70V speakers in danger?


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Old 01-25-08, 12:43 PM
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70V speakers in danger?

In our church meeting hall, we have 14 JBL 26CT ceiling speakers paralled on a 70V distribution line.

Each speaker's level switch is set at 7.5W. The system is driven by a TOA power amp with 70V output.

When the volume is cranked up, the speakers get REALLY loud. I fear for the voice coils, especially the coax tweeters.

My question: Will the 7.5W switch setting strictly limit the power going to each speaker? Am I being overly concerned?

Thanks.

BTW The speakers sound great. I don't want to blow 'em.
 
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Old 01-25-08, 08:48 PM
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If the speakers aren't distorting or crackling, you probably don't need to worry about them. But if you step down the speakers to a lower wattage, they won't get as loud. Of course you could just turn down the amplifier. How many watts is the amplifier?
 
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Old 01-26-08, 05:56 AM
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JBL stands for "Just Be Loud."

JBL is one of the oldest and best names in the business. They have a long-standing reputation for quality, efficiency, and reliability. In fact, a vast majority of the biggest concert performers use JBL components in those huge line-array speaker systems. Take a look at www.jblpro.com.

That's a very nice choice of speaker. Whoever installed it did you right.

7.5 watts is the lowest tap setting on those speakers. They are capable of handling 140 watts, but the maximum transformer tap on the CT models is 60 watts. They are running barely above idle at 7.5 watts. Still, a distorted signal (red "Clip" or "Overload" lights on the amp) can damage them. Avoid lighting the amp's red lights at all costs.

14 speakers each tapped at 7.5 watts will present a total load of 105 watts to the amplifier. A properly designed system should have 20% headroom, so the amp should be rated at 130 watts or more. (As I recall, TOA doesn't make a 130-watt amp. 120 is the closest, which should work fine forever as long as you don't add more speakers or push it into clip/overload.)

In a constant-voltage system, the amp doesn't "push" any power. Instead, the speakers draw their rated power. You can put a 1,000 watt amp on that system and the speakers will still draw only 7.5 watts each.
 
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Old 01-28-08, 03:04 PM
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Gentlemen, thank you for your replies and insight.
The TOA is a P924 (?) rated at 240W RMS per their specs.
I don't recall ever seeing the clip light flashing but I'll certainly watch from now on.
Rick, what you stated about the 7.5W x 14 = 105 (+20%=126W) somewhat concerns me in that we're about to augment the 26CTs with 4 (maybe 6) 19CST subs.
If they too are set to 7.5W that adds up to about 180W draw. That doesn't leave much amplifier headroom.
With regular (8 ohm, etc) outputs, I've always tried to have available at least 2x the speaker rating. But the 70V system is new to me.
Do you think we're OK adding the subs?
If it all works, this should be one B_A system. Can hardly wait to hear it.
 
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Old 01-29-08, 04:30 AM
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240 watts? They really did you right!

Three reasons I wouldn't add subs to the existing system:
(1) Transformers start to look like a direct short to the amp at low frequencies. This will tax the amp. All of our systems are high-pass filtered at 100Hz for that reason.
(2) Subs usually need to have 6dB - 10dB more power than their high-end counterparts. If you tap them at 7.5 you'll be wasting them.
(3) Both sets of speakers will receive the same signals and possibly interfere with each other at the overlap.

I'd put in an electronic crossover --which feeds highs to the 26's and lows to the subs -- and a second amp. The dbx 220i is a wonderful little speaker processor. Run the subs without transformers in a series/parallel configuration to match the amp's output impedance. Use individual 12-gauge cable runs to the head end and make your series/parallel connections there.
 
 

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