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Author Topic: 70v and conventional speaker system voltages...  (Read 9232 times)

ryancousins

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70v and conventional speaker system voltages...
« on: April 18, 2014, 01:09:33 pm »

I'm trying to understand two different but related things.

After reading about 70v speaker systems, the explanations make it sound as though there is something quite different about this setup that allows for a nearly endless number of speakers to be driven by an amplifier without loading it down too much. However, it seems to me that the only difference is the transformers that step up the voltage over the transmission line and back down at the speakers to reduce energy loss over long distances. Why wouldn't such a system be hindered by the same loading effects of multiple speakers? A paper on the Crown website says many 70v systems operate at 50 ohms. Obviously higher impedance than 8 ohms, but not that much. Clearly I'm missing something here.

This also makes me wonder what the average voltage of a conventional speaker system. I've read things that say large PA systems could have 100volts peak to peak at full power. This seems strange for two reasons- if that were the case, then why are 70v systems talked about as such a much higher voltage than conventional systems, and we'd have to be a lot more careful when handling speaker-level cables to avoid shock.
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John Roberts {JR}

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Re: 70v and conventional speaker system voltages...
« Reply #1 on: April 18, 2014, 01:28:43 pm »

I'm trying to understand two different but related things.

After reading about 70v speaker systems, the explanations make it sound as though there is something quite different about this setup that allows for a nearly endless number of speakers to be driven by an amplifier without loading it down too much.
Yes, but not endless. Multiple speakers can be hung in parallel as long as the "total" power they are pulling added together does not exceed the nominal power output capability of the 70/100V amp.
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However, it seems to me that the only difference is the transformers that step up the voltage over the transmission line and back down at the speakers to reduce energy loss over long distances.
Yes, that is the secret sauce for low wire losses over longer distances.
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Why wouldn't such a system be hindered by the same loading effects of multiple speakers?
It is... but Ok as long as the total load does not exceed the amp capability (plus a little fudge factor for losses in wire and magnetics).
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A paper on the Crown website says many 70v systems operate at 50 ohms. Obviously higher impedance than 8 ohms, but not that much. Clearly I'm missing something here.
That 50 ohms sounds arbitrary... I've seen 5W 70V systems that would be fully loaded by several hundred ohms.
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This also makes me wonder what the average voltage of a conventional speaker system. I've read things that say large PA systems could have 100volts peak to peak at full power. This seems strange for two reasons- if that were the case, then why are 70v systems talked about as such a much higher voltage than conventional systems, and we'd have to be a lot more careful when handling speaker-level cables to avoid shock.
The average voltage of conventional amps is a simple function of their rated output power into 8 ohms or whatever impedance they specify the power at.

There should be multiple white papers on the WWW about constant voltage systems, maybe read a few more.

JR
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Steve M Smith

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Re: 70v and conventional speaker system voltages...
« Reply #2 on: April 18, 2014, 01:29:52 pm »

A 70v (more likely 100v in the UK) speaker is just a speaker with a higher impedance.  Taps on the transformer allow its impedance to be adjusted to vary the output level.

It's easy to work out the voltage for a 'conventional' output stage.  Just use rearrange the formula Power = Voltage squared / Resistance.

Rearranged, Voltage = the square root of (Power x Resistance) so for example, 8 ohms at 100 watts = sq. root of 800 = 28.28 volts.

4 ohms at 1000 watts = 63.25 volts.
8 ohms at 1000 watts = 89.44 volts... etc.

The Speakon connector was developed with safety in mind so that you cannot come into contact with the terminals like you can with 1/4" jack plugs.  I don't know what the rules are in the US but in the UK, the maximum voltage a human is allowed to come into contact with is 48 volts.  A high power amplifier is capable of exceeding this.

 
Steve.
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ryancousins

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Re: 70v and conventional speaker system voltages...
« Reply #3 on: April 18, 2014, 01:33:42 pm »

So if a standard audio system can have 80 volts, then why do people talk about 70v systems being such high voltage?
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John Roberts {JR}

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Re: 70v and conventional speaker system voltages...
« Reply #4 on: April 18, 2014, 01:47:54 pm »

So if a standard audio system can have 80 volts, then why do people talk about 70v systems being such high voltage?

