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Tech Tip Of The Day: 70.7 Volts Of Confusion

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By PSW Staff March 7, 2014

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Provided by Sweetwater.

Q: I help out with the audio at my church, and when our choir director asked me to add a loudspeaker for the choir loft, I thought it sounded a little challenging but not overly difficult.

Well, when I connected the wire to one of the loudspeakers in the hallway, the entire system went dead! The plate on the back of the loudspeaker said something about 70.7 volts. What’s going on, and what the heck is the 70.7 volts all about?

A: Indeed, this situation is a bit more complex than you’d anticipated, and the answer is actually quite involved. However, we’ll save the detailed explanation of constant-voltage distributed audio systems for another time and just provide the information that’s pertinent to your situation.

In your case, you’re going to need a “70.7 volt line transformer” to add a new loudspeaker to the system.

First, however, note that there are two types of loudspeaker wiring systems. The first is the type you’re familiar with, the “voice coil” system, which typically connects a loudspeaker (or loudspeakers) directly to the power amplifier. The speaker wire length is usually less than 30 feet.

The second type is called “70.7V” (called “70 volt”), which uses a special transformer at each loudspeaker and a special 70-volt output on the amplifier. 70-volt systems are used in multiple loudspeaker installations where 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 loudspeaker, which doesn’t necessarily have anything to do with the power rating of said loudspeaker.

They also have output taps to match the impedance of the loudspeaker (4, 8, 16 ohms) being used, and this does need to be matched to the speaker for proper operation. It’s important that the total power requested (the load) does not exceed the total power available from the amplifier.

To calculate the “load,” add together the power requested by each loudspeaker transformer in the system.

For example: If you have a 100-watt amplifier and your system has two 20-watt loudspeakers and ten 5-watt loudspeakers, you have a 90-watt load. (2 x 20 watts = 40 watts; 10 x 5 watts = 50 watts; 40 watts + 50 watts = 90 watts) Therefore, you can safely add a 10-watt loudspeaker, or two more 5-watt loudspeakers.

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Dennis Redman says

In determining the watts of amplifier power required for a 70V sound system, it is necessary to consider that most line transformers are very reactive at higher and, especially, lower frequencies.  As a result, the impedance of the system can vary widely as program content changes.  (From 1 ohm at 63 hertz to 24 ohms at 1000 hertz).  This is a reason why good system design will always allow at least 25-percent reserve between the calculated speaker load and the power available from an amplifier.

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