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Author Topic: 70v distributed systems troubleshooting  (Read 25543 times)

John Roberts {JR}

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Re: 70v distributed systems troubleshooting
« Reply #30 on: January 31, 2015, 01:08:56 PM »

Yes - but at speaker level, it's of no benefit.


Steve.

Actually something we learned the hard way is that the install business insists on floating (ungrounded) 70-100V outputs. Peavey tried to use auto-formers that are smaller, lighter, (cheaper) and have better response all else equal, but are grounded. The industry rejected them.  :'(  Apparently fully floating speaker outputs are more forgiving of inadvertent shorts between one leg or the other to sundry building grounds.

The instal industry has their methods and reasons. Avoiding a few service calls can make the difference between profit or loss on a job.

JR 
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Ivan Beaver

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Re: 70v distributed systems troubleshooting
« Reply #31 on: January 31, 2015, 02:06:34 PM »

toa impedance meter zm-104

http://www.toaelectronics.com/products/signal-processors/manuals/zm104a_mt1e.pdf

another thing to consider is the transformers are also converting the amp output from an unbalanced output to a balanced line.  Balanced line will CMR ?  Correct ?
The problem with this meter is that it is only at 1KHz.  If there is a peak or dip in the impedance of the speaker at 1KHz, then  you could be VERY  far off (depending on the size of the "change")-by more than 100% easily.

That is the reason you need a CURVE or AT LEAST 4 or 5 different freq to attempt to get some sort of "average"

The CMRR is not on the balanced or unbalanced line-but rather on the circuit of the input amplification stage.

There is no "input amplification stage" of a loudspeaker
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Ivan Beaver
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Stephen Swaffer

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Re: 70v distributed systems troubleshooting
« Reply #32 on: February 01, 2015, 03:56:28 PM »

The problem with this meter is that it is only at 1KHz.  If there is a peak or dip in the impedance of the speaker at 1KHz, then  you could be VERY  far off (depending on the size of the "change")-by more than 100% easily.

That is the reason you need a CURVE or AT LEAST 4 or 5 different freq to attempt to get some sort of "average"

So impedance is frequency dependent-understood.  But that means a spec that says a speakers has an "impedance of 8 ohms" is meaningless unless we know the frequency that it is determined at.  No doubt there is  standard test method used in industry?
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John Roberts {JR}

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Re: 70v distributed systems troubleshooting
« Reply #33 on: February 01, 2015, 04:12:56 PM »

Speakers are characterized as having a nominal impedance. Amplifiers are designed to drive nominal impedance speakers. Design engineers worry about the minutiae so you don't have too...

If this bothers you consider using powered speakers where this is all inside the box.

Life is complicated and the human condition is to try to simplify everything  past where simplification is useful. Resistors have simple resistance, loudspeaker have impedance, and worse than that the impedance is influenced by the cabinet design, passive crossovers, and to a lesser extent room placement.

Experts on the WWW will blind you with all the science they know.. relax, these are not new concepts and design engineers have worried about it already. 


JR
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Stephen Swaffer

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Re: 70v distributed systems troubleshooting
« Reply #34 on: February 01, 2015, 04:39:49 PM »

I am not bothered by the minutiae as you say-in my day job as an electrician it is common to refer to "460 volts" when in fact I may be dealing with anywhere from 440-495 volts.

More asking from a tinkering standpoint.  Perhaps the "average of 4 or 5 measurements" is the only answer for my question.  I wondered of there were a standard set of frequencies that were measured/averaged etc. 

Real world of course you take manufactures specs and work off of them.  In most cases I do that in my day job-but I know how to do far more detailed calculations than are usually necessary.  Is that knowlege always necessary?  No-but neither is climbing that mountain over there.
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Ivan Beaver

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Re: 70v distributed systems troubleshooting
« Reply #35 on: February 01, 2015, 05:44:07 PM »

So impedance is frequency dependent-understood.  But that means a spec that says a speakers has an "impedance of 8 ohms" is meaningless unless we know the frequency that it is determined at.  No doubt there is  standard test method used in industry?
The rating is usually the nearest "standard number" that the speaker has-and usually the lower standard number.

THe actual impedance may dip a little bit below the rating-but it is more of the "average across the freq band" that you are looking at.

Here is a good example.  I originally gave a "rating" of 6 ohms to the attached loudspeaker because I felt that it "best described" the overall impedance load it would place on the amplifier.

We got LOT'S of calls with people saying they did not know what amplifier to use because they could not find one with a 6 ohms rating.

So we kept the speaker the same-and changed the rating to 4 ohms on the spec sheet (figuring going lower would be safer than going higher-to 8 ohms) and the calls stopped.

Of course we always gave the "standard rating" and ALSO the minimum impedance the cabinet would reach and at what freq that was at-for people interested in such things.

As with all things audio-it depends on how you look at the specs to come up with a number.

You can give the same freq response or impedance curve to different people and they will come up with different "specs" depending on how you look at the curve.

This is EXACTLY why Danley provides not only the "simple specs" but ALSO the curves from which the numbers were determined.  So the user can determine their own values if they wish.  Different people look for different things.

Without the curves-you have NO IDEA where the numbers come from. 

