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Old 19th June 2009, 07:14 AM   #1
FerisB is offline FerisB  United States
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Question Intercom sound system help

This one school I work at has on old intercom that died. At other schools we have used 100w amp / 70v output > line matching transformers > 8 ohm speakers, which I understand. But this school has 5w 45ohm speakers, about 40 of them. 40 @ 5w would be 200 watts, snap crackle pop. Is there a way I can use 8ohm multi tap transformers that I have on a 45 ohm speaker ? How does that change the watt rating. I know if you connect at 25v, the watt ratings are different ont the multi taps.

I'm So Confused ! Thanks !
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Old 21st June 2009, 03:21 AM   #2
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I'm sure there is a way, but for now I can't remember how that stuff works... Did you do a search on google (or my favorite, altavista) for 70V system? It will give you technical info on what the theory is.

I can tell you this: Yes, 40 speakers running at 5W each is demanding ofr an amplifier, however, I'm pretty sure that the amplifier is never to MAX volume... Probably 3W all the time. So, 40X3W is 120W, not that big of a deal. Was it a Dukane amplifier (that old system)?

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Old 21st June 2009, 04:12 AM   #3
imix500 is offline imix500  United States
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Do you have access to an impedance meter or bridge? There are no step down transformers at all?
There's bound to be people on here who installed or maintained a system like you have. I know a guy who definitely would have- my father. I'll ask him and report back if someone doesn't chime in.
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Old 22nd June 2009, 12:25 PM   #4
brig001 is offline brig001  United Kingdom
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I haven't dealt with anything like this, but I would split them into two parallel circuits of 20 speakers in each (2.25 Ohms) and connect the two circuits in series (4.5 Ohms). Should be easy enough to drive from a standard amp.

The only reason I can see where transformers would be needed is if different volumes were required from different speakers, but I don't think standard 100V line transformers would be suitable because they would step down too much, even at maximum. You could certainly try one without risk to amp or speaker, or wire a 50 Ohm 10W resistor in series with any that needed their output reducing.

HTH,
Bri.
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Old 22nd June 2009, 02:20 PM   #5
star882 is offline star882  United States
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Default Re: Intercom sound system help

Quote:
Originally posted by FerisB
This one school I work at has on old intercom that died. At other schools we have used 100w amp / 70v output > line matching transformers > 8 ohm speakers, which I understand. But this school has 5w 45ohm speakers, about 40 of them. 40 @ 5w would be 200 watts, snap crackle pop. Is there a way I can use 8ohm multi tap transformers that I have on a 45 ohm speaker ? How does that change the watt rating. I know if you connect at 25v, the watt ratings are different ont the multi taps.

I'm So Confused ! Thanks !
Connect the speakers in parallel in groups of 5, which will yield 9 ohms of impedance, which will work with your transformers. (I guess it is a tube amp as it uses transformers?)
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