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Does anyone have a schematic or instructions on how to build a small speaker capable of being used for testing siren units. I want to build one so that I can bench test several units that are going out of service and don't want the full volume of the units Any help on this is appreciated.

Try putting a regular siren speaker inside a small box, then pack the small box inside a larger box with styrofoam peanuts, or perhaps foam rubber. You might have to "layer" several boxes this way, but you should be able to get the volume down to a "comfortable" level in this way.

We use an atlas driver without the cone or bell on it. I fabricated a cap for the threaded part and then mounted it in a plastic box filled with foam rubber. drilled a hole for test leads and stuck it under the end of the shop bench. Its still loud but tollerable.

Here at the shop we just took a standard speaker and tied a 100watt light bulb in series with the siren output. It allows us to check the output level and hear it at the same time. when the siren hits full wattage the light gets brighter

techtonics wrote:Here at the shop we just took a standard speaker and tied a 100watt light bulb in series with the siren output. It allows us to check the output level and hear it at the same time. when the siren hits full wattage the light gets brighter

That's about what I was going to suggest: using a load resistor with a small speaker or headphone (NOT WORN!) in parallel. I'd just wonder about the resistance of the bulb vs. a power resistor, as the bulb's resistance is going to vary as the bulb goes from cold to hot.

Wowbagger wrote:

techtonics wrote:Here at the shop we just took a standard speaker and tied a 100watt light bulb in series with the siren output. It allows us to check the output level and hear it at the same time. when the siren hits full wattage the light gets brighter

That's about what I was going to suggest: using a load resistor with a small speaker or headphone (NOT WORN!) in parallel. I'd just wonder about the resistance of the bulb vs. a power resistor, as the bulb's resistance is going to vary as the bulb goes from cold to hot.

That would make sense, its been a while since we built our test box, heh it is an old dispatcher speaker with a load resistor, pot. and with a 100w old school traffic light bulb on it.

I've thrown together a circuit that might do the job, I haven't tried it so you'd have to experiment a bit. Resistors and control should be available through Radio Shack.



The six 8ohm/20W non-inductive resistors form a 12ohm/120W load, and the other resistors and speaker pick off a bit of the audio. The load resistors should get quite warm, so you'd have to take that into consideration in the construction. No guarantees, try it at your own risk.

One thing to remember if you try Jackhackett's design is that all resistors have to be, as Jack said, non-inductive. This means that they can not be wire-wound. 20 watt, non-inductive resistors may be a bit hard to find, and may be a little expensive, but Jack's circuit is a proven concept. There are companies that make such things for use with electric guitars, to accomplish the same thing you are trying to do. They call them a "power brake".

Radio Shack part numbers:
271-120 8ohm/20W NI resistor $2.49each
271-135 100ohm/2W resistor $1.99/2pack
271-265 25ohm/3W pot $3.99

The 100 ohm ones aren't non-inductive, but it shouldn't matter.
The speaker part of the circuit could stand some tweaking, I just went for some values I figured would cut the power to a speaker down enough. You can do some calculating and use other values, figure with it tapped across 1/3 of the load you'll get around 12volts AC with 100watt siren.... study up on Ohm's Law and get out the calculator.

120watts worth of resistors is cutting things a bit close... should be okay for short tests, but you wouldn't want to run a siren into it for any length of time.

I actually did make something similar to this for my guitar amp, didn't sound so great (lost a bit of treble, the pro ones must add some caps or something) but it helped when practicing in an apartment with 100W Amp!

jackhackett wrote:Radio Shack part numbers:
271-120 8ohm/20W NI resistor $2.49each

OK, I stand corrected on availability. They look like a wire-wound to me, but I guess appearances can sometimes be deceiving. On a side note, you won't be able to speak any language but English through the PA with it hooked to a circuit built with the Radio Shack resistors. According to the specs on Rat Shack's website, these resistors only support English!

haha.. english only resistors.. didn't notice that. I did find that they have the 2W resistors listed as 10W, while the image shows the 2W value on them.

Technically the non-inductive ones are wirewound, but they fold the wire in half before winding it, which cancels out most of the inductance, not perfect, but useable.

That's a easy circuit...will have to look into that.

What about something in-line pad the volume between the siren amp and a 100w speaker, so both can be bench-tested together for function? I have a need for such a rig. I'm looking for something that I can put in a project box and have an input and output.