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[Josh Millward]

Excellent advice Josh!  Having troubleshot many such systems I can almost guarantee that It's either a mis-wired volume control or someone, somewhere, sometime, replaced (or installed) an 8 ohm speaker.  Just perused this thread, so if someone already mentioned this, I apologize. The best tool I've found for diagnosing constant voltage systems is the Gold-Line ZM1: http://www.gold-line.com/zm1.htm

Thanks Alan!

Good call on mentioning the Gold-Line ZM-1. I had intended to do that in my post since that is what I use for an impedance meter when I am doing this kind of work.

Having the right tools makes all the difference in the world.

[Stephen Swaffer]

And to repeat myself so soon, the nominal speaker impedance is mainly useful for end users to match up to a nominal amplifier's rated drive impedance.

The folks that make speakers and amplifiers generally try to make it easier for customers to hook them together, and get good results. 

Customers trying to second guess design engineers are being optimistic.

JR

I might go so far as to say that customers second guessing design engineers are being foolish.  However, Ivan's plot is a very convincing argument against relying on an impedance meters reading based on a single 1 kHz test.

Useful perhaps only from the standpoint that it illustrates the complexity of the issue.

[Craig Hauber]

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.

Parts Express sells a "Woofer Tester" kit that in the process of determining a driver's Theill-Small parameters generates a full-frequency impedance plot.  It's a USB dongle with a couple alligator clip leads and is only $100.
I have found that feature works excellent for blind diagnosing of 70V runs terminating in a rack closet.
Bennett reviewed it over at the other site
https://soundforums.net/threads/11506-Parts-Express-Dayton-Audio-DATS-V2-Impedance-Tester.

One way of finding the one "problem" 8-ohm speaker among hundreds is to hook up a huge 70V amp (1000+ W) blow-out the offending speaker then replace the normal 100W amp and be on your merry way. 
(Sarcasm, -but observed a customer doing that once!)

[Hal Bissinger/COMSYSTEC]

One way of finding the one "problem" 8-ohm speaker among hundreds is to hook up a huge 70V amp (1000+ W) blow-out the offending speaker

Never tried it but I heard connecting the speaker line directly to 120v is pretty effective too. 

-Hal

[claude cascioli]

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.

what you should do is buy an impeadance meter  parts express sell them gold line make sone model is the zm 1 if you are going to do installs this is a must
the meter has setting for 25-50-70 volt load hook across the speaker line try starting with the 70 volt setting if it seem the wattage load is very high that means the speakers have 25 volt transformers  10 watts at 70 volt line is 500 ohms 20 watts is 250 ohms and so on 100 watt 70 volt load is 50 ohms

[Kevin Graf]

You can measure speaker impedance with:
a] tone generator
b] power amplifier
c] Volt/Ohm meter
d] adjustable power resistor
Connect the resistor in series with the speaker, adjust the resistor value until the voltage across the resistor and the speaker are the same. Disconnect everything and measure the resistor value, the speaker impedance will be the same.
Any Volt-meter that has a low voltage range will work. Frequency response and voltage accuracy don't matter as the two readings will be identical.

[ShaunRay]

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.

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.

[Cailen Waddell]

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.

Thanks - we got that established about 7 months ago when this thread was created. 


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[Scott Carneval]

Thanks - we got that established about 7 months ago when this thread was created. 


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Not only that, but he copied Ivan's post from January 27th verbatim!  Can we say troll?

[Cailen Waddell]

Not only that, but he copied Ivan's post from January 27th verbatim!  Can we say troll?

Oh moderators...


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