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Author Topic: 70v distributed systems troubleshooting  (Read 25617 times)

Cailen Waddell

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70v distributed systems troubleshooting
« on: January 27, 2015, 05:42:42 PM »

Hi all,

First, I have tried to read all the previous threads pertaining to this, as well as reading Crown's most helpful pdf, available here:
http://www.crownaudio.com/media/pdf/amps/138905-1_10-05_constant_voltage.pdf

I'm the Production Manager of the theater in a municipally owned arts center.  Our building has a building wide distributed speaker system.  Lots of basic in ceiling speakers, mounted in drop ceiling tiles, and a 100w 70v Bogen wall mount amp in the basement.  It goes to all the classrooms, not the theater, and is used for basic announcements.  We did get a box that connects to our voip system that lets me dial into an amp input and make an announcement that way, but I'm getting on a tangent.  There are very large number of home runs, all connected through jumpers on a terminal strip to the amp.  The 100w bogen amp has failed, and through some investigation, I have determined that no one paid attention to the total wattage of the transformer taps during installation (im not surprised).  I am hypothesizing that this is the cause of failure, or certainly a contributing cause.  Clearly a larger amp is required when we replace it, just to be safe.  I have a couple older model amps that output 600w at 8 ohms, so we will try one of these, however, from what I am reading, I also need to be concerned with impedance.  I just can't quite wrap my head on how I am going to calculate that in a constant voltage setting.  If I have to measure it, can I use a basic multimeter?  Is there special equipment required?  I come seeking guidance and some education on this.

Please note this system is non essential to the building operations, and does not serve any required life safety purposes...  Important I know in some modern day 70v systems. 

Many thanks in advance.
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Ivan Beaver

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Re: 70v distributed systems troubleshooting
« Reply #1 on: January 27, 2015, 07:04:15 PM »

You cannot measure IMPEDANCE with a simple volt/ohm meter.

It can only measure DC resistance of a normal speaker and the wire of the transformers in a 70V system.

There are 2 ways to measure the impedance.  You need something (there are various types) than can measure impedance.

I would stay away from the "simple units" that only measure at 1Khz.  That is better than nothing-but it could easily be more than 100% off in either direction.

You really need something that can produce an impedance curve.

I am not aware of any "cheap" units.  But there may be some.

The other way is to take down each speaker and see where it is tapped and simply add them all together and then add another 10% or so.
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Ivan Beaver
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Steve M Smith

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Re: 70v distributed systems troubleshooting
« Reply #2 on: January 27, 2015, 08:06:08 PM »

I just can't quite wrap my head on how I am going to calculate that in a constant voltage setting


People need to stop using that term.  The voltage is no more constant than it is from a 'normal' amplified system.


Steve.
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Ivan Beaver

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Re: 70v distributed systems troubleshooting
« Reply #3 on: January 27, 2015, 09:07:34 PM »


People need to stop using that term.  The voltage is no more constant than it is from a 'normal' amplified system.


Steve.
EXACTLY!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

That is a hold over from many decades ago and it only serves to CONFUSE people.

There is NOTHING constant about it-never has been.

I think somebody saw the term and "assumed" that it was like the voltage at the wall socket.  Not even close!

All it means is that when the voltage on the line gets to 70Volts the impedance of the speaker will be such that the tapped power is being delivered to it.

It is REALLY very simple and much easier than "normal" loudspeakers and more flexible.

BUT it has some drawbacks-but we won't go there now---------
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Cailen Waddell

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Re: 70v distributed systems troubleshooting
« Reply #4 on: January 27, 2015, 09:18:47 PM »


EXACTLY!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

That is a hold over from many decades ago and it only serves to CONFUSE people.

There is NOTHING constant about it-never has been.

I think somebody saw the term and "assumed" that it was like the voltage at the wall socket.  Not even close!

All it means is that when the voltage on the line gets to 70Volts the impedance of the speaker will be such that the tapped power is being delivered to it.

It is REALLY very simple and much easier than "normal" loudspeakers and more flexible.

BUT it has some drawbacks-but we won't go there now---------

Right - so clearly I used some outdated terminology.  Am I off base on the impedance concerns?  I can't just hang an unlimited number speakers off the amp as long as I'm under the wattage right?


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John Roberts {JR}

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Re: 70v distributed systems troubleshooting
« Reply #5 on: January 27, 2015, 09:27:26 PM »

I would stop trying to change ingrained industry jargon and perhaps try to understand it.

Yes a more accurate name would be "normalized" voltage system. Where the output voltage is normalized at 70V or 100V for signal distribution.

The term constant voltage was probably borrowed from the utility power distribution industry that use similar step-up and step-down transformers to reduce wire losses.

JR
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Stephen Swaffer

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Re: 70v distributed systems troubleshooting
« Reply #6 on: January 27, 2015, 09:39:29 PM »

Thinking out load here-most modern quality dvm's are accurate up to to a few kH, and as well clamp meters.

Why could you not use a signal generator or test tone (.wav/CD whatever) played through the system and measure the voltage and current and calculate an impedance? Certainly not as convenient as a purpose built piece of test equipment, but for a one off would this not be workable?
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Steve Swaffer

Steve M Smith

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Re: 70v distributed systems troubleshooting
« Reply #7 on: January 28, 2015, 01:39:38 AM »

If you have the capability, an accurate way would be to work out the turns ratio of the transformer, multiply it by itself then multiply it by the speaker impedance.  That will give you the speaker/transformer combination impedance.

You can measure the turns ratio by putting a small (about 6 volt) ac voltage on the secondary and measuring the voltage on the primary (without the speaker connected).  The turns ratio is the same as the voltage ratio (I used to use the 6.3v heater winding from a valve/tube radio for this).

It's not the most convenient method, but if all of your transformers are the same, you only need to do it once (or once per tap).


Steve.
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Cailen Waddell

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Re: 70v distributed systems troubleshooting
« Reply #8 on: January 28, 2015, 07:27:24 AM »


If you have the capability, an accurate way would be to work out the turns ratio of the transformer, multiply it by itself then multiply it by the speaker impedance.  That will give you the speaker/transformer combination impedance.

You can measure the turns ratio by putting a small (about 6 volt) ac voltage on the secondary and measuring the voltage on the primary (without the speaker connected).  The turns ratio is the same as the voltage ratio (I used to use the 6.3v heater winding from a valve/tube radio for this).

It's not the most convenient method, but if all of your transformers are the same, you only need to do it once (or once per tap).

Thanks Steve - that's quite helpful.  This is certainly something I can do.   


Steve.


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Ivan Beaver

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Re: 70v distributed systems troubleshooting
« Reply #9 on: January 28, 2015, 07:36:48 AM »

Am I off base on the impedance concerns?  I can't just hang an unlimited number speakers off the amp as long as I'm under the wattage right?


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You are correct.  You cannot just keep on adding speakers.

The total wattage of the speaker taps should be no more than 90% of the wattage rating of the amp. 

IF it is less, no big deal, the amp just does not work as hard.

HOWEVER on tube 70V amps that is a different story.  You want the wattage to be close to that 90% mark.  Because a lower wattage will mean that the impedance the amp "sees" is higher, and tube amps will not last as long when driving a high impedance-while solid state amps really like high impedance loads.
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Ivan Beaver
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ProSoundWeb Community

Re: 70v distributed systems troubleshooting
« Reply #9 on: January 28, 2015, 07:36:48 AM »


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