70V Distributed Audio

Distributed Audio

By John Vellone

There are two types of audio amplifier on the market. Constant Current Amplifier (like your home stereo amplifier), which have a low impedance output of either 8 ohms or 4 ohms, and Constant Voltage Amplifier (used for commercial distributed audio systems) which have a high impedance out put identifies in 25 volt, 70 volt, and 100volt output. 70 volt systems is the most commonly used for commercial distribution, 25 volt is used in institutional intercom systems, and 100 volt system is the European standard.


Constant Current Amplifiers

Because of their low impedance output they can accommodate one to two speakers on a short run of cable, typically less then 50 feet using gauge 18 wire. Speakers must be connected directly to the amplifier and your maximum impedance load in ohms must not exceed your rated impedance load of your amplifier. Several speakers may be connected to these amplifiers to create a small distributed audio system. In general when using 8 ohm speakers the total amount of speakers used is a multiple of 2x, ex. 1,2,4,8… 2x . In order to obtain these quantities you must use ohms law and combine your speakers in a series and parallel configuration.

Constant Voltage Amplifier

Constant Voltage amplifier have a high impedance output and can accommodate large quantities of speakers on a long cable run of more than 50 feet up to several thousand feet. Line matching transformers are mounted on the 8 ohm speakers to convert them, (the speaker) to high impedance, matching them, (the speakers) to the high impedance, 70 volt out put of the amplifier. The speaker and transformer assembly are then simply connected in parallel to the output of the amplifier.


Line Matching transformers offer different wattage taps. This allows you to select the desired power output of the speaker for different locations. You can use different wattage taps on a same distribution line.


The following conditions must be observed at all times when designing and installing a constant voltage distributed audio system.

· The sum of the total transformer wattage taps used for all speakers in a distribution line must never exceed the amplifier’s rates output.

· It is preferred to load a constant voltage amplifier to 80 % of it’s rated output. This gives the system a 20 % headroom which increases system performance. Over loading an amplifier will cause the power or output fuse to blow. In some cases overloading can blow the output stage or even the output transformer of the amplifier. In any case over loading will reduce the life expectancy of the amplifier.

· In a constant voltage distribution system it is important to observe speaker phasing. Especially when more then one speaker are installed in the same area.

· If a line matching transformer offers both 25v and 70 volt taps, make sure the proper line voltage is used. Wrong voltage may result in over-loading an amplifier or it will give you reduced or increased sound level.

You can build a distributed audio system using both Constant Current or Constant Voltage amplifiers. Your choice of which system you will choose depends on (1) How many speakers you will nead to install and (2) The cable length you nead to run.