipt as proof of purchase date to any RadioShack store. RadioShack will, at its option, unless otherwise provided by law: (a) correct the defect by product repair without charge for parts and
labor; (b) replace the product with one of the same or similar design; or (c) refund the purchase
price. All replaced parts and products, and products on which a refund is made, become the property of RadioShack. New or reconditioned parts and products may be used in the performance of
warranty service. Repaired or replaced parts and products are warranted for the remainder of the
original warranty period. You will be charged for repair or replacement of the product made after the
expiration of the warranty period.
This warranty does not cover: (a) damage or failure caused by or attributable to acts of God, abuse,
accident, misuse, improper or abnormal usage, failure to follow instructions, improper installation or
maintenance, alteration, lightning or other incidence of excess voltage or current; (b) any repairs
other than those provided by a RadioShack Authorized Service Facility; (c) consumables such as
fuses or batteries; (d) cosmetic damage; (e) transportation, shipping or insurance costs; or (f) costs
of product removal, installation, set-up service adjustment or reinstallation.
This warranty gives you specific legal rights, and you may also have other rights which vary from
state to state.
RadioShack Customer Relations, 200 Taylor Street, 6th Floor, Fort Worth, TX 76102
We Service What We Sell
RadioShack
A Division of Tandy Corporation
Fort Worth, Texas 76102
12/99
32-2001
811081640B
02A00
Printed in China
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