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Title: 70v speakers on 8/4/2 ohm amp
Post by: David Allred on July 27, 2015, 12:16:17 pm
"The integrated 70V ‘matching transformer’ allows multiple speakers to be
connected together without danger of overloading the power amplifier
due to impedance drop."

Is this Yorkville speaker description blurb saying that I could connect a bunch to a QSC RMX2450, PLX 3102, or other similar Crown, Peavey, or other standard amp?  Doesn't seem right.  Please advise.

David
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: John Roberts {JR} on July 27, 2015, 12:26:24 pm
"The integrated 70V ‘matching transformer’ allows multiple speakers to be
connected together without danger of overloading the power amplifier
due to impedance drop."

Is this Yorkville speaker description blurb saying that I could connect a bunch to a QSC RMX2450, PLX 3102, or other similar Crown, Peavey, or other standard amp?  Doesn't seem right.  Please advise.

David

There are many decent white papers on the WWW describing "constant voltage"  or 70V sound distribution systems so I won't repeat the whole story. Short version is that nominal amp output voltage is normalized for 70VAC which is generally high voltage swing for a modest sized power amp, while some high power amps get up there in voltage swing.

The 70V loudspeakers are likewise scaled up in impedance to pull only modest watts of power from a 70V feed.

So yes a bunch of 70V speakers could be hung off a conventional amp without hurting the amp, but they may not get very loud if the amp doesn't put out anywhere near 70V nominal voltage.

JR
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: David Allred on July 27, 2015, 12:59:44 pm

, but they may not get very loud if the amp doesn't put out anywhere near 70V nominal voltage.

JR

I have an Excel spreadsheet that converts watts to volts at a given load. 
How do I determine the load presented at the amp by the "x" number of 70v speakers connected? 
The spreadsheet also lists V RMS and V Peak.  Is V RMS the critical value for this?  As you can tell this is from a limiter calculator.

thanks
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: John Roberts {JR} on July 27, 2015, 01:09:19 pm
I have an Excel spreadsheet that converts watts to volts at a given load. 
How do I determine the load presented at the amp by the "x" number of 70v speakers connected? 
The spreadsheet also lists V RMS and V Peak.  Is V RMS the critical value for this?  As you can tell this is from a limiter calculator.

thanks
There are different flavors of 70V speakers. They will generally offer a power selection or be set for a fixed power.  So a 70V speaker that is tapped for 5W, means you can work backwards from that 5W number to determine effective impedance load at 70V to make that many watts of power. When you determine the impedance of each speaker then you need to calculate the parallel impedance.

It will be easier to add up the power from all the 70V speakers together then calculate that one effective impedance.

Or just use a 70V amp... (Peavey used to sell a step-up transformer called auto-match so you could convert a normal amp to 70V).

JR




Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: David Allred on July 27, 2015, 01:32:20 pm


It will be easier to add up the power from all the 70V speakers together then calculate that one effective impedance.



JR

So if I need to change the number of 70v speakers it alters the load to the amp.  Therefore, it changes the needed specs of the amp.  Are 70v amp so much more expensive because of demand, electronics, or something else?
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: John Roberts {JR} on July 27, 2015, 01:40:24 pm
So if I need to change the number of 70v speakers it alters the load to the amp.  Therefore, it changes the needed specs of the amp.  Are 70v amp so much more expensive because of demand, electronics, or something else?

70v amps are more expensive than what???

Fixed install amps for constant voltage systems are generally pretty inexpensive...

While the modern value MI amps are getting obscenely cheap (I am so old I remember when 1$/Watt was cheap)  8) ... compared to the cheapest MI amp the install amp with an output transformer may be a few $ more, but they are NOT expensive amps, far from it.

JR
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: Ivan Beaver on July 27, 2015, 01:54:59 pm
So if I need to change the number of 70v speakers it alters the load to the amp.  Therefore, it changes the needed specs of the amp.  Are 70v amp so much more expensive because of demand, electronics, or something else?
With any amp-the more speakers you add, the lower the impedance.

