App. #3
Application Notes

Application Note #3

Multichannel Paging and Background Systems with the MPR-450

A Cost-effective Approach

Audio systems with many amplifier channels are especially expensive if the equipment is not chosen carefully. Correctly matching the equipment to the job will result in optimum per-formance and significant cost savings. In addition to the purchase price of the equipment, the instal-lation and operating costs should be considered.

The MPR-450 is ideal for multi-zone paging and background-music systems. For applications where each zone needs a continuous rating of 75 watts, the MPR-450 is the most cost-effective amplifier system available. It is also price-competitive where 150-watt and 300-watt zones are needed, and its outputs have the flexibility to operate at various combinations of power levels.

In addition to providing many channels at modest cost, the MPR-450 saves space. Each MPR-450 provides six channels of amplification in only two rack spaces (3.5 inches).

How Much Power?

The MPR-450 provides six separate outputs with a simultaneous continuous power level of 75 watts each. In practice the operating levels of the different zones are not likely to be identical. This causes the MPR-450 to operate with higher output capability, since additional power is available to the channels which need it. Where zones are addressed individually or driven at different levels, each channel can produce up to 130 watts continuously. This capability provides excellent headroom for demanding situations.

If the output zones need a continuous power level of 150 watts or 300 watts, the MPR-450 can easily provide that in bridge mode. This versatility avoids the needless initial and operating expense of an "over-designed" system. When addressed individually, each bridged output can produce 250 watts continuously; a double-bridged output can produce 400 watts continuously..

Often there is an advantage to utilizing many smaller zones rather than a few large ones. The levels of each zone can be adjusted individually with the gain controls for optimum sound balance in different rooms or different parts of a room.

If the system requirement includes combinations of power levels, the MPR-450 not only meets the specified power requirement; in addition, it provides considerable output headroom, well above many alternative solutions.

Installation Considerations

In many cases a significant part of the system cost is in its installation. The MPR-450 power amplifier and MPT-70 isolated 70-volt distribution system are designed to minimize this cost by reducing the amount of equipment needed and the amount of labor required to complete the installation.

System wiring is simple and straightforward. Amplifier outputs can be connected according to the power requirement. All connections and controls are conveniently located on the back panel, and the necessary configuration information is clearly screened next to the terminals.

Operating Cost

Since amplifiers in this application are normally on all the time, power consumption during idle or low-level output is a significant factor in the operating cost of a system. This is especially true in larger systems with hundreds of channels where the power usage is multiplied accordingly.

The MPR-450 uses less than 6 watts of idle power per channel. Compared to other amplifiers, this not only reduces the electric bill, it also helps to keep the equipment room cool.


The MPR-450 is designed for maximum reliability. Unlike most competitive amplifiers, it is designed to drive any load impedance including a shorted output. Therefore, it is practically indestructible even under adverse or fault conditions.

The fan normally idles slowly, keeping the entire unit at room temperature. It operates at higher speed only when a temperature rise is detected at the heatsinks. The fan uses a special rotor technology which is far superior to sleeve or ball bearings; it is rated by the manufacturer for over 20 years of continuous service at full speed.

70-Volt Operation

For 70-volt operation, the MPR-450 is used with the MPT-70 audio distribution system. This rack-mount device includes six 75-watt transformers for isolated 70.7-volt lines with a total continuous output of 450 watts.

High-power outputs are created using bridge mode or double-bridge mode (two pairs of channels bridged). Each MPR-450 and MPT-70 can create any of these combinations of 70-volt outputs:

1. Six 75-watt lines

2. One 150-watt line and four 75-watt lines

3. Two 150-watt lines and two 75-watt lines

4. Three 150-watt lines

5. One 300-watt line and two 75-watt lines

6. One 300-watt line and one 150-watt line.

Fig. 1

The MPT-70 utilizes high-quality isolation trans-formers with a 1:4 turns ratio to step up the voltage. Fig. 1 shows the design of each transformer.

For higher power, transformer secondaries are wired in parallel while the optimum 4-ohm amplifier loading is maintained. All outputs are always isolated from the amplifier and ground reference.

75-watt Connection

The basic connection for each 75-watt channel is very simple, as shown in Fig. 2. Each MPR-450 output connects to the + and - terminals at the corresponding MPT-70 input. The MPT-70 output terminals provide the 70-volt line. Each terminal can accept two wires as large as 12 gauge.

Fig. 2

150-watt Connection

Fig. 3

The connection for each 150-watt output is shown in Fig. 3. An MPR-450 bridged output (make sure the bridge switch is on) connects to terminals +A and -A at the MPT-70 input, and a ground wire and jumper are connected as shown. The black binding posts on the MPR-450 are all internally connected together, to ground. MPT-70 outputs 1 and 2 are connected in parallel.

25-Volt Operation

The MPR-450 can directly drive a 25-volt line at each output. If isolation is required, the MPT-25 may be used similarly to the MPT-70. Many local electrical codes do not require conduit for 25-volt systems, so the installation may prove less costly even if heavier cable is used to reduce power loss.

More Information

For more information call 1-800-280-8572.

Application Notes