Q: I help out with the audio at church, and thought I’m not overly knowledgable, I try hard.
So, when our choir director asked me to add a speaker for the choir loft, I thought it sounded a little challenging, but not overly difficulty.
Well, 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, 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 and just get you 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 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 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.
They also 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: If 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.”
As always, we welcome input from the PSW community and would love to know your thoughts on this situation. Feel free to let us know in the comments below.
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