Basically think of a 70V system as a high impedance system.

A 1 watt tap is roughly equal to 5000 ohms
10 watts is 500 ohms
100 watt is 50 ohms

600 watts into 8 ohms is 70V
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: David Allred on July 27, 2015, 01:57:22 pm


 (Peavey used to sell a step-up transformer called auto-match so you could convert a normal amp to 70V).

JR

Still do.  For $125-$150, it seems like an option.
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: Mac Kerr on July 27, 2015, 02:07:08 pm
Still do.  For $125-$150, it seems like an option.
With any amp-the more speakers you add, the lower the impedance.

Basically think of a 70V system as a high impedance system.

A 1 watt tap is roughly equal to 5000 ohms
10 watts is 500 ohms
100 watt is 50 ohms

600 watts into 8 ohms is 70V

To expand on Ivan's comment, just make sure you have an amp capable of 600W into an 8Ω load. When you have close to 600W of 70V speakers on the amp you can start worrying about more amps.

Mac
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: David Allred on July 27, 2015, 02:23:37 pm
70v amps are more expensive than what???

Fixed install amps for constant voltage systems are generally pretty inexpensive...

While the modern value MI amps are getting obscenely cheap (I am so old I remember when 1$/Watt was cheap)  8) ... compared to the cheapest MI amp the install amp with an output transformer may be a few $ more, but they are NOT expensive amps, far from it.

JR

Maybe I am missing something.  QSC CX series 70v amp 400w is $1770 MAP.  QSC RMX series 400-500w (depending on preferred rating) @ 8 ohm is only $750 MAP.
Granted, as the number of speakers added increases, the 70v amp's cost per speaker goes down.  If using many low wattage ceiling speakers the 70v can work out really cheap.  If using 100w taps on larger format speakers the 70v is really expensive. 
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: John Roberts {JR} on July 27, 2015, 02:47:13 pm
Maybe I am missing something.  QSC CX series 70v amp 400w is $1770 MAP.  QSC RMX series 400-500w (depending on preferred rating) @ 8 ohm is only $750 MAP.
Granted, as the number of speakers added increases, the 70v amp's cost per speaker goes down.  If using many low wattage ceiling speakers the 70v can work out really cheap.  If using 100w taps on larger format speakers the 70v is really expensive.
400W is relatively large for a 70V amp... I think the largest install amp I recall was 300W (and mono). A 100W tap on a 70V line sounds a little loud to me. For larger than a few hundred watts we would just use a conventional amp.

FWIW the RMX series is a value (cheap) amp series.  I looked at the CX series and did not see a 400W 70V model... in fact most of the CX amps were stereo low Z, and the 70V amps had hooks for computer control (not-cheap amps).

Maybe look at basic install amps from an install manufacturer, while it appears QSC has models intended for that market. 

JR 
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: David Allred on July 27, 2015, 03:35:48 pm
400W is relatively large for a 70V amp... I think the largest install amp I recall was 300W (and mono). A 100W tap on a 70V line sounds a little loud to me. For larger than a few hundred watts we would just use a conventional amp.

FWIW the RMX series is a value (cheap) amp series.  I looked at the CX series and did not see a 400W 70V model... in fact most of the CX amps were stereo low Z, and the 70V amps had hooks for computer control (not-cheap amps).

Maybe look at basic install amps from an install manufacturer, while it appears QSC has models intended for that market. 

JR

CX602V is the amp.  I was basing the power needed on larger speakers covering larger areas in a foreground music situation.  85-95 db required
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: Luke Robinson on July 27, 2015, 04:14:20 pm
CX602V is the amp.  I was basing the power needed on larger speakers covering larger areas in a foreground music situation.  85-95 db required

Look at the CMX series from QSC it is a much more affordable 70v amp
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: Mark Wilkinson on July 27, 2015, 04:19:04 pm
To expand on Ivan's comment, just make sure you have an amp capable of 600W into an 8Ω load. When you have close to 600W of 70V speakers on the amp you can start worrying about more amps.

