Mpa-90 P.A. Amplifier
(320-2024A)                Connections                Faxback Doc. # 37935

Before making any connections, be sure the POWER switch is off and the AC
line cord is not plugged in.

To prevent him and noise pickup, use a low capacitance shielded cable for all
connections but speakers.  For speakers, use a speaker cable or two-conductor
wire of appropriate size.


For best results use 4 to 16 ohms speaker loads.  To use other systems, please
contact your local Radio Shack store.  You must consider impedance matching
and phasing when connecting the speakers.  Keep the speaker cable as short
as possible (less than 200 feet).  The total impedance of your speaker system
must be at least 4 ohms.  See SPEAKER CONNECTIONS on FB Doc. #   and 
SPEAKER PHASING on Faxback doc.  #


Connect the turntable cables to the PHONO jack.  If the turntable has a ground
wire (usually black or green), connect it to the phono GND screw.

Be sure to set the PHONO/MIC 4 switch to the PHONO position when you use the
PHONO input.

Note:  If the turntable has a ceramic or crystal cartridge, connect it to the
       AUX jack.


Connect any high level source such as a tape deck, tuner, ceramic or crystal
phono cartridge to the AUX jack.

Use the AUX source mixing control to adjust the level of the high level
source connected to the AUX jack.


Connect a frequency equalizer or other external signal processor's input/
output to the MPA-90's EQ IN/OUT jacks.  When you use the equipment, set the
EQ/BYPASS switch of the MPA-90 to the EQ position.  If the equipment connected
has a bypass switch, be sure you do not activate it.  In the non-activated
position the external equipment has no effect on your systems sound.  You can
also use the EQ OUT jack as a tape output jack.


You can connect up to four microphones to your MPA-90 at the same time.  The
microphones can be either the balanced or unbalanced type.  The unbalanced
type are equipped with a 1/4" plug.  The balanced type (30-500 ohms) terminate
in a 3-pin XLR connector.  The microphones can be either high or low impedance
For example, a low impedance, balanced microphone lets you use long connecting
cables without hum and noise pickup.

Note:  If you connect both an unbalanced and a balanced microphone to the 
       same input the unbalanced microphone has priority.


This allows you to connect a second MPA-90 in your system.  For details, see
MIX BUS FUNCTION on Faxback Doc.  #


The FEEDBACK FILTER removes or greatly reduces any extraneous noise.  Set this
control to the OFF position prior to hookup.  Adjust the control during your
pre-performance testing.

Adjust the levels of MIC 1, MIC 2, MIC 3/PHONO and AUX source mixing controls
to mix up to five sources.  A little experimentation and practice will soon
give you the required "feel" for smooth mixing and fading of various sources.

Set the MASTER VOLUME to reference number 12 and then adjust the individual
source mixing controls to obtain the desired levels.  Then you can vary the
MASTER VOLUME to adjust the overall gain of the system.  Adjust the MEMORY
ring to remind you of this predetermined volume level for your sound system.

The equalizer/bypass function provides a hookup for a frequency equalizer 
(such as a five-band mono frequency equalizer).  Connect an equalizer to the
EQ/BYPASS IN and OUT jacks.  The equalizer adds further dimension in the 
sculpturing of sound to suit its space.  To use this feature, set the EQ/
BYPASS switch to the EQ position.


You can connect two MPA-90s together to double the size of your PA system.
You can use up to 8 MIC inputs, 2 PHONO inputs, or 2 AUX inputs.  The MASTER
VOLUME control on each MPA-90 affects its own output.  Thus, you can have a
system running a full 100 watts in one area and use another running less 
power in another area.

Use a shielded cable with RCA plugs at each end for the MIX BUS connection,
such as Radio Shack's 42-2367.  For best results, do not use a cable longer
than 6 feet (2m).

The individual source mixing controls the output from the MIX BUS jack, not

You can also connect other PA amplifiers that have a mix bus jack.


The "trough" of a specific frequency can be shifted within the range of 300 to
3000Hz as you rotate the FEEDBACK FILTER control.  This function eliminates
many troublesome noises - especially the squeal caused by a feedback frequency

After adjusting the source mixing controls and the MASTER VOLUME, test the 
microphones to confirm proper operation.  If you have any unwanted noise,
turn the FEEDBACK FILTER control until the noise disappears.

The FEEDBACK FILTER control decreases the unwanted part of the frequency by
12dB and reduces feedback remarkably.  If it does not eliminate the problem
completely, the addition of a frequency equalizer can help you reduce the
signal even more.


The MPA-90 provides two methods of making speaker connections.  If your 
speakers usually use insulated wire, between 16 and 22 gauge, use the push
terminals on the rear panel of the MPA-90.  Press the terminal and insert
the bared end of the wire.  Then release the terminal.

