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Author Topic: 70-volt speakers  (Read 4762 times)

Stephen Beatty

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Re: 70-volt speakers
« Reply #10 on: February 27, 2019, 10:24:50 AM »

We have always used 70V at the University. If you suspect there is a speaker that does not have a transformer in the line and is causing issues then just plug the speaker feed into a 120V outlet for a minute. That will tell you were the offended unit was.

I got that from that great tome Sound System Engineering by Don & Carolyn Davis.
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Craig Hauber

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Re: 70-volt speakers
« Reply #11 on: February 27, 2019, 10:27:04 AM »

IIRC school installations used 35V tap.

JR
I see 25V taps on older equipment all the time when out on service calls, never seen a 35V?
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John Roberts {JR}

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Re: 70-volt speakers
« Reply #12 on: February 27, 2019, 10:42:46 AM »

I see 25V taps on older equipment all the time when out on service calls, never seen a 35V?
I've been working hard to forget stuff like that,,,  yes, my bad 25V.

JR
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Mike Sokol

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Re: 70-volt speakers
« Reply #13 on: February 28, 2019, 06:54:49 AM »

I've been working hard to forget stuff like that,,,  yes, my bad 25V.

JR

Like I tried to forget RS232 data control of modern Video projectors? RXD and TXD and DTR, oh my... 

Rob Spence

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Re: 70-volt speakers
« Reply #14 on: February 28, 2019, 12:18:20 PM »

Like I tried to forget RS232 data control of modern Video projectors? RXD and TXD and DTR, oh my...

McNamara. Want a copy?



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Mike Sokol

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Re: 70-volt speakers
« Reply #15 on: March 01, 2019, 12:19:42 AM »

Joseph D. Macry

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Re: 70-volt speakers
« Reply #16 on: March 01, 2019, 09:53:00 AM »

IIRC school installations used 35V tap.

JR

Nearly all the school installations I've done and seen (for modern, school-wide paging/intercom in Texas) are nominal 25V systems. In particular, I mean Telecor and Bogen systems. Local systems (those confined to one or two rooms) are either 70V or low-Z (4 to 16 ohms).
P.S. The word "tap" in this sense refers to the wattage setting on the speaker transformer.
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John Roberts {JR}

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Re: 70-volt speakers
« Reply #17 on: March 01, 2019, 10:12:30 AM »

Nearly all the school installations I've done and seen (for modern, school-wide paging/intercom in Texas) are nominal 25V systems. In particular, I mean Telecor and Bogen systems. Local systems (those confined to one or two rooms) are either 70V or low-Z (4 to 16 ohms).
P.S. The word "tap" in this sense refers to the wattage setting on the speaker transformer.
I went back and corrected my erroneous post so you sweethearts can stop quoting my mistake.  :o

By tap I meant which winding of the output transformer the voltage was taken from. Install amps routinely had a series of different output voltage "taps", ranging from direct low Z up to as high as 70V (domestic) or 100V (international). For school system installs they used the lower voltage tap, no doubt as a safety concern, while still realizing some elevated voltage benefit.

Since schools still used 120V lighting maybe they should have re-evaluated that decision, to reduce cost from specialty components.  ::) 

JR
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Dave Garoutte

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Re: 70-volt speakers
« Reply #18 on: March 01, 2019, 12:18:14 PM »

I went back and corrected my erroneous post so you sweethearts can stop quoting my mistake.  :o
JR

But I read it on the internet, so it must be true! ::)
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Mike Sokol

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Re: 70-volt speakers
« Reply #19 on: March 02, 2019, 08:30:06 AM »

I went back and corrected my erroneous post so you sweethearts can stop quoting my mistake.  :o

Now I have to correct my initial thread header to read 70.7 volts and NOT 70 volts. I wonder in any of you know why that seemingly arbitrarily chosen number was derived. Here's the reason which I remember reading about perhaps 20 years ago.

I'm not sure of the exact code section, but apparently there's a 100-volt maximum limit on this class of wiring (Class 3, I think) in this type of installation. But that's a 100 volt "peak" rating, not RMS. Of course, you need to multiply the RMS voltage by the square root of 2 to get the peak volts. Since the square root of 2 (Root Mean Square for RMS) is 1.414213562373095, and 70.7 volts times 1.414213562373095 equals 99.984898597778 volts, this is just UNDER the 100-volt maximum peak.

I'm sorry, but that's all the decimal places on my iPhone app or I would be more exact.  :o

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Re: 70-volt speakers
« Reply #19 on: March 02, 2019, 08:30:06 AM »


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