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Speakers and 70.7 Volts?? What’s Going On?

We’ve recently had a number of people asking questions similar to this one from an inSync reader: “Our choir director, at church, asked me to add a speaker for the choir loft. When I connected the wire to one of the speakers in the hallway the entire system went dead. The plate on the back of the speaker said something about 70.7 volts. What’s going on?”

We went to our in house audio guru and general “figure it out” guy Mark (McGyver) Phillips for the answer. Here is what he says:

“You will need a “70.7 volt line transformer” to add a new speaker to the system. There are two types of speaker wiring systems. You are familiar with the “voice coil” system where you connect your speaker, typically 8 1/2, directly to the amplifier. The speaker wire length is usually less than 30ft. The “70.7V” (called 70 volt) system uses a special transformer at each speaker and a special 70 volt output on the amplifier. The 70 volt system is used in multiple speaker installations where the wire lengths may be long, typically hundreds of feet. 70 volt transformers have INPUT “taps” (1,2,5,10 watts) to select the amount of power you wish to take from the system for the speaker, which doesn’t necessarily have anything to do with the power rating of said speaker. Then they have OUTPUT taps to match the impedance of the speaker (4,8,16 ohms) you are using, and this does need to be matched to the speaker for proper operation. It is important that the total power requested (the load) does not exceed the total power available from the amplifier. To calculate the “load” you add together the power requested by each speaker transformer in the system. For example: You have a 100 watt amplifier and your system has two 20 watt speakers and ten 5 watt speakers. You have a 90 watt load. You can safely add a 10 watt speaker.”

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