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[Steve M Smith]

There is a transformer at your house that steps it down for the house to use.

You have a transformer per house?  Over here we have big transformers which each serve two or three streets.
 
I agree with you about the confusion 70v (100v here) causes.  It would be much easier just to work out or know the impedance of the speaker with the transformer.


Steve.

[Geoff Doane]

I agree with you about the confusion 70v (100v here) causes.  It would be much easier just to work out or know the impedance of the speaker with the transformer.

I agree that it is confusing, but that's because we have been conditioned in the MI world to think about watts, impedance and then perhaps voltage, in that order (many people never get past watts!).

"Constant voltage" systems, OTOH, are usually installed by electricians.  They can design them and hook them up in much the same way they hook up strings of light bulbs.  If it is determined that 50 ceiling speakers, each tapped at 5 watts, are required for an area, the designer can easily figure that he needs an amplifier of at least 250W, so probably would spec the next size up.  If the ceiling is lower in some places, and only 2W is required in those speakers, they can be re-tapped, without changing anything else in the system, the amplifier just doesn't have to work quite as hard.

Compare that to a lighting system of 20 100W light bulbs.  A 2000W power source is required to run it (20A, 120V circuit, running at 80%).  If some areas are too bright, just swap in 50W bulbs for the 100s, and nothing else has to change, you just use less power.

To take the analogy a bit further, assume that instead of a 20A circuit powering the string of light bulbs, you have a 20A (2400W) dimmer.  You can bring the voltage up and down (same as an audio signal, just not as fast), and all the light bulbs will follow that voltage, the same as all the speakers in the 70V system follow the audio input.

GTD

[John Roberts {JR}]

You have a transformer per house?  Over here we have big transformers which each serve two or three streets.
 
I agree with you about the confusion 70v (100v here) causes.  It would be much easier just to work out or know the impedance of the speaker with the transformer.


Steve.

Actually 70/100v systems are far easier to work with, if you learn to ignore needless details like speaker impedance. You start with an X watt constant voltage (not really constant) amp, and hang as many Y watt speakers on the output as long as the speaker watts don't exceed the amp watts.  It doesn't get much simpler than that. 

Constant voltage systems are useful for sending modest amounts of power, over long distances to multiple destinations with reduced wiring losses. Perhaps not the OP's application.

I repeat there are many white papers around the WWW describing how these work.

JR   

PS I used to have my own mains power transformer, but since hurricane Katrina took down my power pole, I've been sharing one transformer with 2 other neighbors. 

[Mac Kerr]

But the amp has to push 70V for the system to be "stable".  In other word, the volume HAS to be controlled post amp?  Not by a mixer or by the amp?

What are you talking about? What is "stable"? A 70V amp is an amp just like any other, but at full output the signal voltage is 70V. Turn it down and you get less signal, less volume. A 100W 70V amp can only be loaded down to a minimum of 50Ω, a 600W to 8Ω.

READ THIS

Mac

[Ivan Beaver]

You have a transformer per house?  Over here we have big transformers which each serve two or three streets.
 
I agree with you about the confusion 70v (100v here) causes.  It would be much easier just to work out or know the impedance of the speaker with the transformer.


Steve.

In our neighborhood it is 1 transformer for 2 houses.

It is much easier to simply add numbers (wattage) than to go through more complex calculations-especially if you have different impedance loudspeakers.

Once you get to 3 speakers of different impedance-it gets harder for the average person to figure it out-especially without a calculator.

That is the reason simple addition for different wattage taps works.

[David Allred]

What are you talking about? What is "stable"? A 70V amp is an amp just like any other, but at full output the signal voltage is 70V. Turn it down and you get less signal, less volume. A 100W 70V amp can only be loaded down to a minimum of 50Ω, a 600W to 8Ω.

READ THIS

Mac

Someone above had said that the amp needed to capable of a least 70 volts output.  I interpreted that as the speakers can't draw their tap power unless 70v are applied.

[Mac Kerr]

Someone above had said that the amp needed to capable of a least 70 volts output.  I interpreted that as the speakers can't draw their tap power unless 70v are applied.

What does that mean? No speaker, whether 70V or 8Ω draws any specified amount of power. They are driven to various volume levels by an amplified audio signal which varies in voltage as the input signal varies in level. Generally speakers are rated as having some specified maximum power handling capability, but they don't "draw" that power.

With a 70V distributed system you set the maximum power of the speaker via the transformer tap at the speaker. That maximum will only be possible if the amp is driven to its full output of 70V by the necessary input signal level.

If you have speakers you want driven to high levels you are better off not using a 70V system and sticking to low impedance speakers (8Ω) and regular power amps. Generally the transformers that are needed to change the speaker to a high impedance load are not super high fidelity, and may saturate at high levels, or with lots of bass. For distributed low level BG music and voice paging 70V may be the way to go, depending on the circumstances. If you have a 70V system design where you need significant power using a 600W/8Ω amplifier will behave exactly the same as a 600W "70V" amp. They both will be capable of a maximum output of 600W (70V) into a 70V system. When driven to lower levels they will both deliver less than 70V and the audio will be less than the full maximum output of the system.

I don't know how to say it any more clearly than that. Re-read the article I linked to in my last post.

Mac

[Joseph D. Macry]

One point that nobody has made here regarding 70V systems:
They generally don't do bass freqs well, because the transformers get quickly saturated. (There are some 70V subs that have specially designed transformers to handle bass. You'll see them in the ceilings of sports bars.) Most 70V systems want a high pass filter on the source.

[John Roberts {JR}]

One point that nobody has made here regarding 70V systems:
They generally don't do bass freqs well, because the transformers get quickly saturated. (There are some 70V subs that have specially designed transformers to handle bass. You'll see them in the ceilings of sports bars.) Most 70V systems want a high pass filter on the source.

Indeed it is unclear why the OP is considering constant voltage systems at all.

Not only do the transformers in the subs need to be oversized for extended bass response, the amplifiers likewise need to be transformer-less  (capable of 70V direct drive) or special amps. Install amps routinely roll off low bass response.

JR

[Mike Sokol]

I generally just use a bridged power amp rated for 200 watts per channel at 8 ohms. A little quick ciphering shows that 40 volts squared = 1,600. And 1,600 divided by 8 ohms = 200 watts. As in -  Voltage squared divided by impedance equals wattage. So in bridge mode a 200-watt/8-ohm amplifier will drive around 80 volts RMS or so. Because it's bridged and the voltage add together, this size amplifier will send around 600 watts into an 8-ohm load, so all is well. (Each side of the amp "sees" a 4 ohm load due to push-pull output)

So just roll off everything below 100 Hz to limit bass saturation of the speaker transformers, and don't hook up more than 600 watts worth of 70-volt tapped speakers. Easy...

[ProSoundWeb Community]