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All About 70-volt Speaker Systems

Posted by: Steve Lampen on August 09, 2012

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.


Blog Image Speaker Cable Distance Chart


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.

70 volt speaker transformer

Photo used by permission from 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 steve.lampen@belden.com and I will print them in a following blog.

Tags: Speaker Cable, Speaker, Distributed Speakers, 70 Volts, Speaker Transformers


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Comments

 
    Steve Lampen on October 17, 2013 at 10:40
    Jeff- Not sure if I ever replied to you. Apologies! I don't think the 4 ohms or 8 ohms has a lot to do with what you are hearing. Certainly some speaker are more efficient (louder) than others. This is something that 70-volt distributed systems are designed to fix. The transformer on every speaker usually has multiple outputs. These directly affect the amount if power (watts) driving the speaker. You can either move the quiet speakers to a louder setting (higher wattage) or move the louder speakers to a lower setting (less watts) until their level is the same in every speaker. If you want the whole system to be louder, turn up the volume at the power amplifier . If you hear distortion, it probably means that the amplifier cannot deliver the appropriate amount of watts to drive everything to the level you need. You need a more powerful amp. Does this make sense? Do some experiments and get back to me with what happened.
    Steve Lampen on October 17, 2013 at 10:38
    Brian- It is extremely unlikely that the speaker, volume control or cable could be introducing the hum. Since it gets worse with time, this sounds exactly like the capacitors in the power supply section of the amplifier. (As it warms up, the capacitors get worse - very common.) Put in a new amp or have the old one fixed. That should solve your problem.
    Steve Lampen on October 17, 2013 at 10:36
    Ash - If you are running 70-volt distributed speakers, you should have no problem running 20 ft. or 150 ft. You can find a chart of distances at 70v here http://www.belden.com/docs/upload/Speaker-Cable-Selection-Guide.pdf and this lets you choose what wire gage you can use.
    Brian W on September 08, 2013 at 01:25
    Hello, have a 70 volt system in our church foyer and have a hum that only becomes apparent after the system has been on for about 45- 1 hr. There are about 10 ceiling grills on the amp that is being used. I am not sure if the amp is causing the hum or if maybe there is a bad 70 volt speaker transformer or volumn control. Any thoughts?
    ash on August 26, 2013 at 09:05
    I need comments re an outdoor speaker system for background music and Public Address system in a church/temple. The system will have 12 outdoor (soffitt mounted) speakers.. The shortest run is 30 feet and the longest is 120 feet from the amplifier. Any suggestions will be greatly appreciated. Ash
    Steve Lampen on April 30, 2013 at 05:13
    Antonio- Can we reprint your comment in a future blog? 8 AWG and 6 AWG were originally suggest by Crown, to wire up their 5-kilowatt and 10-kilowatt power amps in large venues (stadiums etc.). Of course you can use 4x14 AWG to get to the same 8 AWG. (Every time you add two identical conductors, the resultant gage is 3 below what you started. So 2 x 14 AWG = 11 AWG and 2 x 11 AWG = 8 AWG.) The one thing that is missing however is a safety rating. Power amplifiers over 1 kW can be lethal if you touch the speaker terminals with audio present. Most of those amps should have a notice over the speaker terminals that says "Class 1 Wiring" or something similar. This is a UL safety rating. We have an entire line of Class 1 speaker cables: 14 AWG, 12 AWG, 10 AWG, 8 AWG and 6 AWG. The part numbers are "88" followed by the gage size (i.e. 8814 to 8806). I'll bet that your 4x14 AWG cable is not Class 1. Most traditional Class 1 cables are like house wiring, solid conductors, and very stiff. They don;t lend themselves very easily to speaker wiring. Hope that answers your question.
    Antonio on April 29, 2013 at 02:31
    Hi Steve What would be the case that according to the distance and power requirements in a 70Volt system I need to run a 8 AWG, but since comercially is not very available I use 4x14 insted. Since is the same copper area apparently it should work, but is there any inconvenience regarding other factors, change of impedance, distortion, etc?
    Steve Lampen on April 15, 2013 at 03:21
    Dave- What a great question! I sent an inquiry to one of my "guru's", Bill Ruck: Bill- I remember reading somewhere that you can directly drive a 70v speaker system by using a high-power amplifier. I assume the voltage output is somewhere around 70 volts. If P=E sqd/R then the amp needs to be above 600 watts. Correct? Do you know anything about this? I had an inquiry from a customer. Here is his response: Yes, your math is correct. At lower power a step-up transformer is commonly used on the output of the amplifier. Those are generally rated in "Watts" rather than impedance. But if you took the nominal 8 Ohms up to 100 Ohms then 70 Volt line would be only 50 Watts. Speaker systems are not constant impedance. They're designed in Watts for convenience. Say you want 12 speakers at 5 Watts each. You select a "70 volt" transformer that matches the rated speaker impedance and use the "5W tap". That is a load of 60 Watts so you find an amplifier that has at least that rated power with an output transformer for 70 volt line or find an amplifier that has at least that rated power and use a 70 volt step-up transformer with the "60 Watt" tap. If I remember correctly "70 volts" is an arbitrary number that just falls under the NEC high voltage requirements.\ The higher the voltage and the higher the system impedance the lower loss in copper. Same thing for the power company. -Bill So, Dave, the answer is YES. Your 900-watt amp would be more than adequate, assuming the total wattage of the speakers you're driving is no more than around 900 watts. - Steve
    Dave on April 11, 2013 at 02:10
    Is there a way to temporarily use a 900 watt amp to drive a 70 volt speaker system?
    Jeff on February 18, 2013 at 04:09
    We ran into an installation where a 70 v system was installed at 4ohms conversion along with some earlier installed 8 ohm cabinets (non-70 v system). Result is the older 8 ohm/45W cabs are a lot softer than the new 70 v ceiling system but the clinet wants these older cabinets to be as loud or louder to fill the area they were designed for where no other cabinets exist. the cabinets TOA - F-10W -no transformers added but they are tied into the whole 70 volt system and not easy to access. What it s the best way to remedy this? change out the cabinets and/or add transformers if possible?
 

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