Mac

I recently tried a 400W into 8ohm amp, on some 70V speakers with taps that added to about 220watts.
I was wanting to see if less than 70^2 / 8 would work...
It didn't, too under powered....by more than i expected
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: Robert Healey on July 27, 2015, 04:44:49 pm
Maybe I am missing something.  QSC CX series 70v amp 400w is $1770 MAP.  QSC RMX series 400-500w (depending on preferred rating) @ 8 ohm is only $750 MAP.
Granted, as the number of speakers added increases, the 70v amp's cost per speaker goes down.  If using many low wattage ceiling speakers the 70v can work out really cheap.  If using 100w taps on larger format speakers the 70v is really expensive.

In QSC world, CX install series = PLX and PL3 portable series. CMX install series = RMX portable series. The ISA series is also cost effective 70V amps.
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: Mac Kerr on July 27, 2015, 06:10:53 pm
Maybe I am missing something.  QSC CX series 70v amp 400w is $1770 MAP.  QSC RMX series 400-500w (depending on preferred rating) @ 8 ohm is only $750 MAP.
Granted, as the number of speakers added increases, the 70v amp's cost per speaker goes down.  If using many low wattage ceiling speakers the 70v can work out really cheap.  If using 100w taps on larger format speakers the 70v is really expensive.

If you're using 100W taps you probably don't want to use a 70V system. If you want that much power to each speaker you are better off will regular 8Ω speakers. You can use any amp, and you don't have that extra transformer to cut the low end.

If you want a lot of low power speakers all paralleled together 70V is the way to go, but you don't need a special 70V amp, a 600W into 8Ω amp will be fine.

Mac
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: Stephen Kirby on July 27, 2015, 06:26:24 pm
CX602V is the amp.  I was basing the power needed on larger speakers covering larger areas in a foreground music situation.  85-95 db required
That's not a job for a 70v system.  Those are mainly used for distributed background music and paging.  If you need 95dB somewhere with music playing then you are talking about a fairly decent normal PA or the sort of thing used in loud restaurants.  That might be an application for medium sized powered speakers depending on distance from an amplifier closet.  You can also use a companion 70v system to pipe sound into halls and auxiliary rooms as required.
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: John Roberts {JR} on July 27, 2015, 06:28:05 pm
If you're using 100W taps you probably don't want to use a 70V system. If you want that much power to each speaker you are better off will regular 8Ω speakers. You can use any amp, and you don't have that extra transformer to cut the low end.

If you want a lot of low power speakers all paralleled together 70V is the way to go, but you don't need a special 70V amp, a 600W into 8Ω amp will be fine.

Mac
like button?
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: David Allred on July 28, 2015, 08:08:11 am

 but you don't need a special 70V amp, a 600W into 8Ω amp will be fine.

Mac

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?

Are there any problems (for the 70V speaker system) if the amp is driven beyond 70V?  Thereby allowing a min-max by the system controller for volume adjustments.
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: Ivan Beaver on July 28, 2015, 08:17:59 am
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?

Are there any problems (for the 70V speaker system) if the amp is driven beyond 70V?  Thereby allowing a min-max by the system controller for volume adjustments.
NO.

The voltage is not "constant" as in you will read 70V if nothing is going on.\

You can turn it up or down as needed up stream.

It is simply a typical musical signal that when the voltage is at 70V (70.7V to be exact) the loudspeaker will be delivering the rated tap wattage (give or take a bunch for impedance) to the loudspeaker.

If you drive it to more than 70V, you risk having the amp clip or saturating the transformers on the speaker end.

What really surprises me is that a system that is sooooo simple (70V) causes so much confusion.

All it is is a high impedance transfer system.

Just like the high voltage lines coming to outside your house.