70/100V systems are called "constant voltage" not "high voltage" . They are high voltage relative to the modest power per speaker drop.

Also keep in mind constant voltage systems have been around a lot longer than modern high power amplifiers.

JR
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Mac Kerr

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Re: 70v and conventional speaker system voltages...
« Reply #5 on: April 18, 2014, 02:36:01 pm »

So if a standard audio system can have 80 volts, then why do people talk about 70v systems being such high voltage?
70/100V systems are called "constant voltage" not "high voltage" . They are high voltage relative to the modest power per speaker drop.

Also keep in mind constant voltage systems have been around a lot longer than modern high power amplifiers.

JR

They are not necessarily higher voltage than a high powered amplifier, and they are also not "constant voltage" although they are called that often. Like any audio signal the voltage varies with signal level. The transformers on each speaker raise the impedance of the speaker so that many of them can be run in parallel on a single amp. That amp often has a transformer output, although there are also many modern high power amps capable of driving the higher voltages of a 70V system without an output transformer.

The impedance of the transformer on each speaker determines the power delivered to that speaker. Commonly the transformers have multiple outputs so that you can use 10W, 5W, 2W, 1W, or .5W depending on how much output you need from any given speaker in the system. The transformers are labeled in watts instead of ohms to make it simple to add up the total load on the amplifier to be sure it does not exceed the rated output.

Mac
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John Roberts {JR}

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Re: 70v and conventional speaker system voltages...
« Reply #6 on: April 18, 2014, 02:44:27 pm »

The constant voltage refers to how the systems are engineered. Instead of different power level amplifiers making different output voltages those output voltages are all normalized to 70/100V and just put out more or less current/power. So they are constant voltage only in the context of how rated power is specified.

Constant voltage is easier to grasp than "normalized voltage" or something more accurate.

Despite the apparent confusion for people more accustomed to constant speaker impedance, constant voltage systems are relatively simple to engineer and apply.

JR
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Ivan Beaver

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Re: 70v and conventional speaker system voltages...
« Reply #7 on: April 18, 2014, 03:09:51 pm »

So if a standard audio system can have 80 volts, then why do people talk about 70v systems being such high voltage?
The 70V comes from a number of places.

First of all 70V is the highest voltage that is still considered "low voltage" so a low voltage contractor can install them-legally.

A 100V system would require an electrician.

A 70V system DOES NOT have 70V on it-as many people think-since it is considered a "contstant voltage" system.

All it means is that if the loudspeaker is tapped at 10 watts, when 70V is applied to the input, then the loudspeaker will be dissipating 10 watts (more or less-but we won't go into the different impedance at different freq issue).

If only 35 volts is on the line, then the power would be 2.5 watts and so forth.

Or thinking of it another way an easy way to remember is this.  Not exactly accurate-but close enough for guesstimation. 

A 1 watt tap (on 70V) has an impedance of 5000 ohms.  A 10 watt tap is 500 ohms and a 100 watt tap is 50 ohms.

You should never load up a 70V amplifier to more than 90% of the capability-as a general rule.

600 watts at 8 ohms is 70V.  So you can use a 600 watt @ 8ohm amp without a transformer to drive the line. 
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Kevin Graf

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Re: 70v and conventional speaker system voltages...
« Reply #8 on: April 19, 2014, 09:47:45 am »

RaneNote

"Constant-Voltage Audio Distribution Systems"

Dennis Bohn, Rane Corporation
RaneNote 136 written 1997; last revised 3/07

    25, 70.7 & 100 Volts
    U.S. Standards
    Just What is "Constant" Anyway?
    Voltage Variations -- Make Up Your Mind
    Calculating Losses -- Chasing Your Tail

http://www.rane.com/note136.html
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Hal Bissinger/COMSYSTEC

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Re: 70v and conventional speaker system voltages...
« Reply #9 on: April 20, 2014, 11:22:32 pm »

I'm trying to understand two different but related things.

After reading about 70v speaker systems, the explanations make it sound as though there is something quite different about this setup that allows for a nearly endless number of speakers to be driven by an amplifier without loading it down too much. However, it seems to me that the only difference is the transformers that step up the voltage over the transmission line and back down at the speakers to reduce energy loss over long distances. Why wouldn't such a system be hindered by the same loading effects of multiple speakers?

