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Author Topic: 70-volt wiring  (Read 8440 times)

John Roberts {JR}

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Re: 70-volt wiring
« Reply #20 on: July 23, 2006, 10:26:08 am »

Rick Johnston wrote on Sun, 23 July 2006 09:02

John Roberts  {JR} wrote]And let's not forget I'm talking about refusal to use 70V auto-formers instead of 70V transformers. The primary difference being isolation from ground or amplifier common. Isolation does provide the benefit of tolerating inadvertent grounding of one of either + or - speaker line. Whether this benefits the customer or installer is a judgement call. /quote


Is there an electrical advantage to using an autoformer instead of a transformer? Both cost about the same AFAIK.


Regards,
Rick Johnston



Yes.... more efficient (less loss), better freq response, smaller, lighter, less expensive.

JR
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Don Boone

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Re: 70-volt wiring
« Reply #21 on: July 23, 2006, 02:50:52 pm »

Rick you might be right about the 100v thing unless telephones
were exempt. Talk battery is 48 volts and ring voltage is 90 volts.
But I heard the 36 volt thing sometime during my education at the
Burn & Learn School of Electronics.

Don
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Rick Johnston

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Re: 70-volt wiring
« Reply #22 on: July 23, 2006, 05:37:13 pm »

Don Boone wrote

Rick you might be right about the 100v thing unless telephones were exempt. Talk battery is 48 volts and ring voltage is 90 volts. But I heard the 36 volt thing sometime during my education at the Burn & Learn School of Electronics.


Telco ring voltage maxes at 100 volts @ 20Hz AC on some systems, and I've never heard of a code that says telco lines have to run in conduit. The current is so low that it won't cause a 24-gauge wire to burn, and there are protections at the CO or switch to shut it off if an over-current condition is detected in the system.

Talk battery is typically between 40 and 50 volts DC if the phone is on hook, and it drops to a max of 9vdc if it's off hook. Most phones will work off hook with as little as 4vdc.

The issue I have with the 36-volt number in a 70.7-volt CV distributed sound system is that it's not a real-world scenario. The center tap of a transformer does nothing for the speaker circuit. (In fact, I've never even seen a center tap connected unless it's a school system, where a call switch in a classroom shorts a floating CT to ground to produce a call-in at the head end.)

In your example, there are three wires in the circuit. One is carrying the "hot" or in-phase signal. A second is carrying the "cold" or out-of-phase signal. A third is center-tapped and is referenced to ground. If you measure the voltage across the hot & ground or cold & ground, you'll see a 6dB voltage drop versus the measurement between hot & cold. That's because half of the voltage (-6dB) is being taken to ground by the center tap -- which typically isn't connected to anything in the field.

The real-world measurement of voltage is taken from the hot & cold, where a peak-to-peak reading on a 70.7-volt speaker line will be 100 volts.

Again, though, the vast majority of distributed systems never come close to the full 70-volt potential in day-to-day operation. The highest I've ever measured hit 28 volts on my trusty Simpson meter, and it was crankin' loud!

Regards,
Rick Johnston
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Hal Bissinger/COMSYSTEC

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Re: 70-volt wiring
« Reply #23 on: July 23, 2006, 07:16:16 pm »

Again, though, the vast majority of distributed systems never come close to the full 70-volt potential in day-to-day operation. The highest I've ever measured hit 28 volts on my trusty Simpson meter, and it was crankin' loud!

Right you are but you have to remember who writes codes. They take the worst case of an open, unloaded line which could actually have in excess of 70.7 vrms on it, but don't tell them that. If the code says that anything (not just speaker wiring) over 100 volts peak must be in conduit then over 70.7vrms must be in conduit. I don't think they are just picking on us.

The 25 volt system came about the same way. A code somewhere a long time ago (I believe relating to schools) said anything more than something like 50 volts had to be in conduit.

-Hal  
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