I hooked a 12 volt incandescent bulb up to my stereo and got it to light up

Lightbulb responding to music   song is Move Along by the All American Rejects

the light bulb is simply responding to the varying frequency,and current.

There is actually voltage there!

There is actually voltage there!

yes loudspeakers are driven by low voltage and relatively high current,depending on the rated power of the amplifier.

In my case 150 watts per channel, what's that equal in voltage?

im not excatly sure but my guess is that voltage shouldnt be more that 12 or 24 volts.

I'll have to try this with a different bulb one of these days, just to see what happens!

The voltage output depends on the loudspeaker impedance that the amplifier is designed to be operated with. A higher impedance equals higher voltage - for example if it's rated 150w @ 8 Ohms this will provide a higher voltage than an amplifier rated 150w @ 2 Ohm.

Of course the best amplifiers to use for this purpose are the 'constant voltage' ones that are used for PA systems - these can output up to 100 volts, which might (depending on ballast type) even light up a fluorescent lamp!

constant voltage systems allow multiple speakers to be connected to one amplifer,but the sound quality is terrible,and the system is prone to magnetic interference.

I use 12V lamps as test loads for testing amplifier repairs, saves listening to a constant tone for hours during test. Plus is a quick test for noisy switches and loose connections, as well as being cheap and easy to replace if they blow. A 12V 20W lamp is perfect for amplifiers up to 50W, as it will light up easily, but blows if there is any fault very quickly.

The 100V line amplifiers are best tested with a mains lamp ( I have no 110V lamps, so use a 220V one instead, it just glows dully) and using a discharge lamp is most likely to result in a blown amplifier, as most do not handle non linear loads that well.

constant voltage systems allow multiple speakers to be connected to one amplifer,but the sound quality is terrible,and the system is prone to magnetic interference.

I agree on that. Especially when the speakers are of that horrible 'megaphone horn' type, what a horrible sound particularly if music is being played through them 

But for the purpose of lighting up lamps - which is what we are more interested in here of course - CV amps are great. Not only do they provide the voltage but they can be bought much cheaper (especially on ebay) than the 'hi-fi' amps, which is a good thing if you want to experiment with amps and lamps where there is a risk of blowing them!

However, a better option if you are serious about making lamps flash in sync with music is to use a 'light organ' circuit to drive an incandescent lamp directly from the mains in response to an audio signal. This will provide the same effect but MUCH brighter and with no risk of damage to the amplifier or speakers.

In my case 150 watts per channel, what's that equal in voltage?

Ok for one, it is NOT 150W/channel RMS. That "150 watts per channel" is a peak rating. RMS power is probably somewhere around 20-40W/c for that thing. May be even less. Many of them are 15W/c RMS, yet say things like "300 watts total power" on them.

And if you wanna know the voltage.. why not just hook your speakers up and test the voltage on one of them with a voltmeter (set on AC volts) with music blasting? Won't hurt anything.

Like SeanB~1, I've used 12V lamps as dummy loads for amplifier testing/repair before. Cheaper than big resistor dummy loads (or "heaters") and good visual indicators of faults. They can be series-paralleled for larger amps just like "heaters" or large wirewound resistors.

when i do this experiment,i use a 250 watt 4 ohm subwoofer amplifier,which is also good cause the current output is high.