If the impedance is higher-there is less loss across the lines-just like regular loudspeakers.  And since the impedance is high-you can hook up a lot of speakers by simply adding the wattages (and allowing 10% for "fluff") and have them run at different levels easily if you want by changing the taps.

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

The speaker is "your house".
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: Steve M Smith on July 28, 2015, 08:46:52 am
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.
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: Geoff Doane on July 28, 2015, 09:22:08 am
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
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: John Roberts {JR} on July 28, 2015, 11:47:32 am
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.  ;D

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.  :'(
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: Mac Kerr on July 28, 2015, 12:43:31 pm
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 (http://www.crownaudio.com/media/pdf/amps/138905-1_10-05_constant_voltage.pdf)

Mac
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: Ivan Beaver on July 28, 2015, 12:46:20 pm
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.



Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: David Allred on July 28, 2015, 01:02:45 pm
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 (http://www.crownaudio.com/media/pdf/amps/138905-1_10-05_constant_voltage.pdf)

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.
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: Mac Kerr on July 28, 2015, 03:19:42 pm
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
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: Joseph D. Macry on July 31, 2015, 10:39:37 am
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.
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: John Roberts {JR} on July 31, 2015, 10:55:19 am
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

Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: Mike Sokol on July 31, 2015, 11:33:21 am
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...
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: Steve M Smith on September 25, 2017, 03:03:22 am
Basically think of a 70V system as a high impedance system.

A much better way of thinking about it because that's what it is.

Constant voltage is a ridiculous term.


Steve.
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: Ivan Beaver on September 25, 2017, 07:10:47 am
A much better way of thinking about it because that's what it is.

Constant voltage is a ridiculous term.


Steve.
You would not believe the number of people I run into that think there is a "constant 70V" on the line.  Just like the wall voltage.

70V systems are VERY easy, but yet many try to overthink it and make it much harder than it really is.

Once you understand a few (just a couple) basic concepts, the rest is easy.
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: Lafayette Hight on August 09, 2018, 10:43:31 pm
With any amp-the more speakers you add, the lower the impedance.

Basically think of a 70V system as a high impedance system.

A 1 watt tap is roughly equal to 5000 ohms
10 watts is 500 ohms
100 watt is 50 ohms

600 watts into 8 ohms is 70V


Hi,

I'm resurrecting this dead thread, because were I to post a new one, folks (wisely) would direct me to versions of the question and answers that have been posted previously.

Things I know:
-A lot about 70v speakers and 70v amplifiers. Been installing them in commercial applications for 5 years. (Mom and pop company.)
-Everything (not bragging) about 8/4/2 speakers and their respective amplifiers. 24 years experience as a live sound engineer.

Things I don't know:
-Anything about compatibility between the two. But in reading this thread, I think I understand more. Just wanted a piece of clarification.

Background:
In my commercial installs, I always use JBL Control series speakers, tapped to the highest wattage. I use 70v because of the long distances...up to 1,000 feet of speaker cable, in some installs, like football stadiums. In this particular instance, I put two JBL Control 25AV speakers in an outdoor courtyard, set them to the 60w tap, and powered them with a amp capable of 240 watts, both at 70v and 8/4/2.

Three years later the amp blew, long after the warranty ended. I gave them a loaner 1000w 8/4/2 amp, changed the taps to 8 ohm, and told them I'd find a new replacement amp, as the manufacturer discontinued that series. The loaner worked so perfectly, that they wanted to purchase the used one at whatever price I'd sell it. (I think it's a bit of corporate fear, mixed with superstition, mixed with not messing with something that's working perfectly.) Despite the amp being about 15 years old, they didn't want me changing it, because everyone was happy it was working well.

Fast forward to 2018, now, and they asked me to install four more speakers in the same courtyard. Looking back at the original paperwork, I saw the 70v amp. But forgot about the replacement 8/4/2 amp (which is still working beautifully) until a couple of days ago, as I prep to install.

