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Author Topic: 70 Volt questions  (Read 11713 times)

Jeff Heart

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70 Volt questions
« on: September 17, 2007, 12:56:03 am »

I found a site that explained the use of a regular 2, 4, 8 ohm etc... amplifier for powering 70volt systems.

We do sound for a venue that has the main system on a 70v amp but they do not have a back up amp.  If it goes down, the night is over!  This led to me research which stumbled me upon this article :

 http://www.yorkville.com/default.asp?p1=6&p2=17&p_id =73

The lower half of that article details some formulas and methods to use regular power amps without the 70v transformer.  My question is to see if some of you math gurus can validate these methods and give a little further insite into what is left fuzzy still my head, due to a few vague areas in the article.

For example sake, I have 9 70v speakers in which 5 are tapped at 60watts and the other 4 are tapped at 30 watts.  420 watts total.
System impedance is 11.66 (based on formula from that link above)

Amplifier is a Crown MT 2400VZ which at 2,4 and 8 ohms stereo will NOT yield a voltage output close enough to 70v.  However, bridged mono, it's specs are 1650watts at 8ohms which will yield a bit over 114volts (again, based on math from what I read at above link)

Does this mean I can properly power the speakers with the Crown amp since my total speaker wattage is 420, the amp is at 1650 watt and the volatge calculation is above 70?  OR does the votage calc have to be right at 70v and none higher?
 The speakers have a 100v mode too which is auto sensing so I assume I would have problems since the 114v calculation is above the 100v transformer tap rating as well.

Maybe to help make any answers easier, we can keep it real simple and just say, "can I use an amp like a macrotech without a transformer as long as the voltage output calculation is greater than the 70v speaker system, and the wattage is greater than the speaker systems total wattage OR does the voltage yield have to fall on the button of either 70 or 100, just as the speaker transformers expect?"
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Jeff Heart
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Mac Kerr

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Re: Powering a 70v speaker system with a non 70v amp??
« Reply #1 on: September 17, 2007, 01:09:53 am »

Speaker transformers don't "expect" anything. The main thing you lose by using an amp that can't put out 70V into the system impedance you have is the ability to drive the system to its maximum volume. When you are playing low level background music that 70V amp isn't putting out 70V either. When it does put out 70V, each of the speakers will be receiving about the power it is tapped at.
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Jeff Heart

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70v amp question...part 2
« Reply #2 on: September 17, 2007, 01:22:46 am »

Is it true that with a 70volt amp, you do not need to calculate for headroom (other than line loss and transformer efficiency), and that you can literally load on the speakers until you have used up the entire rated wattage of the amp?

For example, a 70v amp rated at 500 watts per channel stereo.  I can tap 5 speakers at 100watts each and use up all 500 watts without worrying about the amp needing more juice for peaks, etc...? (again, taking line loss and transformer efficiency out of the equation)

I'm curious because the venue I reffered to in my other 70volt post has a system in which was just put in very recently and it had an issue the other night.  I wasn't there so I dont know exactly what they heard but... here is the scenerio :

THey have a Crown CH4 70v amplfier capable of 500 watts per side stereo.  THe one side has 9 JBL Control 266 CT's on it.  5@60 watts and 4@30 watts, with a total of 420 watts. 80 Watts of headroom to allow for line and transformer power loss.
   They called me and said that last night they heard some pops at one point and then it sounded like the first speaker in the chain was letting low frequenices through (despite crossover settings) and then randomly getting lower in volume and then louder again, along with sounding muffled.  He said the other speakers in the chain were following suit with different less abrupt degrees of issues.  He said the amp wasn't clipping ( i was on the phone with him during the incident) and the levels of the system were all in the green, no limiting or compression taking place.  I had him turn down the other amp channel and he said the problem seeemed to go away.  Considering the other channel is independent of the problem channel, I think it was just coincidence that the problem stopped.
 At my test there today, all speakers appeared to be fine with no signs of noise, pops, or any indicators at the amp.  However, I could not turn it up (it was dinner hours) to see if it was happening when the amp was serving up some real power.
 This leads me to this purpose of this post which is to ask if anyone knows what happens when a 70v amp is pushed to it's max potential or if it is loaded up with speakers that have a combined wattage that is greater than the limit of the amp channel they are on.  Even though in this case as I said earlier, it is 80 watts under the channel total.  DOes any of the symptoms above sound like anything that could happen if the amp is pushing it's limit or would that cause something entirely different?  I'm gonna check out the Crown CH4 manual as I know it will list the protection features for this particular amp but I'm assuming that it would do many things to protect itself other than the wacky issues described above.


thanks as usual,
jeff
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Jeff Heart
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Jeff Heart

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Re: Powering a 70v speaker system with a non 70v amp??
« Reply #3 on: September 17, 2007, 02:05:43 am »

Not to side track my original questions but, based on what you said, it led me to one other question.  Since the transformers have 70v and 100v settings, but only 1 input....how does the transformer compute which setting I want?  Does it use the 100v setting after the amp is putting out more than 70v?  When the selector on the speaker is pointing to say 60 watts on the 70v side, it is also simultaneously pointing to 30 watts on the 100v side (i'm not certain about those numbers, it's not infront of me).  SOmeone told me in the past that it knows what is being sent to it and uses the setting of the supplied voltage.  WHich is why I referred to thinking it "expected" something.  
  Maybe the selector is not pointing to both sides at the same time and rather just looks that way.  Even still, it seems redundant then to even have a 100v 60 watt setting AND a 70v 60 watt setting.  Why would I ever need the 100v setting if I can reach the same max power output at 70v?
    Again, this is a bit side tracked so I hope any responses don't neglect my original questions but perhaps just add this one to the mix.