If your speaker cable terminates in an RCA phono plug, insert the cable plug
into the RCA speaker input jacks.


Connect the minus (-) side of the speaker to the minus (-) terminal on the
amplifier.  Connect the plus (+) side of the speaker to the plus (+) terminal.

Your MPA-90 delivers maximum power into a 4-ohm load.  However, you can use
any speakers rated at 4 ohms, 8 ohms, or 16 ohms.


When there is more than one speaker in a sound system, you have to observe
impedance matching for maximum power transfer.  This means that the total 
impedance of all the connected speakers must match the output impedance (or 
fall within the acceptable range) on the amplifier.  Your MPA-90 operates 
using speakers within the range of from 4 ohms to 16 ohms.

Impedance is measured in ohms and is designated by the ohm symbol.

When using serial and parallel connections, all the speakers must have the
same impedance rating to ensure equal volume from each speaker.  Notice
that all the speakers used in this discussion are 8 ohms.  Your local Radio
Shack store carries an extensive selection of suitable 8 ohm speakers.

Connecting Speakers in Series

When there is more than one speaker in a sound system, you must determine the
total impedance of the speaker network before you can make the correct 

To determine the total impedance of speakers connected in series, add the
impedance of all the individual speakers.

Connecting Speakers in Parallel

To arrive at the correct impedance for connecting speakers in parallel, divide
the impedance of a single speaker (8 ohms) by the number of speakers (two)

Note:  Remember all speakers must have the same impedance when using this

Combining Series and Parallel Connections

If you must hook-up more than two speakers you might have to use a combination
of series and parallel to obtain a final (total) matched impedance (from 4 
ohms to 16 ohms in your MPA-90).

Remember, improper connections can damage the amplifier.

For example, four 8-ohm speakers connected in series, result in a total of 
32 ohms, which is outside the range allowed by the amplifier (4-16 ohms).

And, if you connect four speaker in parallel, the impedance is less than the
lowest value allowed by the amplifier (4 ohms).

But, if you connect four 8-ohm speakers in a series/parallel combination, you
can arrive at the required impedance.

First connect each pair of speakers in series.  

Next, connect the two pairs of speakers in parallel.  To arrive at the total
impedance of the four speakers, divide the impedance of one pair (of speakers)
by the number of pairs.  Example, the impedance of one pair (16 ohms) is
divided by the number of pairs (2).

Note:  Never use a speaker with an impedance rating lower than 4 ohms.  Never
       use any combination of speakers that has an impedance less than 4 ohms
       or greater than 16 ohms.  If you do so, you might damage your speakers
       and amplifier.


Speaker phasing is the direction the cone moves with reference to the polarity
of the connecting wires.  Out-oh-phase speakers can reduce volume level 
considerably and can create an adverse effect on the base response.  So, it is
critically important when multiple speakers are close together, to have the
speakers properly phased.

In general, speakers are in phase if all the speaker cones move in the same
direction when an equal signal is applied.  Correct phasing is accomplished
by observing the correct polarity when connecting the speakers.

If the speakers are unmarked, or they are not the same model, the following
procedure lets you determine the polarity of the speaker terminals.

1.  Connect one end of a 1.5V flashlight battery to one of the speaker

2.  Momentarily make contact between the opposite end of the battery and the
    other speaker terminal.  Note the direction of the cone movement, inward
    or outward.

3.  If the speaker cone moves outward, the terminal connected to the positive
    side of the battery is plus.

    If the cone moves inward, the terminal connected to the negative side of
    the battery is plus.  Mark the terminal that is positive with a plus

4.  Repeat the same procedure for each sucessve speaker, and be sure to mark
    the correct terminal for the direction of the cone movement in each case.

5.  Connect the terminal in accordance with your speaker wiring.


Each sound system installation is unique.  The best installation for your
purposes can only be found through trial and error.  Microphone and speaker
placement can be especially tricky.  Often moving one of these items just a
few feet can make a big difference.  Remember that your sound system should
be a natural extension of voices and music - the loudest sound possible isn't
always the clearest sound.

You get the best results from your system when you cover the area with a 
constant sound level - avoiding reverberation (echo effects) and preventing
dead spots.

A couple of hints:  Place the speakers so they are mounted slightly above the
heads of listeners and pointed toward them.  When using more than one speaker,
overlap the projection areas of the speakers to prevent dead spots.  Be 
prepared to experiment a little before you obtain optimum positioning.

When you install your MPA-90, don't place it near a heat vent, radiator or
other heat source.  Provide adequate ventilation for your MPA-90.  If
ventilation is inadequate, a thermal overload could result.

If there are any accessories you want to augment your system, you'll find a
wide selection of microphones, speakers, cables, etc. at your local Radio
Shack store.


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