The question:
The amp does 700w into 8 ohms, in mono bridge mode. Does this mean that I'm safe to tap all six speakers at 60w, and run about 300 feet of cable (total cable run) to the remaining three speakers, and that the above rule (about 600w into 8 ohms being the same as a 70v amplifier) applies??
 
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: Wes Garland on August 10, 2018, 07:45:58 am
I'm weak on the theory behind this, but I'm pretty sure you shouldn't do that.

Why not just buy a transformer and do it right?

http://yorkville.com/installation/coliseum_amps/product/lt-70v/

$110 at Full Compass.
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: Pete Erskine on August 10, 2018, 08:27:02 am

The question:
The amp does 700w into 8 ohms, in mono bridge mode. Does this mean that I'm safe to tap all six speakers at 60w, and run about 300 feet of cable (total cable run) to the remaining three speakers, and that the above rule (about 600w into 8 ohms being the same as a 70v amplifier) applies??

NO you need to convert the 8ohm to 70V with a transformer.  Then you can do what you said.
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: John Roberts {JR} on August 10, 2018, 09:34:59 am

Hi,

I'm resurrecting this dead thread, because were I to post a new one, folks (wisely) would direct me to versions of the question and answers that have been posted previously.
Keep in mind that old threads have all the answers, not just the correct ones.  ;)
Quote
Things I know:
-A lot about 70v speakers and 70v amplifiers. Been installing them in commercial applications for 5 years. (Mom and pop company.)
-Everything (not bragging) about 8/4/2 speakers and their respective amplifiers. 24 years experience as a live sound engineer.

Things I don't know:
-Anything about compatibility between the two. But in reading this thread, I think I understand more. Just wanted a piece of clarification.
Just for a short detour before we dig into specifics, all power amps are voltage amplifiers. They take a small voltage signal at their input and boost it to a higher voltage version. 70V amps (or 100V) are normalized or designed to all put out their different rated powers at the same 70V.  OTOH conventional (non-70V amps), put out their different rated powers at different voltages into different load impedances.

One quibble, these non-70V amps typically do not deliver the same power at 8/4/2 ohm loads. They will generally make max power into 2 ohms and proportionately less at 4ohm and 8ohm terminations. Only install amps with transformer outputs can provide same power taps for 2/4/8 ohm.

Since all these amps are similar (voltage amplifiers) you can substitute a non-70V amp into a constant voltage system with some considerations.

 
Quote
Background:
In my commercial installs, I always use JBL Control series speakers, tapped to the highest wattage. I use 70v because of the long distances...up to 1,000 feet of speaker cable, in some installs, like football stadiums. In this particular instance, I put two JBL Control 25AV speakers in an outdoor courtyard, set them to the 60w tap, and powered them with a amp capable of 240 watts, both at 70v and 8/4/2.
sounds reasonable 120W load, on 70V line capable of 240W, we approve.
Quote
Three years later the amp blew, long after the warranty ended. I gave them a loaner 1000w 8/4/2 amp, changed the taps to 8 ohm, and told them I'd find a new replacement amp, as the manufacturer discontinued that series. The loaner worked so perfectly, that they wanted to purchase the used one at whatever price I'd sell it. (I think it's a bit of corporate fear, mixed with superstition, mixed with not messing with something that's working perfectly.) Despite the amp being about 15 years old, they didn't want me changing it, because everyone was happy it was working well.
Just to be clear can I ASSume the 1000W amp is not a 70V install amp? In that case it is likely to make the max 1000W at 2 ohm. That calculates out to something like 44V. Since power is E^2/R the available power drops by the square of the voltage drop so less than 1/2 power available from 70V.

Are you saying you changed the speaker taps to 8 ohm? In that case the math is simply do all the speakers in parallel still load the amp with 2 ohms or higher.
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Fast forward to 2018, now, and they asked me to install four more speakers in the same courtyard. Looking back at the original paperwork, I saw the 70v amp. But forgot about the replacement 8/4/2 amp (which is still working beautifully) until a couple of days ago, as I prep to install.

