Just like the light and power utilities use of higher voltage power lines, between generators and neighborhoods, saves wire costs and allows for greater power carrying distances, the audio distributed power 70-Volt systems, between the amplifiers and speakers, saves wire costs and allows for greater power carrying distances.
25-Volt systems use almost three times the current to carry the same power of 70-Volt systems. Compared to 70-Volt systems, 25-Volt systems have to use larger wires, and won't go as great of a distance before loosing too much power into the wires.
That explains why most audio distribution systems nowadays are 70-Volt systems. To understand why there are 25-Volt systems, a little history is needed.
History
When distributed sound systems were first used, there weren't any 70-Volt systems. Technically, 25-Volt systems were the highest voltage that could be designed to be both reliable and safe. Later, when 70-Volt systems were introduced, 25-volt systems were already established and they were everywhere. It took a while to change over the designs from the 25-Volt systems to 70-Volt systems.
Also, when making minor changes to a sound distribution system, keeping the system at 25 Volts was easier than converting it to 70 volts. Even now, when just adding a speaker to a 25-Volt system, adding a speaker that's set for a 25-Volt system is far cheaper than resetting all the rest of the speakers, plus changing the amplifier, in order to convert the whole audio distribution system to 70 Volts.
Nowadays, when there's a choice, systems are designed using the 70-Volt audio distribution system. However, there are still 25-Volt systems around, mostly because of history.
Douglas Krantz