I think your mistake is thinking in terms of voltage rather than impedance.

 
The constant voltage system was invented to make calculations for an amp supplying multiple speakers easy. Before the constant voltage system, if you wanted to supply say 20 speakers you would have an amp with the usual 8 ohm output. You would hang an 8 ohm to 50 ohm transformer off that 8 ohm output. The 50 ohm winding provides the “line” that runs to each speaker.

To make things easy, let’s also assume you wanted to drive those 20 speakers with an equal amount of power. On each speaker you would install a transformer with a 1000 ohm primary connected to the "line" and an 8 ohm secondary connected to the voice coil. In this simple example the math is simple- 1000 divided by 20=50. You have matched 20 speakers to the amp output each receiving 1/20 of the amps power. Now, try to do the math for a more complex installation with more speakers and different amounts of power to each speaker and you can see the problem.
 
 The constant voltage system works exactly this way but with the math already done for you by applying standards. Amp outputs have different impedances depending on the power of the amp. For a 70.7 volt system this is nominally 5000 ohms per watt, with the amp will delivering 70.7 volts across the output at full rated power.
 
 Then you have the line transformers for the speakers. Instead of a single winding you have a tapped winding on the primary so that you have a selection of impedances by which to adjust the wattage of the speaker. And instead of marking the taps with their impedance they are marked by wattage. The 5000 ohm/watt standard comes into play. A 5000 ohm tap is marked as 1 watt, a 500 ohm tap is marked as 10 watts, a 10,000 ohm tap is marked as 1/2 watt, etc.
 
 So to design a system all you have to do is add up all the wattages of the taps you are using on the speaker transformers and choose an amp with at least that amount of power. As was mentioned, it's general practice to provide an amp with at least 15-20% more power than needed. No complex math needed.
 
 -Hal
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ryancousins

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70v and conventional speaker system voltages...
« Reply #10 on: April 21, 2014, 02:13:17 pm »

That helps clear things up. I just assumed the step up transformer after the amp and the step down transformer before the speaker were the same ratio, just in reverse.

When I learned about transformers in my electrical theory class, I was taught that the coupling between the two coils is nearly 100% efficient, regardless of the turns ratio, so I never thought about a transformer presenting or changing an impedance.


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John Roberts {JR}

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Re: 70v and conventional speaker system voltages...
« Reply #11 on: April 21, 2014, 02:19:11 pm »

That helps clear things up. I just assumed the step up transformer after the amp and the step down transformer before the speaker were the same ratio, just in reverse.

When I learned about transformers in my electrical theory class, I was taught that the coupling between the two coils is nearly 100% efficient, regardless of the turns ratio, so I never thought about a transformer presenting or changing an impedance.


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In fact the impedance transform is the transformer turns ratio squared. So if the voltage is altered by a factor of 2x the impedance shifts 2^2 or 4x. While there are resistive losses in windings, yes generally power transfer is 1x. So the impedance transform can be derived from Ohms law. As the voltage goes up the current must fall for the power to still equal unity.   

JR
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Steve M Smith

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Re: 70v and conventional speaker system voltages...
« Reply #12 on: April 21, 2014, 02:24:37 pm »

The turns ratio of a transformer is equal to the square root of the source impedance divided by the load impedance,

So a transformer to match an 8 ohm speaker to a 500 ohm amplifier output stage would be the square root of 500/8 or about 7.9:1

Transformers are most useful in valve (tube) amplifiers to convert their high voltage, low current (high impedance) output stage to a low voltage, high current (low impedance) to drive a speaker.


Steve.
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Richard Turner

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Re: 70v and conventional speaker system voltages...
« Reply #13 on: April 22, 2014, 12:03:10 pm »

old thread here , some of the links still work

http://forums.prosoundweb.com/index.php?topic=566.0

home page article here

http://www.prosoundweb.com/article/understanding/

Some amplifiers output at 70v or there abouts when loaded with transfortmer speakers, yorkville AP4040 is somewhere about 75v QSC pl340 was about 85 volt, theres an equation somewhere to do the math.

The key is to use good quality transormers or fidelity will suffer, that isnt really an issue if its just a paging system, human voice only but for a background music system it will.
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Re: 70v and conventional speaker system voltages...
« Reply #13 on: April 22, 2014, 12:03:10 pm »


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