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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => Installed Sound/Contracting => Topic started by: Cailen Waddell on January 27, 2015, 05:42:42 PM
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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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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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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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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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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?
Sent from my iPhone using Tapatalk
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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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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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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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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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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?
Sent from my iPhone using Tapatalk
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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All the transformer does is increase the impedance of the speaker.
So if you have an 8 ohm speaker, you might have a transformer which makes it into an 80 ohm speaker.
Connect ten in parallel and you're back to 8 ohms again.
Crown have a good guide here: http://www.crownaudio.com/media/pdf/amps/138905-1_10-05_constant_voltage.pdf (http://www.crownaudio.com/media/pdf/amps/138905-1_10-05_constant_voltage.pdf)
And there is much more than you will ever want to know here: http://sound.westhost.com/articles/line-amps.html (http://sound.westhost.com/articles/line-amps.html)
Steve.
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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.
While indeed solid state amps can be operated with no load, I have heard horror stories about (some) tube amps that become unstable and self-destruct when operated with no load or even switching between speakers if they become disconnected from the load momentarily.
I am aware of the 90% rule of thumb to provide headroom and account for distribution losses.
I never heard that tube amps age faster under light load while I do not know what a safe load is to insure stability (I suspect << 90% but that is just a guess).
The good news is I don't expect people will encounter any new tube install amps, so the ones out there should already be safely loaded.
JR
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With valve amps, My understanding is it's more a question of the load having the right inductance than an actual impedance load.
However, I don't know why I think that or where I learned it so it could be wrong!!
Steve.
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With valve amps, My understanding is it's more a question of the load having the right inductance than an actual impedance load.
However, I don't know why I think that or where I learned it so it could be wrong!!
Steve.
Power tubes have higher internal impedance than power transistors so can not drive loudspeakers directly. Output transformers are generally used in tube amps to step up the speaker load to be a better match for tubes. When negative feedback is connected around a tube amplifier, this higher source impedance will be more susceptible to phase shift from a reactive load. Capacitance will cause lag in this negative feedback that can cause oscillation when that lag causes so much phase shift that the negative feedback become positive. The resistive part of the speaker load will attenuate the negative feedback and shift the phase shift pole higher keeping the amp stable.
I suspect a well designed tube amp could be stable with no load, just like modern solid state amps that compensate for no load, but enough of them are not stable that the general advice to not operate them without load is reasonable.
JR
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While indeed solid state amps can be operated with no load, I have heard horror stories about (some) tube amps that become unstable and self-destruct when operated with no load or even switching between speakers if they become disconnected from the load momentarily.
I am aware of the 90% rule of thumb to provide headroom and account for distribution losses.
I never heard that tube amps age faster under light load while I do not know what a safe load is to insure stability (I suspect << 90% but that is just a guess).
The good news is I don't expect people will encounter any new tube install amps, so the ones out there should already be safely loaded.
JR
That is the reason the old Fender amps had a shorting jack for the speaker output. The output was shorted when the speaker jack was unplugged.
I had an old Fender Champ that I put a series resistor with the speaker to drop the level so I could overdrive it hard.
I burnt out 2 output transformers before somebody mentioned that the load to to high.
One time when the "light bulb" turned on :)
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All the transformer does is increase the impedance of the speaker.
So if you have an 8 ohm speaker, you might have a transformer which makes it into an 80 ohm speaker.
Connect ten in parallel and you're back to 8 ohms again.
Crown have a good guide here: http://www.crownaudio.com/media/pdf/amps/138905-1_10-05_constant_voltage.pdf (http://www.crownaudio.com/media/pdf/amps/138905-1_10-05_constant_voltage.pdf)
And there is much more than you will ever want to know here: http://sound.westhost.com/articles/line-amps.html (http://sound.westhost.com/articles/line-amps.html)
Steve.
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
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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.
Unless your DMM reads AC impedance you just measured the DC resistance. Not the same thing as AC impedance.
Hanging 514 ohms across a 70V line will draw 9.5W each...
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.
68 x 9.5W is more than 100W 8)
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.
DCR not 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.
yup... 100W from 70V line is around 49 ohms (AC 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
34W from 70V is roughly 144 ohm (AC) load.
If smoke does not come out of the 100W amp you're good...
The whole concept of constant voltage system math is to KISS so 68 1/2 W loads is 34W... job finished take a break.
JR
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You really need to find the turns ratio of the transformer and from that, work out the impedance ratio (square of the turns ratio) to be sure of the load you are applying. A dc resistance doesn't tell you much.
Steve.
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Unless your DMM reads AC impedance you just measured the DC resistance. Not the same thing as AC impedance.
Hanging 514 ohms across a 70V line will draw 9.5W each... 68 x 9.5W is more than 100W 8)DCR not load.yup... 100W from 70V line is around 49 ohms (AC load)
34W from 70V is roughly 144 ohm (AC) load.
If smoke does not come out of the 100W amp you're good...
The whole concept of constant voltage system math is to KISS so 68 1/2 W loads is 34W... job finished take a break.
JR
Thanks JR., I'm all for KISS. And I certainly have been known to over think things. I guess part of my supposition was that the smoke was let out of the existing amp and perhaps we were asking too much from it. That doesn't seem to be the case. Maybe we just drove it too hard?
Thanks though, I knew the math didn't quite make sense (or it did, it's just the wrong math)
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People need to stop using that term. The voltage is no more constant than it is from a 'normal' amplified system.
Steve.
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
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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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.
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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.
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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!)
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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. 8)
-Hal
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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
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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.
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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.
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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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Thanks - we got that established about 7 months ago when this thread was created.
Sent from my iPhone using Tapatalk
Not only that, but he copied Ivan's post from January 27th verbatim! Can we say troll?
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Not only that, but he copied Ivan's post from January 27th verbatim! Can we say troll?
Oh moderators...
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Never tried it but I heard connecting the speaker line directly to 120v is pretty effective too. 8)
-Hal
Pat Brown teaches this in his SynAudCon classes. I've always wanted to try it. ;D