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Why light bulb rather than a fuse in a passive crossover?

34K views 35 replies 21 participants last post by  chad 
#1 ·
I have a set of ARC Audio ACS-265 (Rainbow) crossovers and
they use a 12v 20w light bulb to fuse the tweeter rather than a fuse.
Why do they do this?

The previous owner blew one of them. They must have been running
a lot of wattage because the cases are slightly melted near the bulbs :eek:
 
#2 ·
A light bulb is actually a lot better than a fuse as it will start to take the excessive power, using it to light the bulb and creating more resistance in the circuit. I think there are two advantages over a fuse. One, a fuse will just pass all the power until it's rating has been somewhat exceeded and it finally blows, sending all that power to the tweeter before hand, where as a light bulb takes most of the excess power and then uses it to light the bulb and creat more resistance which than lowers the power going to the tweeter. Number two, the light bulb can be used over and over again since it doesn't blow and leave you tweeter-less until it's replaced like a fuse does.

If someone burnt out the light bulb and burned the crossover case, they were getting real stupid with that set of speakers.
 
#4 ·
The sound signal going to a component is AC.... start clipping and you push DC... DC comes down the line, light bulb absorbs an amount of it, acting as somewhat of a filter and/or a "slow blow" fuse... (someone please correct me if I'm wrong here, it's how I was taught)

Sounds like the previous owner was pushing things WAY to hard
 
#9 ·
I have a pair of Type X Alpine components (the older version) that have this same protection feature, and after letting a family member borrow them in a vehicle for some time, they came back to me with their cases bubbled due to the light's heat!

Luckily the tweeters are just fine.
 
#13 ·
I have a pair of Type X Alpine components (the older version) that have this same protection feature, and after letting a family member borrow them in a vehicle for some time, they came back to me with their cases bubbled due to the light's heat!

Luckily the tweeters are just fine.


Do either of you know the value of your bulb?

I'm starting to think 12v 20w is too small, I'm going to pair
these with tweeters that handle 75w rms. The crossover
point is an acceptable 3.2khz.
 
#15 ·
I was running a set of qsd 216's and the first time I seen that light come on it scared the crap out off me. I just had my rear seat flipped down and I saw some flashing coming from my trunk and I thought I was going to have to put out a fire.
 
#16 · (Edited)
A lightbulb is a load, it uses power up ! [ safe and easy use of an idiot light... it lets someone know something is wrong !]


cheap amps clip as you try to get the volume you want [ clipping causes more power under the bridge...kewl , now your amp is making twice the rated power !! ].

downside you blow tweeters with only 25 watts on them and think they are junk :( { the tweeters ].

Lightbulbs that melt thru the plastic case are exposed to clipping ~ [ too much power , over long time periods ].

Excellent for bragging rights by moronic individuals who would be better off playing with Legos ;)
 
#17 ·
cheap amps clip as you try to get the volume you want [ clipping causes more power under the bridge...kewl , now your amp is making twice the rated power !! ].

downside you blow tweeters with only 25 watts on them and think they are junk :( { the tweeters ].

Lightbulbs that melt thru the plastic case are exposed to clipping ~ [ too much power , over long time periods ].

Excellent for bragging rights by moronic individuals who would be better off playing with Legos ;)
In my situation with the bubbled Type-X crossovers, I think it happened due to just too much power overall. He was using my DCX 300.4 bridged...probably a bit too much power looking back on it.
 
#29 ·
It depends. The resistance of a bulb is very nonlinear. And some bulbs have different curves than others. So it's certainly possible to choose a bulb that doesn't have an appreciable resistance until the "right" amount of current is delievered. That's why I said earlier, it can be a very good idea if the design is implemented correctly (and the right bulb/circuit is used to sort of mimic what the speaker is going to do). I'm skeptical that that's the case in most designs, but what do I know?
 
#27 ·
hc_TK
"wouldnt you also dampen the peaks of the music?
in other words, loose dynamics?"

Actually, yes, you do lose dynamics. The light bulb acts as a compressor of sorts due to the temp coefficient mentioned earlier. It's easy to see as the light gets brighter during peaks and starts limiting power to the tweets.

Clipped AC doesn't "push" DC. You can "create" a square wave by adding all odd harmonics of a fundamental signal. A clipped signal is just the summation of the fundamental sine wave + 3rd harmonic + 5th harmonic + etc.

This site "http://en.wikipedia.org/wiki/Square_wave" has an animation of every odd harmonic being added to the fundamental wave.
 
#28 ·
Push DC may be a bad term, but I imagine most are getting at the point that if you look at the area under the curve; a clipped square-ish wave signal holds significantly more power like DC does than a correct musical wave. It will cause any power limiting device to act sooner and will heat any device more. Correct me if wrong.

Not my area but isn't clipping and harmonics/square wave two different things? I know they are similar in result, I've seen both on a scope. I only say that because a given amp will only develop X voltage on the rails and then it clips, so the only way for it to deliver more power is deliver it longer via deformed clipped wave. Now you would have to be at the point your thermal device activates, but the increase in average power would be more clipped than than a moderately larger unclipped wave right?

Here is a page that explains it nicely in the form of heat dissipated: Measurements of AC magnitude : BASIC AC THEORY

Now you might get more music volume out of a clipped wave I don't know guessing it depends on the driver some, but I do know it will not sound as good (as opposed to the proper larger waveform you could use that generated the same amount of heat.)

I should try to test that on a scope some time, see what the waveform is after a bulb. I bet it looks a lot nicer than a clipped one.

I guess in the end all this proves is that you would get the most clean output through a bulb to your driver without clipping the amp, or using an amp large enough not to clip when the bulb lights. In fact if it clips with the bulb lit, then you would have sound distortion from the clipping AND the bulb, not just the bulb.
 
#34 ·
Oh I am, but I like having the flexibility to change things up whenever I want.

$9 shipped for some decent crossovers is hard to pass up, but I can't really
say until after I test them.

For instance I may throw my tweeters in my wife's car off of these crossovers
with some 4 ohm mids (my mids are 8 ohm right now).
 
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