Testing a Unitrol 800, need speaker connection advice

[StonedChipmunk]

I have a very weird Unitrol 800 that seems to defy the old installation manual in every way, shape, and form. The connections are all labeled differently and it doesn't even have a model number. I think it's one of their newer models (wow, that's charitable).

So I went out to the Radio Shack and purchased some average 100 watt speakers with the right resistance to prevent damage from some huge sales lady who obviously didn't have a clue about the thing and didn't know any of the product specifications of anything she was selling. Nevertheless, I hooked it up a few minutes ago but I'm resistant to test it out.

I hooked it up according to the (probably incorrect) manual and everything's ready, but there's a nagging feeling that I'm forgetting something important that might damage the speakers. They're small bookshelf-style speakers (according to their instruction manual) so I just keep thinking that flipping the switch might blow something in the siren when the speakers fail. The speakers are rated at 6 ohms 100 watts and the siren requires 6-9 ohms 75-100 watts... I'm not sure if there's anything else, but I don't really want to blow my first vehicle siren.

So, anything I could have missed here? Or should I go ahead and post a video to YouTube?

(Oh, and thanks again, Franz!)

[Franz?]

I'm looking at the same manual you have, and the answer is on page 5.

Wire the speakers in paralell, and connect to the "common" and "single" terminals on the back of the unit.

Turn the PA function on, and use that to confirm the speakers operate. PA generates much less audio that the siren generates.

After confirming the speakers work, try the siren function.

[StonedChipmunk]

I'm looking at the same manual you have, and the answer is on page 5.

Wire the speakers in paralell, and connect to the "common" and "single" terminals on the back of the unit.

Turn the PA function on, and use that to confirm the speakers operate. PA generates much less audio that the siren generates.

After confirming the speakers work, try the siren function.

Keep in mind that this is the manual that says "A SINGLE 100 WATT SPEAKER SHOULD BE CONNECTED TO COMMON AND DUAL." with the obvious illustration of the speaker connected to the Single terminal. And I'll be damned if I didn't buy a microphone for PA.

Also... does the siren automatically turn on when you switch it to position 3 (as it says in the manual)?

EDIT: Connected as you suggested, but I don't like the sound of this. When the power is connected to the system it has a resounding "thud" that pretty much shows the implications of using the siren indoors. I may have to postpone this test until tomorrow where I can get either a bunch of towels and duct tape or a reasonably open area on campus to test it. In the meantime, do you think that these sort of junky plastic speakers will work?

[Franz?]

Keep in mind that this is the manual that says "A SINGLE 100 WATT SPEAKER SHOULD BE CONNECTED TO COMMON AND DUAL." with the obvious illustration of the speaker connected to the Single terminal. And I'll be damned if I didn't buy a microphone for PA.

Look at Mr Schematic on the back page. The Com to Singl connection only uses half of the output transformer. Com to Dual uses all the output of the transformer and all of the aplifier's power.

No Microphone, no problem. The spec says the 800 will function with a carbon mike so wander about till you find an oldfashioned phone, and borrow the micrtophone cartrige. Let me know if it works.

Also... does the siren automatically turn on when you switch it to position 3 (as it says in the manual)?

Yes and NO, depends on the position the mode switch is in when the slide switch goes to position 3. If it's in "manual" the horn switch circuit must be closed to make the big bad noise come out.

EDIT: Connected as you suggested, but I don't like the sound of this. When the power is connected to the system it has a resounding "thud" that pretty much shows the implications of using the siren indoors. I may have to postpone this test until tomorrow where I can get either a bunch of towels and duct tape or a reasonably open area on campus to test it. In the meantime, do you think that these sort of junky plastic speakers will work?

Put the speakers in a large packing box full of styrofoam peanuts or popcorn. Never played with Radio Shack speakers, I have the real ones. I can definitely tell you the output will blow the filament of a headlight bulb, so they make real short lived dummy loads. Never tried a toaster, but that might work for a dummy load.

the book says the amplifier is capable of operating into a wide impedence range, from 2 ohms on up. It also says the speaker is the weakest link in the system. You'll know soon enough how those speakers work.

[StonedChipmunk]

Thanks for all the help Franz. Do you know if a resistor would work to lower sound output? That's what they do on YouTube for indoor testing, I think.

But yeah, I definitely won't find an old-fashioned phone anywhere around here. I might open up a 3.5mm jack wire and hook up my iPod to the mic connection. That should work, right? Or hook it up to my computer... either way. Hmm... that actually sounds like a pretty good idea...

[pyramid head]

I got my Unitrols today, very nice indeed. My units seem to follow the manuals pretty good, so the only problem I will face is finding a proper 100W speaker. All I have is a 15W speaker that came with my headache rack.

[Franz?]

OK guys, you can use a dummy load on the 800 amplifier, BUT, I haven't yet perfected building one.

In siren mode there is no output control on an 800 and they pump out full power from instant 1. Trust me when I tell you a headlight lamp is not sufficient to work as a dummy load.

I have a couple ideas, one involving some nichrome wire frol a laundry dryer, but I haven't built that yet.
Using wirewound power resistors is another possibility, but those are expensive.
I need to get around to doing some testing on various heat producing devices to determine the resistance they provide as a dummy load. Doubtless there is something cost effective that will work, its just a matter of finding something around 10 ohms resistance that will handle 100 watts. That way a small wattage speaker can be paralelled with the dummy load and it will not drop the impedence to a point that will endanger the oputput transistors.

[JasonC]

The guys at eLightbars use a 500ohm resistor in series with one of the speaker leads. That way, you can connect the output to a small, normal speaker and have it at low volume.

[Franz?]

The guys at eLightbars use a 500ohm resistor in series with one of the speaker leads. That way, you can connect the output to a small, normal speaker and have it at low volume.

500 ohms is only half of the rating information on a resistor. You also need to know the wattage of the resistor and have a resistor of sufficient wattage to discipate the output of the driver amplifier, or the weakest link in the audio circuit is going to fail.

Are you perhaps confusing the use of a 500 ohm potentiometer in a Tpad circuit normally used as a volume control with the load resistor to discipate the output power?

[JasonC]

500 ohms is only half of the rating information on a resistor. You also need to know the wattage of the resistor and have a resistor of sufficient wattage to discipate the output of the driver amplifier, or the weakest link in the audio circuit is going to fail.

Are you perhaps confusing the use of a 500 ohm potentiometer in a Tpad circuit normally used as a volume control with the load resistor to discipate the output power?

Whoops, forgot about that. A 5 watt resistor should be sufficient, but personally, I'd go for something a lot higher. And your right, I wouldn't be surprised if they blew, but I've never tried this myself.