20-Watt AC/Mobile PA (320-2041) Preparation Faxback Doc. # 15375
Amplifier
PLACING THE AMPLIFIER
Before you use your amplifier, be sure you choose a location with adequate
ventilation. Do not place the amplifier on thick carpeting (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).
PRESETTING THE CONTROLS
Setting the Amplifier
WARNING: A sudden high output from the amplifier could damage your hearing
or the speakers connected to the amplifier's output.
Control Setting
POWER OFF
MIC 1, MIC 2, PHONO/AUX.CD MIN
TONE MID
MASTER VOLUME 0
To avoid accidentally overdriving a channel or prematurely amplifying an
audio input, set the amplifier's controls to the levels shown below before
you connect the AC power cord or turn on power.
Setting the Input Sources
To avoid sudden audio from the input sources, set their controls to the
levels shown below before you connect their output jacks to the amplifier's
input jacks.
Audio Device Control Setting
Turntable Power Off
Tape Deck Power Off
CD Player Power Off
Amplifier/Receiver Power Off
Tone Flat
CONNECTING AUDIO COMPONENTS TO THE AMPLIFIER
You can connect optional components (such as microphones, tuners,
turntables, or CD players) to the amplifier to expand the system. Your
local Radio Shack store carries a wide selection of audio components,
adapters, microphones, and speakers.
CAUTION: make sure you make all the connections to the amplifier and
speakers before connecting and turning 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 microphone(s) can be high- or low-impedance and
must have a 1/4-inch plug.
Connecting a Turntable
You can connect an audio input source, such as a magnetic cartridge
turntable to the amplifier's PHONO jack.
CAUTION: 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.
NOTES: To avoid a low frequency hum, connect your turntable's ground wire
(usually black ceramic cartridge turntable or green) to the
amplifier's GND screw.
To prevent a beat or howling sound during monaural operation in the
PHONO mode, separate the PHONO input line from the speaker wire.
Connecting an Auxiliary Audio Source
You can connect an auxiliary audio input source (such as a tape deck, CD
player, ceramic cartridge turntable or tuner) to the amplifier's AUX. CD
jack.
NOTE: To connect a stereo audio source to the AUX.CD jack, use a shielded
Y-adapter, such as Cat. No. 42-2438 (not supplied).
PLACING THE SPEAKERS
Speaker placement depends on your room's size and arrangement. We
recommend you play a wide-range recording and experiment with speaker
placement until you find the locations that result in the best sound. Use
the shortest length of wire possible to connect the speakers. After
placing the speakers, determine the wire length and choose the appropriate
gauge size as follows:
NOTE: If you connect speakers without transformers, the speaker wire
should be no longer than 50 feet.
CONNECTING THE SPEAKERS
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.
Remove about 1 inch of insulation from the end of each wire. Then twist
the exposed wire to secure all its strands.
Connect the speaker wire to the amplifier and the speaker(s) by pressing
down on the appropriate push terminal lever and inserting the end of the
twisted wire into the terminal's hole. Then release the lever to secure
the wire.
Phasing the Speakers
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.
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 (-).
Phasing is correct when you connect + to + and - to -.
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.
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 connected in series, parallel, or a
series/parallel combination.
Speakers are connected in series when the first speaker's positive terminal
is connected to the next speaker's negative terminal. Determine the total
speaker 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.
Determine the total speaker impedance of speakers connected in parallel by
dividing the impedance of one speaker by the number of speakers.
For example, if you plan 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 = 32). This
exceeds the maximum rating.
-In parallel, the total impedance is 2 ohms (8 divided by 4 = 2).
This falls below the minimum rating.
You can arrive at a proper total impedance by combining series and parallel
connections.
Connecting Only 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 speaker's impedance (4 ohm, 8 ohm, or 16 ohm).
Connecting Speakers in Series
Follow these steps to connect speakers in series.
1. Connect the first speaker's positive (+) terminal to the second
speaker's negative (-) terminal.
2. Connect the first speaker's negative (-) terminal to the amplifier's
COM terminal.
3. Connect the second speaker's positive (+) terminal to the amplifier's
4 ohm, 8 ohm, or 16 ohm terminal that matches the total speaker
impedance.
Connecting Speakers in Parallel
Follow these steps to connect speakers in parallel.
1. Connect the speakers' negative (-) terminals together.
2. Connect the speakers' positive (+) terminals together.
3. Connect the speakers' negative terminals to the amplifier's COM
terminal.
4. Connect the speakers' positive (+) terminals to the amplifier's 4 ohm,
8 ohm, or 16 ohm terminal that matches the total speaker impedance.
Connecting Four Speakers in Series/Parallel Combination
Follow these steps to connect four speakers in series/parallel
combination.
1. Group the four speakers into two pairs.
2. Connect each pair of speakers in series. The total impedance of each
pair is 16 ohms (8 + 8 = 16).
3. Connect the two pairs of speakers in parallel. The total impedance of
both pairs is 8 ohms (16/2=8).
4. Connect both speakers' negative (-) terminals to the amplifier's COM
terminal.
5. Connect both speakers' positive (+) terminals to the amplifier's
4 ohm, 8 ohm, or 16 ohm terminal that matches the total speaker
impedance.
Connecting Speakers with Transformers
For the best results when you connect two or more speakers to your system,
we recommend you use a line transformer (such as Cat. No. 32-1031, not
supplied) for each speaker.
Transformers offer these advantages:
You can connect speakers with 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 entire system's impedance.
You can reduce signal loss when you use speaker wire over 50 feet
long.
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 20-watt output power rating.
Avoid multiple connections to the amplifier's 70V and COM
terminals.
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, each speaker in a system uses the same wattage tap. If
you want a particular speaker to have a higher volume level, use a
higher wattage tap on its transformer.
2. Connect a wire from the amplifier's COM (common) terminal to the C
(common) 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 ohms, 8 ohms, or 16 ohms).
4. Connect a wire from the speaker's negative (-) terminal to the C
(common) tap on the transformer's secondary side.
(br/all-05/30/95)
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