MPA-45 PA Amplifier 320-2035 Connecting Speakers

MPA-45  (320-2035)           Connecting Speakers       Faxback Doc. # 15699

For the best results, use speakers that are rated at 4 to 16 ohms.  For 
speaker connections, use 18-gauge speaker wire for lengths up to 25 feet-
7.5m.  Use 16-gauge wire for lengths over 25 feet-7.5mm.  We recommend that
you use the shortest length of wire possible.

To prepare the speaker wire, remove about 1 inch-2.5 cm of insulation from
the end of the speaker wire you intend to connect to the amplifier.  Then,
twist the exposed wire to secure all of the wire strands.

To connect speaker wire to the amplifier, press down the appropriate push 
terminal lever on the amplifier and insert the end of the wire into the 
terminal's hole.  Then, release the lever to secure the wire.

You can connect two or more speakers with or without transformers.

DETERMINING SPEAKER POLARITY

Most speaker terminal 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 minus symbol (-).  If your speakers' terminals do not indicate 
their polarity, you must determine the polarity yourself to ensure that 
the speakers are properly phased.

Proper phasing is important when you use more than one speaker in the same
room or area.  Speakers are in-phase when their cones move in the same
direction when an equal signal is applied to the speakers.  Speakers that
are out-of-phase can lose up to one-half of their potential volume, and can
have a decreased bass effect.

Follow these steps to determine the polarity of the speaker terminals.

1.  Connect one end of a 1.5-volt flashing battery to one of the speakers
    terminals.

2.  Briefly touch the opposite end of the battery with the other speaker 
    cone moves in or out.

If the cone moves in, the terminal connected to the battery's negative 
side is positive.

Mark the positive terminal with a plus (+) sign.

3.  Repeat Steps 1 and 2 for each speaker.

CONNECTING ONE SPEAKER

Connect the speaker's negative (-) terminal to the amplifier's COM
terminal.  Then, connect the speaker's positive (+) terminal to the
terminal that matches the impedance of the speaker.  The following example
shows an 8-ohm speaker connected to the amplifier's 8-ohm terminal.

NOTE:  We recommend 18-gauge speaker wire if the length of wire is 25 feet-
       7.5m or shorter.  For the best results, we suggest you use the
       shortest length of speaker wire possible.

CONNECTING SPEAKERS WITHOUT TRANSFORMERS

If you connect speakers without transformers, the lengths of speakers wire 
should be no longer than 50 feet-15 m.  For lengths up to 25 feet-7.5 m, 
use 18-gauge wire; for lengths from 25 to 50 feet-7.5m to 15m, use 16-gauge
wire.  To ensure equal volume from each speaker, all the connected speakers
should have the same impedance rating.  The following examples show 8-ohm
speaker, which are the most common type sold by your local Radio Shack
store.

CONNECTING SPEAKERS IN SERIES

Before you connect speakers in series, determine the total impedance for 
the speakers.  To do so, add the impedance of all connected speakers.  For 
example, if you connect two 8-ohm speakers, add 8 (impedance of one 
speaker) for a total impedance should match one of the amplifier's 
terminals (4-ohm, 8-ohm or 16-ohm).

CAUTION:  The total impedance must be at least 4 ohms but most not exceed
          16 ohms.

CONNECTING SPEAKERS IN PARALLEL

Before you connect speakers in parallel, determine the total impedance.  To
do so, divide the impedance of one speaker by the number of speaker.  For
example, if you connect two 8-ohm speakers, divide 8 (impedance of one
speaker) for a total impedance of 4 ohms.  The total impedance should match
one of the amplifier's terminals (4-ohm, 8-ohm, or 16-ohm).

CAUTION:  The total impedance must be at least 4 ohms but not exceed 16
          ohms.

COMBINING SERIES AND PARALLEL CONNECTIONS

Before you connect speakers in parallel, determine the total impedance.  To
do so, divide the impedance of one speaker by the number of speaker.  For
example, if you connect two 8-ohm speakers, divide 8 (impedance of one
speaker) by 2 (the number of speaker) for a total impedance of 4 ohms.  The
total impedance should match one of the amplifier's terminals (4-ohm,
8-ohm, or 16-ohm).

CAUTION:  The total impedance must be at least 4 ohms but not exceed 16
          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).  Or, if you connect four 8-ohm speakers 
in parallel, the total impedance is 2 ohms (8 divided by 2 =2).

CAUTION:  A total impedance that is too high or too low can damage your
          amplifier or speakers.

You can arrive at a proper total impedance by combining series and parallel
connections.  In the following example, we show the connection of four
8-ohm speakers.

First, connect each pair of speakers in series.  Each pair has a total 
impedance of 16 ohms ((6+6=16).

Next, divide 16 (the impedance of one pair of speakers) by 2 (the number 
of pairs) for a total impedance of 8 ohms.  After you determine the total 
impedance, connect the two pairs of speakers in parallel.

CONNECTING SPEAKERS WITH TRANSFORMERS

If you connect two or more speakers in you system, you can use a line 
transformer (Cat. No. 320-1031) for each speaker.

Transformers offer these advantages:

    You can use speakers that have different impedances without causing 
    differences in output between the speakers.

    You can add or remove a speaker from the system without having to 
    recalculate the impedance for the entire system.

    You can reduce signal loss when you use runs of speaker wire over 50 
    feet-15m long.

NOTE:  Use 18-gauge speaker wire for lengths up to 25 feet-7.5, and use
       16-gauge wire for lengths more than 25 feet-7.5m.

Line transformers have several connectors called taps.  The primary taps 
are on one side of the transformer, and are marked as 10,5, 2.5, 1.25 and 
0.62 watts.  The secondary taps are on the other side of the transformer, 
and are marked as 4, 8 and 16 ohms.

CONNECTING THE PRIMARY TAPS

Before you connect speakers, be sure the total wattage of the primary taps 
you intend to use does not exceed the amplifier's power rating of 35 watts.
In the following example, the total wattage is 8.75 watts.

1.  Connect the desired primary taps to the amplifier's 70V terminal.

NOTE:  Usually, each speaker in a system uses the same wattage.  If you
       want a particular speaker to have a higher volume level, use a
       higher wattage tap on its transformer.

2.  Connect the C (common) taps on the transformers' primary side to the 
    amplifier's COM (common) terminal.

CONNECTING THE SECONDARY TAPS

To connect the secondary taps of each transformer, follow these steps.

1.  Connect the speaker's positive (+) terminal to the transformer's 
    secondary tap that matches the speaker's impedance.

2.  Connect the speaker's negative (-) terminal to the C (common) tap on 
    the transformer's secondary side.

SPEAKER PLACEMENT

Finding the best speaker placement requires some experimenting.  For the 
best result, point the speakers toward the listeners, and mount the
speakers slightly above the level of the listeners' heads.  The following 
drawings show recommended speaker placements for typical, narrow, and wide 
sound coverage.

Hint:

If you need to place speakers far apart, point them toward the main 
listening area so that the speakers' coverage areas overlap.  This helps 
prevent dead spots (areas not covered by the speakers' sound).


(SP/all-06/28/95)