Most churches have a need for a sound system that can provide sound to many areas. Your church probably has sound in the narthex, the nursery or cry room. The fellowship hall, offices, kitchen, restrooms or hallways may also have need for a loudspeaker system. These needs are solved with a distributed loudspeaker system. They are also called a 25 volt, 70 volt, or 100 volt distributed loudspeaker system. We will illustrate with the 70 volt system, but the principles of operation are the same for the 25 and 100 volt distributed systems.

The 70 volt loudspeaker system can provide a safe, efficient method to connect many loudspeakers to one amplifier. Each area or room can have its own volume control or loudspeaker on/off switch. However, there are several principles which must be observed if the system is to perform correctly. Here are some typical questions we often get asked about distributed loudspeaker systems.

Q:  My loudspeakers are hooked up to the amplifier on screws marked 70.7 volts. What does that mean?

A:  Your amplifier has the ability to power or drive a system which is called a constant voltage 70.7 volt loudspeaker system or 70 volt for short. This system allows for easy hookup of many loudspeakers without the fear of amplifier damage, as long as certain conditions are met. The name is actually a misnomer since 70 volts are not constantly applied to the loudspeakers. The amplifier output voltage is only at 70 volts when the amplifier is operating at full power. A normal operating level may produce an average of only 22 volts, and it is greatly varying with the input signal. The term constant voltage comes from the way the amplifier is designed. The constant voltage amplifier will put out a voltage that is independent of the loudspeaker load (impedance). It doesn't make any difference if one loudspeaker or 20 loudspeakers are connected.

Caution: Do not connect loudspeakers to a 70 volt system if they do not have a 70 volt transformer on them. Damage may result to the loudspeakers or the amplifier. Also, do not connect loudspeakers to the amplifier on both the 70.7 volt taps and the 8 ohm or 4 ohm tap. This can result in a severe overload to the amplifier.

Q:  How many loudspeakers can I hook up to my 70 volt amplifier?

A:  All you need to remember is to not exceed your power amplifier wattage rating by the total sum of the transformer power taps that are used by each loudspeaker. We recommend you also allow a 25% safety margin for losses in the transformers and wiring.

Figure 1
Transformer terminals

For example, each loudspeaker must have a transformer mounted to it. This transformer usually has a primary side (PRI) with the terminals or wires marked in power rating in watts, i.e., 5W, 2.5W, 1.25W, etc. (Figure 1). There is also a terminal marked COM. The other side is the secondary (SEC) and the terminals usually say 8 ohm and common (COM). The primary side is hooked up to the 70 volt line across the primary COM and the power tap which will give you the wattage (power) you need to drive that particular loudspeaker to the needed volume. Let's say you need 2 watts to each loudspeaker, and you have a 60-watt amplifier. That means you could hook up 22 loudspeakers at 2 watts apiece for a total load of 44 watts with a 25% safety margin.

Q:  What transformer tap should I use?

A:  That is a difficult question because there are many variables, such as loudspeaker efficiency, ambient noise level in the room, and how much volume you desire. If these factors are known, a system can be engineered. To give you a general idea, a 1/2 watt tap on a good 8" loudspeaker will usually be more than adequate in an office.

Q:  What gauge wire should I use to hook up my 70 volt loudspeakers?

A:  One of the main advantages of the 70 volt loudspeaker distribution system is that smaller and less expensive cable can be used. You can operate a 10 watt loudspeaker on 900 feet of 22 gauge wire with only a .5 dB or 12.5% power loss. With 18 gauge wire, you can put the 10 watt loudspeaker 2300 feet away!