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John Roberts {JR}

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Re: 70v distributed systems troubleshooting
« Reply #36 on: February 02, 2015, 02:09:06 AM »

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
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John L Nobile

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Re: 70v distributed systems troubleshooting
« Reply #37 on: February 02, 2015, 09:37:10 AM »

Do the ohms at different frequencies affect the level at those frequencies?  In the above chart will the speaker be quiet at 600hz and hot at 1k? Something I've always wondered about.
Or is that too simple of an interpretation? Will cabinet design come into play to compensate?
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John Roberts {JR}

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Re: 70v distributed systems troubleshooting
« Reply #38 on: February 02, 2015, 10:18:14 AM »

Do the ohms at different frequencies affect the level at those frequencies?  In the above chart will the speaker be quiet at 600hz and hot at 1k? Something I've always wondered about.
Or is that too simple of an interpretation? Will cabinet design come into play to compensate?

Amplifiers are voltage sources which means they put out roughly the same voltage independent of the load. The actual source impedance of power amps is what we measure to impute damping factor... i.e. speaker impedance divided by amp source impedance which is routinely 1/100 of the speakers and less.

Speaker wire is in series with the amp's source impedance for damping factor.

If we look at Ivan's speaker impedance plot with several ohms of speaker wire between the amp and speaker, we could see a response error from the voltage divider formed by wire and speaker impedance. That said typical wire resistance is low enough that response errors are insignificant***.

FWIW speaker frequency response is measured when driven by low impedance voltage source (like an audio amp), so even though the current draw changes widely over the frequency range, the output is designed to be flatter. IMO Ivan's impedance plot is too much information for most customers. Of value perhaps when trying to drive multiple speakers from a single amp, but even then the best advice is to take the manufacturers general advice and not try to second guess their engineers.

Note: The larger concern when specifying a speaker's nominal impedance is the low impedance regions, since solid state amplifiers don't care about high impedance loads. Care must be used when looking at driver impedance in free air, since interaction with the box will affect and shift real world impedances.   

JR

*** I recall years ago on this very forum having a discussion with a sound engineer convinced that his snake oil consumer speaker wire really was better because his speakers sounded different (better). After some investigation we determined that his "funny" wire was higher resistance at the lengths he used. He was actually hearing a modest frequency response change from the wire resistance interacting with his speaker impedance at different frequencies. Mystery solved, he didn't hear the wide band power loss from the wire resistance, just the apparent boost where his speaker was higher impedance.
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Alan Clayton

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Re: 70v distributed systems troubleshooting
« Reply #39 on: February 02, 2015, 03:05:34 PM »

Do you have stepped attenuators in various locations to control the loudness of the loudspeakers? If you do, are you ABSOLUTELY CERTAIN that they are hooked up correctly? If one is hooked up backwards it can cause a serious load issue.

Also, if ONE loudspeaker is tapped to the 8 Ohm (or thru) position, it will look like a 600W load to the system. This will often cause one loudspeaker to be louder than the rest and the amplifier to overheat and shut down (if it can't handle a 600W load... your 100W amplifier would behave like this).

Here is my standard method of handling situations like this:
  • Go through a system and unhook ALL attenuators.
  • Turn the amplifier on and go around to all the attenuator locations with a test loudspeaker. Use this test loudspeaker to make sure that the lines marked INPUT to the attenuator do actually have signal on them from the amplifier.
  • Then use an impedance meter to test the OUTPUT wire to see what the load is that that loudspeaker circuit is presenting to the system.
  • Once all this is documented and you are fairly certain that there are no shorts in the cabling nor faulty loudspeaker taps or wiring, turn the amplifier off and reinstall the attenuators and set them to maximum level.
  • Measure the total load of the system at the amplifier with your impedance meter, be sure you have at minimum 10% excess capacity in the amplifier. This will show you the total load of the loudspeakers plus the attenuators plus the wire in the system.
  • Set all attenuators to minimum.
  • Reconnect main feed to amplifier and switch it on. Set the input level on the amplifier so it is just below clipping. This will get you a good voltage swing on the feed line.
  • Go around to all the attenuators and start opening them up. You need ears in the zone being operated so if your attenuator is not local to the zone being adjusted, you will need a second person in contact via radio or telephone so you can tell them to turn it up or down as necessary to set the level appropriately for the zone. If all your loudspeakers are tapped correctly, generally the attenuator will be between 50-75% for normal operation, this gives the end user the option of turning it up a bit louder than normal and it gives a good operational range down to off.

This method will allow you to quickly find and fix all the most common issues with 70V systems by basically breaking it all apart, testing each part, and putting it back together. It is astounding how often "professionally installed" systems that have been problematic for years will simply have an attenuator hooked up backwards somewhere and the cables going in and out will actually be labeled backwards because someone was not as diligent as they should have been during the initial installation. Of course the result is that you can not really rely on the wire labels because they could be labeled wrong. This is why you need to make sure your main distribution feed really is the line labeled "FEED" and not the line labeled "SPKR". Also, it is not unusual to find incorrectly tapped loudspeakers, or transformers with the unused taps shorted together or to something like building steel because someone was in a hurry to get as many loudspeakers installed in one day as possible... perhaps because they are paid by the unit and not for doing a good job. These kinds of things are frustrating to find because it gives everyone in this industry a bad rep.

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 
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Re: 70v distributed systems troubleshooting
« Reply #39 on: February 02, 2015, 03:05:34 PM »


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