ADVANTAGES OF 70V OPERATION
As stated before, a 70V line reduces power loss due to cable heating.
That’s because the loudspeaker cable carries the audio signal as a low current.
Consequently you can use smaller-gauge loudspeaker cable, or very long cable runs, without losing excessive power.
Another advantage of 70V operation is that you can more easily provide the amplifier with a matching load. Suppose you’re connecting hundred of loudspeakers to a single 8-ohm amplifier output. It can be difficult to wire the loudspeakers in a series-parallel combination having a total impedance of 8 ohms.
Also it’s bad practice to run loudspeakers in series because if one loudspeaker fails, all of the loudspeakers in series are lost. This changes the load impedance seen by the power amplifier.
With a 70V system you can hang hundreds of loudspeakers in parallel on a single amplifier output if you provide a matching load. Details of impedance matching are covered later. In addition, a 70V distributed system is relatively easy to design, and allows flexibility in power settings.
Let’s compare a standard low-impedance system to a constant-voltage system. Imagine that you want to provide PA for a runway at an airshow. A low-impedance system might employ 30 speaker clusters spaced 100 feet apart, each cluster powered by a 1000W amplifier for extra headroom. A high-impedance version of that system might use only one amplifier providing 140V. The cost savings is obvious.
DISADVANTAGES OF 70V OPERATION
One disadvantage of a 70V system is that the transformers add expense. Particularly if you use large transformers for extended low-frequency response, the cost per transformer may run $70 to $200. Low-power paging systems, or those with limited low-frequency response, can use small transformers costing around $4.95 each. Many loudspeakers are sold with 70V transformers included.
Another disadvantage is that transformers can degrade the frequency response and add distortion. In addition, a 70V line may require conduit to meet local building code.
TRANSFORMERS
The main component of a 70V system is the loudspeaker transformer.
Its secondary winding has taps at various impedances. You choose the tap that matches the loudspeaker impedance.
For example, if you’re using a 4-ohm loudspeaker, connect it between the 4-ohm tap and common.
The primary winding has taps at several power levels. These power taps indicate how much maximum power the loudspeaker receives. For example, suppose you have a 70V transformer with the primary tapped at 10W and the secondary tapped at 8 ohms. Then a loudspeaker rated at 8 ohms should receive 10W at its voice coil when the primary is connected to a 70V line.
Transformers have insertion loss mainly due to resistance. Precise system calculations should take insertion loss into account. These calculations are covered in the Appendix later in this article.