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  1. #1
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    Adding a light parallel to existing loudspeaker circuit

    I work on a ship which has Public Announcement (PA) system. PA system consists of a couple of zones. Each zone has an a controller, amplifier and a loudspeakers circuit.
    In our case, PA system is only used to make announcements and play alarm, not music.

    What I need to do is to add a light indication which will be activated when, for example, zone A is activated (meaning that loudspeakers in zone A are producing sound).

    I can definitely do it by using one of the control outputs from the controller, but it would take a lot of cable pulling. It would be much more simple if I could just
    connect a light parallel to one of the loudspeakers.

    Now... my concerns are:

    1) When I look at the drawing of one zone, the output from the amplifier to the loudspeakers is 100V. Am I right to suppose this is 100VAC "high" frequency (by "high" I mean higher than 50/60Hz), depending on the input signal?

    2) There is around 50 loudspeakers in a zone where I would like to add the light. Each loudspeaker has a marking 100V, 25W, 101dB. If I add a 220V 20W halogen light (which is purely resistive load) in parallel to the loudspeakers, am I going to disturb i any way the loudspeakers circuit or the amplifier (assuming the amplifier will not get overloaded)?

    3) Is it correct to calculate the approximate current through the individual loudspeaker I = P/U = 25W/100VAC = 0.25A?

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  2. #2
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    Re: Adding a light parallel to existing loudspeaker circuit

    Hi,

    a PA sound system that is used for alarms ... most probably must comply with safety standards.
    I donīt think itīs a good idea to modify a safety alarm system (unless you really know what you do and you know the safety standards)... because you will be responsibe for the proper function in case of an emergency.
    Donīt risk otherīs lifes.

    Klaus
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    Re: Adding a light parallel to existing loudspeaker circuit

    Doesn't look like a good idea to me.

    I won't want the flickering disco light appearance of a lamp driven by the speech signal. The passengers may be laughing on it.
    I would prefer a continuous light activated by a level switch and an off-delayed time relays.

    Consider that an incandescent lamp cold resistance is only 1/10 of the rated V/I value.

    Do you really have 50 speakers wired to 25 W each? Driven by a 1250 W power amplifier? 100V systems have often an option to run the speaker with less power. In any case the actual announcement power level is probably lower, causing only dim light of a 230V lamp.


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    Re: Adding a light parallel to existing loudspeaker circuit

    I agree entirely with earlier comments, however, if you really did want to modify it, a lamp across the loudspeaker would (a) drop the volume considerably and (b) flicker in time with the sound. The only reasonable technical solution would be to superimpose an inaudible signal on the wires and detect it at the loudspeaker end to turn a locally powered light on.

    Brian.
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    Re: Adding a light parallel to existing loudspeaker circuit

    On the other two websites he was told that an old fashioned incandescent light bulb is like a dead short circuit when cold and it would destroy the amplifier.
    Of course the speakers are not 25W/100V= 0.25A. Then the impedance of each speaker would be 100V/0.25A= 400 ohms, instead each speaker has a 100V transformer probably set to maybe only 2W for each speaker.
    The light bulb might not produce any light during the fluctuating levels of speech.


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    Re: Adding a light parallel to existing loudspeaker circuit

    There is a slim chance it's no harm if you steal some signal from the speaker, and run it through a converter to light up led's.

    Click image for larger version. 

Name:	200Hz 5VAC spkr signal 3 Villard stages light 4 orange led's.png 
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    This idea can only work if the output from the transformer is a few volts.
    Unfortunately little led's won't get anyone's attention, unless there's sufficient power so you can use hi-intensity 12V led assemblies, as in modern vehicle turn signals.



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    Re: Adding a light parallel to existing loudspeaker circuit

    Thank you all guys for your quick replies. From what I've seen, it seems much better not to try my idea, but to run the cable to the controller's control output.



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