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70V Systems

70 Volt sound systems

When wiring up a large sound system, its often very difficult to calculate the proper impedances of all of the speakers that are being used. There is an easier way, however, it only really works well for systems that do not use speakers that need much power to appear at individual speakers. It is done by having the output of an audio power amplifier connect to a transformer and the signal is converted to a higher voltage - approximately 70 Volts (hence the name of the system).
From here, any time that you want to add a speaker, you need to have another transformer that allows you to connect to the 70 Volt Signal line, and then, connect the other side of the transformer to the speaker.
70v wiring diagram
You'll note that the above schematic shows colored taps on t transformer that is associated with each speaker. These taps are used to define the amount of power going to each speaker. This allows you to set some speakers louder than others, if there is a need to do that, however, typically all the speakers are set on the same taps (with the same loudness settings).

You will find that most of these systems limit the amount of signal to any individual speaker to no more than 15 watts. Odds are very good that when set, you will use a 2 to 5 watt tap for each speaker.

As you would with any audio system, you will need to figure out how many speakers that you want to use and then divide that into the wattage of your power amplifier - you should not exceed that load, otherwise, you will encounter distortion.

Because of the low wattage limitations of the individual speakers, this set up is normally used in office sound systems and areas where you only want to hear background music, or occasional messages to everyone within a facility. They are rarely used for Pro-Audio systems. Higher powered transformers are available, but they tend to be very expensive. With this in mind, its easy to see that this is not a usable system if you need Sub-Woofers (they need far too much power).

You can't miss the transformer - its stuck right onto the speaker.

For horns that are used outdoors, the transformer will not be as obvious, as it is usually built into to the horn housing to protect it from weather. These often require that a cover be removed to have access to the taps on the transformer. Many outdoor horns do not have a transformer (they are simply a horn that is wired as any other speaker) - you can add one if needed to any 4, 8 or 16 ohm audio speaker/horn. Many electronics suppliers sell the transformers seperately - all will require some form of weatherproof/waterproof protective housing if mounted outside of the horn in a place exposed to the outdoors.

Note: You may find 100 Volt systems as well. These use the same concepts, however, they use 100V transformers instead of 70V transformers. Use all the same type in your system.

Some advantages of 70 Volt systems:

  • You don't need to work out impedance.
  • Because the speakers tend to be fairly far apart, phase of the individual speaker is not very important.
  • Long runs of wire from the transformer on the back of the Power Amplifier are common with fewer losses.
  • Simple installation by untrained crews.
  • Often used when wiring PA systems in buildings, using hundreds of speakers.
  • Often used when wiring outdoor PA systems using 'all weather horns'; ie. sporting events, sales areas, car lots, etc.

Some disadvantages of 70 Volt systems:

  • Low power (typically 10 watts or less) per speaker. Pro-Audio systems may have 100 to 500 watts per individual driver.
  • More expensive - you have to buy transformers for each speaker.
  • Not exceptionally flexable - you can't mix and match speakers to cover your sound needs. The losses in the transformers are fairly high and when added to the losses in Cross-Over Networks could waste 30% or more of your available power.
  • Your power amp has to be able to drive these.