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Author Topic: 70v distributed systems troubleshooting  (Read 25041 times)

Ivan Beaver

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Re: 70v distributed systems troubleshooting
« Reply #20 on: January 28, 2015, 09:29:11 pm »

Well, not exactly. The output of a constant voltage amplifier at full power will be 100, 70.7 or 25 volts regardless of amplifier wattage. So there is your constant and is why the system was named for it.

-Hal
I have never seen the output of any audio amplifier produce a "constant" voltage-unless the source was a SINGLE sine wave. 

The voltages are NOT limited to the 100 or 70V ratings.  With a large amplifier the voltages can easily be higher than what people would think.

All it really means is that when the level is 70 or 100 volts then the tapped power will be delivered to the speaker.

If a higher voltage is applied to "the line", then the wattage to the speaker will be higher.

It is simply a high impedance transfer system.
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Ivan Beaver
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Jason Lavoie

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Re: 70v distributed systems troubleshooting
« Reply #21 on: January 29, 2015, 10:45:21 am »

I have never seen the output of any audio amplifier produce a "constant" voltage-unless the source was a SINGLE sine wave. 

The voltages are NOT limited to the 100 or 70V ratings.  With a large amplifier the voltages can easily be higher than what people would think.

All it really means is that when the level is 70 or 100 volts then the tapped power will be delivered to the speaker.

If a higher voltage is applied to "the line", then the wattage to the speaker will be higher.

It is simply a high impedance transfer system.

70V is a constant more in terms of a 'constant' in a math equation. different use of the word

to increase the wattage capability of an amplifier you can make it capable of more voltage, more current, or both.
in the case of constant voltage systems someone decided to hold the voltage as a constant and make the current the variable that determines the amp's wattage

Jason
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John Roberts {JR}

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Re: 70v distributed systems troubleshooting
« Reply #22 on: January 29, 2015, 11:27:29 am »

70V is a constant more in terms of a 'constant' in a math equation. different use of the word

to increase the wattage capability of an amplifier you can make it capable of more voltage, more current, or both.
in the case of constant voltage systems someone decided to hold the voltage as a constant and make the current the variable that determines the amp's wattage

Jason
LIKE...

Yes this is the constant voltage math that makes matching CV systems amps to loads and vice versa so simple... everything (like amp outputs and speaker transformer inputs) is normalized for the same nominal system operating voltage.

Whining about the nomenclature is pointless since the industry is not going to abandon popular industry jargon to use a far more complex relationship between amplifier outputs and speaker loads.

It is nice that so many here understand how audio amplifiers work literally (more or less) but why complicate this relatively simple install system math with TOO MUCH INFORMATION.

KISS

JR 
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Ivan Beaver

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Re: 70v distributed systems troubleshooting
« Reply #23 on: January 29, 2015, 11:56:03 am »

LIKE...

 but why complicate this relatively simple install system math with TOO MUCH INFORMATION.

KISS

JR
Yeah.

70V (or 100V or 140V) are really much easier to figure loads than "normal" speakers.  Simple addition.

Yet they seem to be complicated to many people.

There are some things that can get to be an issue if you are not careful however-especially on the lower freq.  Some amps don't like the big inductive load of the transformers at lower freq.
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Ivan Beaver
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Josh Millward

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Re: 70v distributed systems troubleshooting
« Reply #24 on: January 29, 2015, 11:59:42 am »

Thanks Steve, I had read the crown publication, but didn't quite have my head wrapped around it.  I think I do now.

lets review my math and findings....

I took one of the spare speakers and started measuring with my DMM.  The 1/2 watt tap is 514 ohms, the 1 watt tap is roughly half that, and so on. 

There are 68 speakers in the facility all tapped at 1/2 a watt.  Now this in theory is only a 34 watts.  Well within the 100w of the amp.  however, when I add the load up (1/total impedance) = (1/speaker 1) + (1/speaker 2) + (1/speaker 3), etc.  I get a 7.55 Ohm Load.  However, using the formula on Crowns website where (safe impedance) = (voltage squared)/(rated wattage), I see that a 100w amp could only support a minimum 50 ohm load. 

Is this correct logic?  If so, while I have plenty of power, the impedance is severely off, possibly indicating why magic smoke escaped the amp.  I have an old 2 channel amp that can do 600w at 8 ohms, so close enough.  If I divide the load into two channels, then I could put about a 15 ohm load on each channel, which I think would be acceptable right?  Do i need to worry about how far off the wattage is?

Sorry for all the maths but I want to make sure I understand this before I try a solution.

Thanks,

Cailen

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.
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Josh Millward
Danley Sound Labs

John Roberts {JR}

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Re: 70v distributed systems troubleshooting
« Reply #25 on: January 29, 2015, 12:48:46 pm »



There are some things that can get to be an issue if you are not careful however-especially on the lower freq.  Some amps don't like the big inductive load of the transformers at lower freq.

It's not purely an amp issue, while dedicated install amps are designed to anticipate such loads. It isn't the inductance but transformer saturation at LF, if the transformer can not support the magnetic flux required to pass high power at low frequency.  For an install amp output transformer to pass full power at 20 Hz it would need to be 3x the size and weight of the power transformer operating at 60 Hz. If the output transformer saturates the primary no longer looks like the reflected load from the secondary side of the transformer, but instead just the primary winding to ground (perhaps in parallel with the output side load), so most power amps will current limit or worse at loud extreme LF. A similar transformer saturation effect at LF can occur in the step down transformers at the speakers too.

Most dedicated install amps use pretty severe HPF to scrub off too much LF content. One of my old Peavey patents while designing gear for this market is a dedicated LF clamp that allows the user to turn up the bass tone control but clamps that bass signal if loud enough to bother the output magnetics.   

This is generally not a problem for dedicated install amps, that are designed to be easy to use, but when customers get too clever for their own good and throw a general purpose audio amp on a 70V distribution line, without HPF, bad things can happen.

A robust well designed amp should just shut down but sending too much LF into an unprotected systems can release the smoke.

Sorry if this is TMI...

JR   
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Hal Bissinger/COMSYSTEC

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Re: 70v distributed systems troubleshooting
« Reply #26 on: January 29, 2015, 07:56:05 pm »

70V is a constant more in terms of a 'constant' in a math equation. Different use of the word.

Sorry if I didn't make that clear.

-Hal
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Jason Lavoie

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Re: 70v distributed systems troubleshooting
« Reply #27 on: January 30, 2015, 01:52:13 pm »

70V (or 100V or 140V) are really much easier to figure loads than "normal" speakers.  Simple addition.

Yet they seem to be complicated to many people.

I think the problem is that most questions come from people who have learned at least a bit about regular speaker systems first and then their first interaction with a 70V system is someone else's install that's not working and they're lost.
If everyone learned CV systems first and then started helping their friend with the band's low-z system I'm sure we'd get the same puzzled questions like "why does the 500W amp cook and shut down when I only have 8x50W speakers hooked up to it?"

Jason
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Jerome Malsack

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Re: 70v distributed systems troubleshooting
« Reply #28 on: January 31, 2015, 11:15:23 am »

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 ? 
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Steve M Smith

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Re: 70v distributed systems troubleshooting
« Reply #29 on: January 31, 2015, 12:40:30 pm »

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 ?

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


Steve.
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Re: 70v distributed systems troubleshooting
« Reply #29 on: January 31, 2015, 12:40:30 pm »


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