Distributed Sound Systems

Distributed sound systems are used in commercial applications as an efficient means to distribute sound. These systems are commonly referred to as constant voltage (CV) systems or 70-volt systems. In some cases, usually schools or correctional facilities, 25-volts is used instead of 70-volts.

1. Source Components - microphones, CD players, tape decks, etc.
2. Mixer Amps - these provide input selection, signal control, and power to drive speakers. They have a 70(25) volt output that is usually just two screw terminals on the back, "+" and "-".
3. Wire - most installations use 18 gauge, two conductor, stranded, jacketed wire. There is no need for a shield.
4. Volume Controls - these are only necessary when sound levels need to be changed in various areas on the same system.
5. Transformers - these are almost always mounted to the back of the speaker frame and the step the 70(25) volt signal down to an 8 ohm signal; commonly called step-down transformers. The input of these transformers is referred to as the primary side and the output is called the secondary side. Therefore, a transformer with a 70 (25) volt primary and an 8-ohm secondary would have a 70(25) volt input and an 8 ohm output.
6. Speakers - these can be almost any kind of speaker available. They need a transformer to change 70(25) volts into the signal they are designed to operate on, usually 8 ohms, sometimes 4 ohms.

1. Amplifier Rule - The sum of the tap settings should never exceed 80% percent of the amplifiers' rated output.
   For example, if there are 23 speakers tapped at 2 watts, the load would be 46 watts (23 x 2 watts = 46 watts). To arrive at the needed power for this number of speakers, simply divide the total load by .8. In this case, 46 ¸ .8 = 57.5 watts. Therefore, a standard 60 watt amp would safely drive this load. To calculate the amount of usable power an amp offers, simply multiply the rated output by .8, i.e., 60 watts x .8= 48 watts.
2. Volume Control Rule - The sum of the tap settings on a volume control should not exceed its power rating.
   If the volume control is rated for 10 watts (i.e., AT10), then it could handle 10 speaker/transformer assemblies tapped at 1 watt. If the same assemblies were tapped at 2 watts each, then the resultant 20 watt load would need to be controlled by the next size up, the AT35.

1. The most common wire used on commercial 70(25) volt systems is 18 gauge, 2 conductor, stranded, and jacketed without a shield. Shielded wire is only needed with low level signals.
2. The wire starts at the amplifier location and is paralleled past each speaker location. One of the two conductors is connected to the positive lead of the step-down transformer on each speaker assembly, and the other conductor is connected to the common or negative lead (usually black).
3. Wire Length -18 gauge is appropriate up to 700 feet with a 100-watt load. If you double the load (sum of your tap settings), you will reduce the footage by half, to 350 feet. Conversely, if you half the load, you may double the acceptable wire length, i.e., a 50 watt load is safe over 1400 feet of 18 gauge. Stepping up to 16 gauge wire extends the allowable run length by approximately 35%. For example, a 100-watt load can go 700 feet on 18 gauge; the same load may be placed on 1100 feet of 16 gauge.