All About 70-volt Speaker Systems
Almost everyone has hooked up a speaker to an amplifier. Two wires from each speaker to two terminals on your amplifier or receiver. If this was in your home, you probably didn't think about how far this cable would go. Likewise, you probably didn't think about how much power you were losing on the cable. The table below is from the Belden website and shows how far you can go before you get to a specific loss.
In this case, we show three values of loss: 11% (-0.5 dB), 21% (-1 dB) and 50% (-3 dB) for 4 and 8 ohm speaker impedance. But you'll notice a third section there, titled 70V (volt) speaker. You will also notice the astounding distances that 70-volt distributed systems can go.
So how do you use this kind of system? First, you need to buy an amplifier that has a 70-volt output. These are commercial-grade products so you won't find one at a hi-fi or home theater store. The second thing is that you will need to buy a special transformer for every speaker running off this system. The picture below was provided by Parts Express.
The real problem is that all your audio is going to run through these transformers, so the quality of the audio is directly related to the quality of these transformers. If you try and buy a high-quality, high-power 70-volt transformer, so if you can find one you are going to spend a bunch of cash.
So the 70-volt distributed speaker systems are mostly confined to low audio-quality applications such as background music or paging. Supermarkets, hotels, hospitals, airports are where you will find 70-volt distributed loudspeaker systems; anywhere you have more than a dozen speakers. While the power amp might be a few hundred watts, most speakers have a transformer that is designed for maybe 5 or 10 watts.
Even with all these drawbacks, there are things that 70-volt systems can do that regular systems cannot. One, as you can see from the chart above, is distance. You need a speaker out in the parking lot a mile away? No problem! In fact, within a building, the gauge of the wire almost doesn't matter at all.
Note that 24 AWG wire, like some bell wire or spare Category 5, is only 1 dB down at 900 feet!! The cost of the wire in such an installation becomes incidental. Another unique feature is that the transformer on each speaker has a number of taps on the output. It might be a 10-watt transformer but it will allow you to hook up that particular speaker at a number of lower power levels. This means you can decide as you're installing, or even afterwards, if a particular speaker is too loud or not loud enough.
I put a 70-volt system in a radio station. The level in the reception lobby was a gentle low volume. In the bathroom, I put that transformer/speaker at full throttle, so the DJ could hear his cut running out! And once you have set up the entire system, with soft and loud speakers, the master volume control on the power amp takes the entire system up or down with the softest speakers still being the softest, and the loudest still the loudest.
There are a number of other tricks you can do with 70-volt systems. Do you know some of them? Send them to me at firstname.lastname@example.org and I will print them in a following blog.