Multiple Speaker Public Address and Intercom System

The amplifier is a Realistic MPA-20 Solid State Amp. This is used instead of an old stereo receiver due to longevity of operation. This unit must remain powered on 24/7 to facilitate as the amplification for the alarm siren as well as serving as a PA, Audio, and Intercom system.

Alarm

The Alarm component will produce a siren loud enough to resinate though an entire city block. If an intruder breaks into the residence, the alarm will cause enough attention that the intruder will surely leave before police arrive as the result of someone phoning the police in the neihgborhood.

Public Address and Music

Audio from various sources can be piped into the system. These sources include satilight TV audio, MP3 music, or a stereo receiver.

Intercom

This is tied into the home intercom system so that from any intercom station within the residence someone may be paged that is outdoors near the house.

There will be a hybrid of outdoor horn speakers, outdoor 2-way speakers, and indoor speakers all powered by the MPA-20 amplifier. Different audio levels will be needed for the various speakers.

The loudest speakers need to be the outdoor horn speakers, which serve as the main volume when the alarm siren is activated. The horn speakers are also ideal for voice, easy to hear over the sound of a lawnmower when being paged.

Moderately loud will be the 2-way outdoor speakers. Their purpose is to supply some level of musical fidelity as horn speakers are not able to produce a wide range of sound. However, they need to be limited as to prevent damage when the high powered siren is sounded.

The relatively quiet indoor speakers serve only to supplement the alarm and also music. These speakers not only need to be volume limited, but also require an off function.

Multiple Speaker Handling

Without Line Transformers

Here are some sample configurations without using transformers.

With Line Transformers

The output connectors on the Amp serve only a single speaker, or speakers in a series or parallel circuit of the same resistance. More complex speaker arrangements may be achieved with the aid of transformers.

The MPA-20 Amp uses a 70v system. When using transformers, power from the 70v terminal is being supplied to the transformers to "localize" the amplification power to the speaker, when the transformer is at or near the speaker.

Transformers are not meant to be used local to the amp, but are to be used local to the speakers. One transformer is meant to be used per speaker, in a 1:1 ratio with the speakers.

It is wrong to think that one transformer near the amp can be used to solve multiple speaker of multiple resistance configurations, as there is obviously already a transformer in the amp itself with leads going to the connectors of the different resistance ratings on the back of the amplifier.

• Determine what type of transformer is needed.

There are several advantages to using transformers.

1. connect speakers with different impedances without causing differences in output between the speakers.
2. add or remove a speaker without having to recalculate the entire system's impedance.
3. reduce signal loss on long distance wire runs (like 50ft or more)

• Ensure the total wattage of the primary taps doesn't exceed the amp's 20w max power output.
• Avoid multiple connections to the 70v terminal and the com terminal.

1. Connect wire from amp 70v to each transformer pri tap that matches the speaker watts.
2. Connect wire from amp com to each transformer com.
3. Connect speaker + to each amp + on secondary tap matching the ohms.
4. connect speaker - to the com on the secondary side of transfomer.

Examples of 20W 70V Line Transformers

This unit is called the T-20 and has a sticker on one side so that it may be adheared to the place where it will be mounted.

This transformer handles 20W RMS, and has primary taps for 5W, 10W, 15W and 20W. Secondary connections accommodate 4ohm or 8ohm speakers.

Speakers Staged in PA System

• Qty 2 Marine Horn Speakers
• Qty 2 Two Way Outdoor Speakers
• Qty 2 Unknown Indoor

Marine Horn Speakers

Three Way Outdoor Speakers

Dual LU43PW 3-Way 100 Watt Dynamic Indoor/Outdoor Speakers

Speaker Volume Control

This may be accomplished using a Potentiometer on each speaker. All the Potentiometers can be centralized in the same area as the amplifier and PA microphone.

• Determine what resistance value and type is necessary for the potentiometer.

10K Ohm potentiometer - seems to be what is used on RS home intercom

The Realistic MPA-20 is a 20 watt Solid State PA Amplifier sold by Radio Shack at one time.