sundance-communications.com/forum Forums Wiring - Telecom Carriers - Trunks & Services Security, Alarms and Paging Amp Size and Number of Horns

I'm not too strong on paging design. I know it depends on the amp, and I'm trying to figure out how big the amp must be if I have x number of horns.

We'll be using a Bogen amp (model yet to be determined). The customer has told me they have some 20 watt horns, no taps or adjustments. They also said they have another model horn but I don't have that info yet. I'm hoping we're looking at something like a Bogen SPT-15A.

Can I just add up all of the watts for each horn? If I have 5 - 20 watt horns = 100 watts? If I have 5 - SPT-15 = 75 watts? So together I should use a 200 watt amp?

How many horns per wire run?

Thanks for your help.
Jim

The customer has told me they have some 20 watt horns, no taps or adjustments.

Well, right off those sound like straight voice coil 8 or 16 ohm speakers. They are not something you can use unless you are only using one or two on an amp by themselves.

The Bogen SPT-15 is what you do want to use. They have a built-in line transformer which allows many speakers to be used with one amp. You would use the 70.7 volt output and simply daisy chain your wiring around to your speakers, however is easiest. You usually won't need more than 18ga with 70.7 volts either unless you have really long runs or a lot of power. Make sure you use the proper wire- CL2 for class 2 wiring (it will say on the back of the amp by the outputs) and not lamp cord or "speaker wire" from HD or Radio Shack.

At each speaker you select the tap or wattage (I belive it's a screwdriver adjustment) according to how loud you want the speaker to be.

Since power is cheap, to be on the safe side use the maximum wattage of your speakers even though you may not be using the 15w tap on all when you calculate your amp wattage. Add all your speakers up then add about 10% as a safety factor to determine your amp wattage. You can't go wrong with bigger is better philosophy either.

Remember, it's better to use a lower wattage setting on the speakers and crank the amp up to just below clipping to get the volume level you want than to set each speaker at max and turn the amp down. This way you can add speakers without making any adjustments to the existing speakers or going to a bigger amp.

Try the lowest wattage tap first and turn the amp level up. You may find that you don't need as many speakers as you think. Those horns are very efficient so they are loud!

-Hal

I agree with everything Hal said, but I typically leave 20% on the amp for future expansion. But to each their own...just remember not to load the amplifier to it's power rating...always leave some extra spare power.

Yeah, I should have said at least 10% but I think you would be very lucky to find that your calculations land exactly at the amp power less 10%. Like I said, power is cheap and you usually find that the next size amp up from what you require is probably twice what you require.

-Hal

Hal pretty well covered it. I usually set the horns at about 2-3 watts each to start and try to only use 60-70% of amplifier power. If the 20 watt horns are voice coil only you can use them with a external transformer.

If the 20 watt horns are voice coil only you can use them with a external transformer.

They used to make a transformer housing so that you could do just that. Unfortunately I haven't seen one in 20 years. Hanging a bare transformer off a horn IMO is just a hack job at the least and not possible if the horn is outdoors. Tell the customer that those horns they have can't be used. Do the job right and save yourself the aggravation.

-Hal

Thanks for the great info Hal, and everyone.

I found out the customer provided horns are Algo model 1185. I don't know how they came to have them, or why. I'll recommend a Bogen model and make the connections and settings as mentioned above.

Thanks, Gentlemen.
Jim

Hal, if you had to do it, couldn't you mount the transformer on a blank plate screwed to a 4" square box, similar to what sparky uses for a doorbell transformer? All the wires could be neatly spliced inside. Just a thought.

I would get a big enough box and put it inside with the proper connectors or bushings for the cable in and out. You also have to think about where and how you are going to mount it next to the horn. But now you're talking extra work and speakers with transformers cost about the same as those without. I suppose if the guy had 50 of those speakers that he wanted to use it might be worth it but how many are we talking about here?

-Hal

I could never understand why the transformer housings disappeared. As a substitute, I’ve used plastic project boxes with the metal covers. Transformer bolts onto the cover; easy enough to drill a hole and insert a grommet for the cable.