Photo by Possessed Photography at Unsplash.
There's no question that many of today's younger church sound engineers can dial-in a great sounding ‘verb on any of five or six different console brands in a matter of seconds and then go on to seamlessly mix monitors. There's also little question that most are technically unfamiliar with: 70 V audio systems.
What is a 70V “Constant Voltage” system and what do I need to know anything about it?
Apart from pure curiosity, it's useful to know how our industry has evolved from early days of extremely low power systems, to the behemoths that are in use today.
70 V systems go way back the days were when amplifiers and loudspeakers were in the 5- to 35-watt range. The idea was to raise the amplifier's output voltage (at the amplifier) to a nominal 70V and then reduce the voltage at the loudspeaker to provide a proper level that the driver(s) can handle. This is the exactly the same idea that the power company uses to minimize wire size (wire is expensive, you know). This all worked rather well, as simple as it is, and was a true Godsend back when amplifiers could barely put out 10 to 25 watts. Loosing even a few watts in the speaker line was too much to loose.
By good fortune a side benefit was soon discovered. By equipping different loudspeakers with different transformer taps, each speaker on the common 70V line could be adjusted for level to optimally fit the location it is used in. Usually the taps fall within 3 dB of one another, and four or five taps are pretty normal in a system.
Nowadays, 70 V systems have become so good they can be used for high-fidelity reproduction. I've heard some great-sounding, modern 70V systems reproducing music just as well as loudspeakers twice to five times their purchase price. Newer models from long-term 70V players like Community Professional not only sound great, but look handsome, are easy to install. Levels can often be adjusted from a front panel access switch, which makes it very easy to re-tune levels in just seconds. The switch selects from among four transformer “taps “ to provide a very usable range of levels for balancing out a system.
Later technology from some manufacturers has replaced the transformers with autoformers. An autoformer works like a transformer but with less insertion loss and markedly less distortion.
Keep in mind that all these speakers (as few as three or four, or maybe as many as 100) are all connected to the same amplifier – providing an interesting form of level control in which loudspeaker can be manually changed individually.
This is because the loudspeakers themselves almost always have adjustable taps, sometimes even accessible from the front panel, allowing (typically) 3dB changes up or down, in 3 dB increments. For a distributed paging system, or even adjunct speakers for a larger main system, what could be better? Run a pair of wires in a loop wherever you want to place loudspeakers; wire them all in parallel; then set their attenuators to balance out the sound in the church.
So what?
Well, in your career in church tech you may very well encounter one or more of these systems. It might be a meeting in the fellowship hall where half the drivers disintegrated from age. It might be a new, high-powered, quite lovely system that is providing support for the FOH system. It might be system that provides sound to the back stage, green room or dressing room areas. The great thing about 70 V systems is you can turn any individual ‘node' or loudspeaker up or down or off, without needing to adjust the load on the other loudspeakers in the system.
In large performing arts centers 70 V is always used for such things as back-stage page, dressing room call times, and general announcements before or after a show.
Interestingly, 70V systems are making a comeback in certain applications. For a quick MNEC (Mass Notification and Emergency Communication) one or more high-power (say 500 watt) 70 V amplifiers can feed a conglomerate of high-power loudspeakers strategically located around a facility.
It's important to realize however, that a system of such type probably hasn't been engineered for uniform coverage, battery (or other) power backup considerations. Still, it's better to start with something than nothing.
What are 100V and 140V “Constant Voltage” systems?
100V and 140V “Constant Voltage” systems are essentially the same as America's standard 70V system, but run at a higher voltage to further minimize cable loss. You'll rarely see 140 V systems for general-purpose use, but they are common in large venues, fair grounds or a motor speedway perhaps, where cable distances can be miles long.
100 V is the European equivalent to our standard 70 V. It functions identically, but with slightly lower cable loss.
And what does “Constant Voltage” Mean?
That one is confusing, as you would expect some kind of limiter to keep the amp from trying to put out more than 70V. That's not really what's happening. The term “constant voltage” was the concept of how a 70 V system might work. But we all know that music and speech is dynamic, so there's no point in trying to keep the max output at a level of 70V. It's more likely to be all over the place, but centering around the 70-V output marker.
So now when you're confronted with an ancient remnant in your church that needs sorting out, or you're part of a new high-end 70V installation (and there are plenty of very impressive 70 V installations these days), you'll know something about what is being referred to and perhaps how to trouble shoot any problems.
# # #
