...is it possible? What is the voltage that drives a PA soundsystem? I presume it fluctuates +-5v? So if I took a feed from the speaker I could light an LED if I chose the right resistor.

Basicaly I want an LED to 'throb' to the beat, but powered from the output of an amplifier, is it possible? what calculations would i need?

Thanks.

 

you could also do it with no connection to the system, by using a coil stuck onto the side of the speaker to either drive the LED, or act as an input to an op-amp that drives the LED...

 

Interesting, but a bit technical for me

 

wind a coil of magnet wire... attach to an LED... heh

doesn't get much simpler than that :-P

you'll just have to experiment with how big a coil...

but i know it has to be possible. they make little stickers you can put on cell-phones that do just this, light an LED with a small coil when the phone rings. and given the large electric field generated by the coil in a PA speaker, you should have no problem getting a small LED to light... or feed it into some circuit that will light the LED

 

thats a VERY cool idea, im gonna ahve 2 give that a shot :lol:

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I would use a cap of a certain size to match frequency you are aiming for in series with a resistor to adjust amperage then to the LED to ground. High voltage caps are needed. With that kind of amperage a highpass filter is needed to drop dB and voltage/current. zener diodes can even come in handy to make the Led react the way you want. A step down tranformer will work but the cost??

Try a 1.6 uF just a safe guess if not lower, but must be 50V+ paper or mylar. Think like this load is like a tweeter.

Using an inductor is asking for noise to be introduced to the system.

I forgot to metion that the positive terminal is the signal, so basically this makes a signal ground.

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The voltage is going to be considerably higher than that!.

The formula for power is W=V*V/R.

So V = Square Root(W*R).

Assuming your system is 600W RMS into 4 ohms (which if it doesn't actually state RMS is probably a great deal less!) it works out at 49V RMS.

Which is about +/- 59V using your original method.

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it's 600(rms) watts into 8ohms , so that would be 69v +-?

 

No, 69V RMS, you need to multiple by 2.828 to get the p-p voltage, it works out at 195V, giving +/- 97V voltage swing.

This is a pretty fearsome HT rail!, as it's specified into 8 ohms, rather than 4 ohms, I'm presuming it's probably a bridged amplifier? - two 300W 4 ohm output stages bridged to give 600W into 8 ohms.

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How often, if ever, will the amplifier be operating flat out, at maximum power? Do you want the LED to light the same as the amp's clipping indicator? Does it ever light?
You could choose a series current-limiting resistor (and of course, a reverse diode across the LED) today, but tomorrow somebody turns the sound up and blows the LED (and your ears).
Maybe it would be best to have a current source limiting the current to the LED and a voltage-divider so that it is active only above say, 60W.

 

A bridge rectifier, a series resistor*, a transistor C and E to return
Across the transistor C & E, connect an LED and a resistor(to return). The value of the resistor will be 0.6 divided by LED current. Connect the drop voltage to the base and emitter through a series resistor of 100 ohms.

The excess current will be bypassed by the transistor.
Select appropriate value for the resistor* and the transistor to suit the resulting current.

Gajanan Phadte