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[Billy Wood]

"Huh"?    70/100V systems are called "constant voltage" systems, and impedance is indeed a useful measure of how heavily that amp is loaded for that nominal (not really constant) system output voltage..

It is fairly simple math to convert 70V at X watts, into a minimum Y ohms, and unlike your clamp on ammeter you can debug the system when it's turned off or broken.

Of course do what works for you...


JR

Sorry JR.  I should have rephrased that.  I was really only addressing a building short.  For example the all to common light fixture placed on top of a cable in some ceiling tiles and has a earthing problem from the wire being smashed but not completely cut.  A handheld or isolated testing equipment would not follow through the building because it is isolated from it.  It would still follow through the wire as if nothing was wrong. But a amp being connected through the same ground as the lighting fixture surely will.  You are completely correct.

And the only reason I mention it is because alot of times I work on 70v the amp is to small or on the other side of a transformer and it will not burn through a cable leaving an open circuit. 

I was not trying to smash you guys at all. I was only answering one side of his question about building short.  A short between hot and nuetral of a system would plainly show near 0 ohms where a building short would not be so apparent.

I find that I never seem to come out and service a distributed audio system when it is simple and we all no electricians never pay any mind to our speaker wire near their light fixtures or a plumber and their fire systems.

[Hal Bissinger/COMSYSTEC]

...Alot of people buy 100w volume knobs assuming that it is related to its durability and max load but it limits that zone to said wattage.  You can put 1000w worth of speakers on the other side and it will only get 100w.  It is just a multi-tap transformer with a knob on the front to pick the multiple taps.

Ok with that said testing at the amp will ussually show you nothing because you have volume knobs at every zone.  Everything past the volume knob will not show at the AMP.  That is usually where most of your problems lie due to them being above ceiling tiles that everyone and their mom gets into!

Impedance will not tell you much also. 70v is not a impedance based system. That is why it is used in distributed audio.  You need to turn every zone off at the zone controls and then run signal through your system.  If you are getting amperage draw over the hot line at the amp you have a leak to building ground somewhere (ie. - a short). 

The impedance remark is only applicable if you do not have a step up transformer at your amp and it is a transformerless amp system.  This is saying you don't have a regular 2/4/8 ohm amp at the start and a step up transformer feeding your 70v.  I have not ran into this in any recent installations. 

 
Billy Wood
www.avprogroup.com
The Woodlands, TX

A direct drive amp (transformerless) is no different than one with an output transformer in that the voltage at the output will be 70.7 volts at the rated output. That's the basis for the constant voltage system.
 
Actually those volume controls are a tapped auto transformer. The taps represent some value 1/10, 1/5, etc of the connected impedance and as such the input will represent the connected load. A 100 watt control is capable of handling a 100 watt load and if you use it to control a 1000 watt load it will deliver the power- until it burns up.
 
Also, with any controls that I have ever seen the highest setting is 1:1 and the autotransformer is bypassed. Since at the maximum setting the control is out of the circuit set all your controls to max while doing impedance measurements.
 
The most sensible way to find a short to ground is to just disconnect the wiring from the amp and use an ohm meter to check each side to ground.
 
-Hal

[Devin Rice]

Actually those volume controls are a tapped auto transformer. The taps represent some value 1/10, 1/5, etc of the connected impedance and as such the input will represent the connected load. A 100 watt control is capable of handling a 100 watt load and if you use it to control a 1000 watt load it will deliver the power- until it burns up.
 
Also, with any controls that I have ever seen the highest setting is 1:1 and the autotransformer is bypassed. Since at the maximum setting the control is out of the circuit set all your controls to max while doing impedance measurements.
 

 
-Hal

I have had an architect spec for 15w ceiling speakers with a 6w volume control. Does this mean there is underutilization of the system? This is for a 100v system.

[Brad Weber]

I have had an architect spec for 15w ceiling speakers with a 6w volume control. Does this mean there is underutilization of the system? This is for a 100v system.

Many 'constant voltage' speakers have multiple taps, for example a nominal 15W speaker may have 15W, 7.5W, 3.75W and 1.875W taps, any of which could be used.  A 6W volume control could simply indicate that the speaker is to then be tapped using the 1.875W or 3.75W taps on its transformer.  However, if the speaker is specified to be tapped at 15W then a 6W volume control is undersized.

[Devin Rice]

Many 'constant voltage' speakers have multiple taps, for example a nominal 15W speaker may have 15W, 7.5W, 3.75W and 1.875W taps, any of which could be used.  A 6W volume control could simply indicate that the speaker is to then be tapped using the 1.875W or 3.75W taps on its transformer.  However, if the speaker is specified to be tapped at 15W then a 6W volume control is undersized.

I have made a mistake, the ceiling speakers I am told are 6w. Tapped at 6w. Is this a common size or does it seem underpowered? The ceiling height is 11 feet.

Trust it is OK for a 6w speaker to have a 6w volume control.

[Charlie Zureki]

I have made a mistake, the ceiling speakers I am told are 6w. Tapped at 6w. Is this a common size or does it seem underpowered? The ceiling height is 11 feet.

Trust it is OK for a 6w speaker to have a 6w volume control.

   Yes.     He's installing a 100v system..surely you're in Europe or the East ? 

   Hammer

[Hal Bissinger/COMSYSTEC]

I have made a mistake, the ceiling speakers I am told are 6w. Tapped at 6w. Is this a common size or does it seem underpowered? The ceiling height is 11 feet.

I have no idea. It all depends on how big the room is and other factors.
 
-Hal

[Devin Rice]

   Yes.     He's installing a 100v system..surely you're in Europe or the East ? 

   Hammer

South Asia to be precise

[Ivan Beaver]

I have had an architect spec for 15w ceiling speakers with a 6w volume control. Does this mean there is underutilization of the system? This is for a 100v system.

One of the "misunderstood" aspects of a 70V/100V etc system is the taps.  Basically they are just impedance taps (they change the impedance load that the loudspeaker presents to the amp).  Let's look at a 6 watt tap.  The only time that 6 watts is "coming" out of the loudspeaker is when the voltage on the line is 70V.  When the level is turned down, the voltage on the line is lower-therefore less power is being "dissapated" by the loudspeaker.   But the impedance load is still there-as far as the amp is concerned.

The design may be just fine with a 6 watt volume control.  Or not-hard to say without a lot more information.  The choice of the 15 watt speaker may be because of coverage pattern (they are NOT all the same), freq response, cost, physical depth, mounting weight or other factors that have to be considered in an system of this type.

Just because a loudspeaker is rated for 15 watts does not mean it has to be run that hard.  Just as with "regular" loudspeakers, it if far beter to have a more powerful loudspeaker and run it at a lower level, than to get a lessor loudspeaker and run it to the max.  Guess which one is going to sound better and last longer?

How high does the speedometer on your car go?  Do you drive it to the limit all the time?  I doubt it (unless you are racing).  So that means that you are "underutilizing your car".  Does that bother  you?  If it bothers you with loudspeakers-it should bother you with your car as well.

I hope there is a specific reason the loudspeakers-the taps involved-the size of the volume controls etc are choosen for a particular job.  That is all part of the DESIGN process

[g'bye, Dick Rees]

How high does the speedometer on your car go?  Do you drive it to the limit all the time?  I doubt it (unless you are racing).  So that means that you are "underutilizing your car".

Bennett P excepted.....

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