The question:
The amp does 700w into 8 ohms, in mono bridge mode.
700W @ 8 ohm is roughly 75V so right in the ballpark for 70V line.
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Does this mean that I'm safe to tap all six speakers at 60w, and run about 300 feet of cable (total cable run) to the remaining three speakers, and that the above rule (about 600w into 8 ohms being the same as a 70v amplifier) applies??
6 x 60W = 360W which seems safely less than 700W.   360W @ 70V is equivalent to a 14 ohm load so safely larger than 8 ohm amp is rated at. The only concern about driving a 70V with a bridged amplifier is both lines are hot. In the install business many installers get a little sloppy about keeping the speaker 0V return line isolated. Since that speaker 0V line is actually being driven with opposite polarity half voltage, shorting it to ground would be problematic. 

I have designed amps for that industry (last century), and it is no accident that they insist on fully floating transformer isolated 70V outputs. We could make amps for that market cheaper, lighter, and smaller using direct coupled outputs (or even autoformers) but the industry rejected them.

So yes your amp will probably work, as long as the wiring is good (speaker 0V isolated and floating from ground). It is not good install industry practice, and some future hack wiring work that inadvertently shorts speaker 0V to ground could release smoke from your amp.

JR

PS: I'm still working on my first cup of coffee so check my math.  8)
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: Lafayette Hight on August 10, 2018, 08:54:37 pm
sounds reasonable 120W load, on 70V line capable of 240W, we approve. Just to be clear can I ASSume the 1000W amp is not a 70V install amp? In that case it is likely to make the max 1000W at 2 ohm. That calculates out to something like 44V. Since power is E^2/R the available power drops by the square of the voltage drop so less than 1/2 power available from 70V.

Are you saying you changed the speaker taps to 8 ohm? In that case the math is simply do all the speakers in parallel still load the amp with 2 ohms or higher. 700W @ 8 ohm is roughly 75V so right in the ballpark for 70V line. 6 x 60W = 360W which seems safely less than 700W.   360W @ 70V is equivalent to a 14 ohm load so safely larger than 8 ohm amp is rated at. The only concern about driving a 70V with a bridged amplifier is both lines are hot. In the install business many installers get a little sloppy about keeping the speaker 0V return line isolated. Since that speaker 0V line is actually being driven with opposite polarity half voltage, shorting it to ground would be problematic. 

Hi John,

You are correct. The 1,000w amplifier is not a 70v install amplifier. It does a bridged 700w @ 8 ohms, 1,000w @ 4 ohms. Doesn't do 2 ohms in bridged mode.

Yes, the current speaker taps are at 8 ohms (the two installed speakers) and my original plan was to simply run all of the speakers in 8/4/2 mode, three on one channel, three on the other. But then the placement of the speakers changed, and I need about 200-feet of cable to get to what would be the first speaker on Channel 2.

I've never run speaker wire that far in 8/4/2 mode, so I started thinking about how I could convert the amplifier output to 70v, since my speakers have that capability.

Note: These speakers are strictly used for paging, not music. There's a wireless Sennheiser microphone connected directly to the amplifier.

To the other poster, thanks for the link to the Yorkville transformer. I was actually doing a Google search for transformers when I happened upon this forum, and discussion about the same topic.
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: Ivan Beaver on August 11, 2018, 06:13:42 pm
The short simple answer is that you will be fine.

Back in my install days, I used to use amps that would produce 600 watts into 8 ohm loads all the time for 70V speakers.

That way you avoid the output transformer issues (core saturation, low freq loss etc).

As a general rule you should subtract 10% from the output capability as the maximum taps/loads.

With the speakers you are talking about, you are fine, with room to spare
Title: Re: 70v speakers on 8/4/2 ohm amp
Post by: Lafayette Hight on August 24, 2018, 11:30:06 pm
The short simple answer is that you will be fine.


Update: Did the install last weekend, everything is working and sounds beautifully. Total cable run was about 500 feet, with four speakers spaced across that distance.

Thanks! Appreciate the advice