Adding a PA feature to a siren not currently equipped?

[losangelescop]

Hi board,

Wondering if at all it can possibly be done. What I'm interested in is whether or not a PA function can be added (either internally or externally) to a remote mount siren which was not designed for PA use (long shot, I know). Examples of such units include the Carson SA-361, Federal MS4000U, etc etc.

I ask based on curiosity, but if this could be done It would be very beneficial for upgrading. It would need obvioulsy the mic attached to the siren amp or an extra user supplied amp, then through to the siren speaker. However, it would also be beneficial (for obvious reasons) to somehow wire it so that if the siren is on and you push the PTT, it stops and overrides the siren until you release the PTT.

Again, I am not looking to do this personally, Just wondering if it could be possible (hopefully easily) at all. Besides my own curiosity I know a couple of VFF and others that may find the information useful.

Nic.

[thebigphish]

I doubt it could be done "easily" and i also doubt that it would be worth doing for anything other than a really really RARE antique piece of apparatus that absolutely cannot have the siren head upgraded. I mean how much money are you going to spend on a new PA head? About a hundred thirty around these parts...at that point, i look at a new Whelen HFS295L100 for $199 and just put it together that i would put in a new head, controller & siren driver for a price that is similar & MUCH easier. And what does a 361 cost, a hundred bucks? Makes sense to me to just sell the 361 and upgrade to a much smoother install of a HFS295

If you look at what you would have to do, i ask you to consider the following. If you want to run two drivers into one speaker, you have to consider backfeed. What happens when both drivers are running? You say you want to override one with the other, sure this is possible with some relay logic between the PA PTT switch and the siren outputs, but then you have to wonder how that 361 driver is going to tolerate going from full load to no-load - and then right back to full load while the siren is rippin' and you are shouting "PULL TO THE RIGHT!!!" at the soccer mom in the volvo? In terms of the mechanics of what you will have to do, i see $199 for a fully functional head & driver as the MUCH easier install rather than some mickeymouse work to "save money" and do it yourself. I would be willing to bet that if you mapped out the cost of doing it yourself, and installing your own standalone PA and then all the electronic work you wanted to try, you would definately bust the price of a new unit with all your requested features...in one package.

and a warranty.

(for when you let the smoke out)

(...if you do)

[jackhackett]

It wouldn't be impossible, all they do in a siren with PA is switch the input of the power amplifier from the siren signal to the PA mic signal when the mic is keyed.

Where you'd probably run into trouble is the biasing on the power amp. In siren mode they run them well into class B bias, which gives the efficiency needed to get 100W with little more than the chassis as a heat sink, but distorts a voice signal pretty badly. When in PA or Radio ReBroadcast modes they usually switch the biasing on the output transistors to run closer to class AB, giving less distortion. This would require a good knowledge of power amps, and unless you've got a pretty good idea of what you're doing you could kill a siren pretty quickly.

Unless you really had some good reason to try it, I'd go along with upgrading to a siren with PA built in.

[losangelescop]

Thanks guys!