MPA-50 40-Watt PA Amplifier
(320-2002) Speakers Faxback Doc. # 51511
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.
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 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
impedance. In determining the total speaker impedance, you must first
determine if your speakers are connected 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.
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).
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 (not supplied; available at your local
RadioShack store) to complete the connections to your amplifier. Be sure
to use 16-gauge wire or larger.
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.
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.
One Speaker
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 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
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 you can make the correct connection
The total impedance of all the speakers must match one of the amplifier's
output terminals (4 Ohms, 8 Ohms, or 16 Ohms).
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.
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 (16 Ohms 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 speakers)
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
terminal.
5. Connect the speakers' positive (+) terminals to the amplifier's
terminal that matches the total speaker impedance (4 Ohms, 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.
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).
3. Connect the pairs of speakers in parallel.
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.
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
amplifier's COM terminal.
6. Connect the positive (+) terminals of all pairs of speakers to the
amplifier's terminal that matches the total speaker impedance (8-Ohm,
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), such as RadioShack's Cat. No. 32-1031B, 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.
You can connect speakers with different 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.
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.
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 40-watt output power rating.
Avoid multiple connections to the amplifier's 70V and COM
terminals.
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.
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.
(EB 3/1/99)
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