Mac Kerr wrote on Mon, 17 September 2007 06:09

Speaker transformers don't "expect" anything. The main thing you lose by using an amp that can't put out 70V into the system impedance you have is the ability to drive the system to its maximum volume. When you are playing low level background music that 70V amp isn't putting out 70V either. When it does put out 70V, each of the speakers will be receiving about the power it is tapped at.

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Jeff Heart
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Dave Dermont

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Re: Powering a 70v speaker system with a non 70v amp??
« Reply #4 on: September 17, 2007, 10:14:40 am »

A quick rough calculation of the total system impedance is 11.9 Ohms. That makes the short answer to your question, Yes, you can use an amp like the Macrotech without a transformer.

This is based on a 30w 70V tap being 163.3 Ohms and a 60w 70V tap being 81.6 Ohms

Check my math before you hook anything up. I don't have a lot of confidence in my own math.

These things are nice for guys that don't work with the formulas enough to know them by heart:
index.php/fa/11365/0/


I am not so sure about those "auto sensing" speaker taps. I'd take a closer look at that.

A 25V, 70V, or 100V speaker distribution system is basic Ohms Law stuff, and not nearly as mysterious as what some people make it out to be.
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Ken Freeman

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Re: Powering a 70v speaker system with a non 70v amp??
« Reply #5 on: September 17, 2007, 10:45:29 am »

Hey Jeff,

This is not rocket science, but does require some math as Dave noted above.  I direct drive 70 Volt all the time.  I often explain this little slice of audio as a lawn sprinkler system, which works just fine as long as you keep the pressure (voltage) up and don't draw too much water (watts) from the system.  Our rule of thumb is to use an amp that can generate more than 700 watts into an 8 ohm load.  For large scale systems, you do need to figure in the wire as part of the load that the amplifier is seeing.  If you do not, your calculations for actual delivered SPL may be off.

Ken
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Jeff Heart

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Re: Powering a 70v speaker system with a non 70v amp??
« Reply #6 on: September 17, 2007, 12:56:16 pm »

Ken Freeman wrote on Mon, 17 September 2007 15:45

Hey Jeff,

This is not rocket science, but does require some math as Dave noted above.  I direct drive 70 Volt all the time.  I often explain this little slice of audio as a lawn sprinkler system, which works just fine as long as you keep the pressure (voltage) up and don't draw too much water (watts) from the system.  Our rule of thumb is to use an amp that can generate more than 700 watts into an 8 ohm load.  For large scale systems, you do need to figure in the wire as part of the load that the amplifier is seeing.  If you do not, your calculations for actual delivered SPL may be off.

Ken


Ok, so for purely example sake, could I take an amp that is waaaay over rated such as an Itech 8000 and then, make sure the total wattage of all speakers on the channel do not surpass the total wattage of the connected amp channel at the 8ohm spec, send a pink noise signal through the system and then measure voltage on the output.  Slowly turn up gain on the amp until voltage is about 70v and then call it a day and be sure the amp is not "supplying too much water" as you put it?
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Jeff Heart
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Bob Lee (QSC)

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Re: Powering a 70v speaker system with a non 70v amp??
« Reply #7 on: September 17, 2007, 01:31:14 pm »

No, transformers don't "compute" anything and can't change ratings around. Basically, a transformer changes an input voltage to an output voltage. The ratio of output voltage to input is equal to the ratio of the number of turns of wire in the secondary and the primary windings, respectively.

A 70-volt amp can put out a maximum RMS voltage of about 70 volts. (How much current it can put out at 70 volts determines its 70V power rating.) A power amp rated at about 600 to 650 watts into 8Ω, or about 1200 to 1300 watts into 4Ω, will fit this description.

A 100-volt amp, similarly, can put out a maximum RMS voltage of about 100 volts.
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Ken Freeman

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Re: Powering a 70v speaker system with a non 70v amp??
« Reply #8 on: September 17, 2007, 02:13:38 pm »

Yes, that would be watering the garden with a Fire Hydrant.

Far and away, my favorite 70 Volt amp is an EX-4000....or MLA-1100 in a another form.

Will you build me 12 more on a special run?

Ken
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Jeff Heart

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Re: Powering a 70v speaker system with a non 70v amp??
« Reply #9 on: September 17, 2007, 02:29:17 pm »

Ken Freeman wrote on Mon, 17 September 2007 19:13

Yes, that would be watering the garden with a Fire Hydrant.


Ken


But I could do it right?  Despite Niagra Falls power, as long as I measure the output of the amp with pink noise and turn it up until it reaches 70v, and no more?
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Jeff Heart
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