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[70sRocker]

Our football stadium has 5 horns mounted outside the press box.
We HAD been using an old amp that had 70v and 4 ohm outputs.
Both seem to work, but the amp overheats.

How can I tell if the speaker array should be connected to 4 ohms or to 70v???

I need to know in order to buy the right amp.
(While the amp is working - it sounds fine!)

Please direct me to the right forum if this post is misplaced.

[Michael 'Bink' Knowles]

70sRocker wrote on Tue, 14 September 2004 15:30

Our football stadium has 5 horns mounted outside the press box. We HAD been using an old amp that had 70v and 4 ohm outputs. Both seem to work, but the amp overheats. How can I tell if the speaker array should be connected to 4 ohms or to 70v??? I need to know in order to buy the right amp. (While the amp is working - it sounds fine!) Please direct me to the right forum if this post is misplaced.

The forum you need is the install forum. But I've used 70v systems for temporary live gigs and the info is common between both worlds.

Most likely you have 70v horns and will need a 70v amp. However, you have to ask yourself why the amplifier is overheating... If it has been overheating ever since day one then someone screwed up and the amp should have been more powerful. But if the overheating is a recent development then you might have a cable or speaker problem or you might have an amp problem. Getting a new amp could be an expensive way to find out that the old amp WASN'T the problem. How will you explain to the money folks that the new amp is overheating just like the old one?

The best solution is to hire a firm that has a lot of experience in 70v installations and have them go through the whole system to see what is going on. Money well spent! Good Luck -

-Bink

[Mac Kerr]

Since a 70 volt system implies high impedance speakers, you can measure the impedance of the speaker lines. If your amp is specified to provide 100 watts at 70 volts, the minimum impedance of the speaker lines is about 50ohms. You can check out other combinations at:

http://www.angelfire.com/pa/baconbacon/page2.html

When you change the "wattage" taps on 70 volt speakers so that they add up to the power of the amp, you are really adjusting the impedance so that the amp delivers it's rated power at 70 volts.

Mac Kerr

[Al Limberg]

Just to expand a little on Mac and Bink's good advice,  you may find that your 70v horns have taps at perhaps 1,2,4,8,15,30, and 60 watts.  If you set them all to 60 watts but your amp delivers less than the aggregate wattage,i.e., less than 300 watts in this example, then your amp will probably overheat (or worse).  Best to check the amp's specs then climb up on the roof and check out the tap used on each horm.

HTH,
Al

p.s. Careful on that roof!!!!

[70sRocker]

Thanks all!

I'll try to check the taps (?) though I know nothing about that.
I've only used band PAs!

Do I understand that if it's a 70v system, the ohmage should test near 50 ohms?  
If it's 3.6 or so, does that mean it is NOT 70v?

TIA

[Al Limberg]

3.6 would be awfully low for a 70v system.  To expand a little more, on a 70v system, each driver will have a transformer inside the housing.  Typically (Atlas for instance) there will be a clear plastic cover over a circular pattern of screws on the back side of the driver.  One screw will be labelled 'Common' and the rest will have a wattage figure and/or an impedance figure.   According to the Ohm's law calculator that Mac pointed you to, a combo of 70v and 3.6 ohms would be asking your amp for over 1300 watts so I doubt seriously that figure would be correct.  On the other hand, you don't mention if you are measuring just one driver or all 5!  More info please.


Al

[70sRocker]

Last I checked it was about 3.6 ohms on the wires coming into the pressbox. (from all 5 horns)

I don't know how the horns are hooked up.  They aren't over 30 feet total distance.

I always wondered why its sounds about the same on the 4 ohm or 70v output???

Is checking the ohmage on the wires conclusive?

[Mac Kerr]

I don't know about conclusive, but if all 5 drivers are working, they are probably 16 ohm drivers. It is unlikely they want to see more than 20-30 watts each, so you want a 100-150 watt amp, and you want to use the 4 ohm output.

What kind of amp do you have now? How hot is overheating? Amps can be pretty hot to the touch and still be within their operating limits.

Mac Kerr

[70sRocker]

I've been plugging into 70v.  It overheats after 30 to 40 minutes.

Maybe it SHOULD be run at 4 ohms.

It's a "bogen" type 100 watt amp.
Since I'm not looking at it right now, I think it says US Sound or something like that.  It's your standard - medium-quality amp.

Soundtronics?  Maybe University Sound???

[John Roberts {JR}]

In a 100W utility amplifier the 70v tap is essentially the 4 ohm output passed through a voltage step up transformer. Connecting a low impedance speaker load to this high voltage tap could work until the amplifier overheats (may current limit prematurely).

While the advice to actually measure the load impedance sounds good, to do it properly you need an impedance meter. A typical VOM will only measure the DC impedance of the voice coils or transformer windings and may not accurately represent true audio band impedance.

One question that comes to mind, how does the system act if you connect the speakers to the 4 ohm tap? It will obviously not get as loud but does it deliver a workable level? Driving a 70v line from a 4 ohm tap would be pretty puny, driving a 4 ohm line from a 70V tap would kick ass for a very short time   .

JR

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