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Question:

How many speakers on a 70v transformer?

Hello. I am setting up a 70 volt sound system. It is primarily outdoors, so I have weatherproof horn speakers. I have a 120 watt 70v amp. I also have a 70v 15 watt step down transformer. Now, I want to run 2 horns running at 15 watts rms at 8 ohms on one 15 watt step down transformer. I was wondering, is this safe? I know it works, because I have tried it. But, can it run long term and can this hurt the amp?

Answer:

I don't think the person doing the first answer understands the open voltage PA type system that you are describing (and I just barely do) but the intention is that you basically can plug in the correctly equipped speakers just like we plug in appliances on a 120 volt line. I don't think what you are describing with the step down transformers works quite the way you say either unless you want lower volume in a specific area.
You should run 8 each 15 watt speakers on a 120 watt amplifier, but the system is reasonably flexible, and probably self protected, so you can connect several speakers to one of the 8 transformers. The volume on these speakers will be much reduced, but negligible effect on the other 7 transformers. Possibly the transformer with multiple speakers will over heat, but that is unlikely for ordinary music and speech. I'll guess 5 transformers with two speakers each will be ok, and all ten speakers will be the same volume. You can also connect 9 speakers in series to the 70 volt output, without using any transformers, but I understand this lowers the sound quality a little, and means no sound if there is an open circuit anywhere.
Remember that the 15 watt tap on the step down transformer is a 15 load if and only if the secondary is connected to 8 Ohms. If you put 3 8 Ohm speakers on one transformer then the 15 Watt tap becomes a 45 watt load. Also be sure you are not using 70V unless the speaker wiring is at least 200 feet. Any transformer looses about 1.5dB so between the output transformer and the speaker transformer, you are loosing 3dB ie half the audio power. Transformers are also a serious handicap to frequency response and distortion. A cheap transformer is a short to low frequencies. 70V speaker systems are a logistic solution in buildings where there are dozens of speakers that need to be individually adjusted but should never be used to service a single room or space.

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