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Author Topic: 70v speakers on 8/4/2 ohm amp  (Read 13782 times)

Steve M Smith

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Re: 70v speakers on 8/4/2 ohm amp
« Reply #30 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.
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Ivan Beaver

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Re: 70v speakers on 8/4/2 ohm amp
« Reply #31 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.
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Lafayette Hight

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Re: 70v speakers on 8/4/2 ohm amp
« Reply #32 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??
 
« Last Edit: August 09, 2018, 10:51:41 pm by Lafayette Hight »
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Wes Garland

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Re: 70v speakers on 8/4/2 ohm amp
« Reply #33 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.
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Pete Erskine

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Re: 70v speakers on 8/4/2 ohm amp
« Reply #34 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.
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John Roberts {JR}

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Re: 70v speakers on 8/4/2 ohm amp
« Reply #35 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.
Quote
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.
Quote
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)
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Lafayette Hight

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Re: 70v speakers on 8/4/2 ohm amp
« Reply #36 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.
« Last Edit: August 10, 2018, 08:56:42 pm by Lafayette Hight »
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Ivan Beaver

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Re: 70v speakers on 8/4/2 ohm amp
« Reply #37 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
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Lafayette Hight

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Re: 70v speakers on 8/4/2 ohm amp
« Reply #38 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
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Re: 70v speakers on 8/4/2 ohm amp
« Reply #38 on: August 24, 2018, 11:30:06 pm »


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