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



## mcsoul (Jul 23, 2008)

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


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## 89grand (Nov 23, 2006)

mcsoul said:


> 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?
> 
> ...


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.


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## aznboi3644 (Jan 25, 2009)

edit: beat me to it lol


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## Aaron'z 2.5RS/WRX (Oct 24, 2007)

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


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## mcsoul (Jul 23, 2008)

Aaron'z 2.5RS/WRX said:


> 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


That must be why only the crossovers where for sale as-is. I hope (and think) the crossovers survived.


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## mcsoul (Jul 23, 2008)




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## 89grand (Nov 23, 2006)

Aaron'z 2.5RS/WRX said:


> 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



Actually, you don't have to be clipping and sending DC for the light bulb to protect the tweeters. Even with clean power, if there is too much, the bulb does it's job. The light bulb is matched to the tweeter and the protection you want based off it's rating. It's not just a random small bulb.


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## 14642 (May 19, 2008)

Aaron'z 2.5RS/WRX said:


> 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


Has nothing to do with DC and clipping isn't DC.

The light bulb has a positive temperature coefficient--that means it's resistance rises as it gets hotter. The harder you drive the speaker (through the bulb) the brighter the bulb gets, which prevents all that power from reaching the tweeter. Nice as a protection device and doesn't affect the sound as long as the bulb doesn't light.


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## Lanson (Jan 9, 2007)

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.


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## AceX (Dec 15, 2008)

MB Quart uses this same set-up


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## mjgonegm (Jun 21, 2008)

I would make a small foil heat shield if i were you.

But yeah he had to being pushing some serious Power LOL


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## fredridge (Jan 17, 2007)

I put a set of speakers in the youth room at a church that had this feature, you could see the light through the port.... was easy way to tell the kids to turn it down


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## mcsoul (Jul 23, 2008)

AceX said:


> MB Quart uses this same set-up





fourthmeal said:


> 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.


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## chad (Jun 30, 2005)

fredridge said:


> I put a set of speakers in the youth room at a church that had this feature, you could see the light through the port.... was easy way to tell the kids to turn it down


Peavey uses a unique scheme where there's a polyswitch and a light bulb in parallel that's in series with teh horn, the lamp does not engage till the polyswitch opens up


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## zwc0442 (Oct 24, 2008)

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.


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## Oliver (Jun 25, 2007)

mcsoul said:


> 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?
> 
> ...


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


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## Lanson (Jan 9, 2007)

a$$hole said:


> 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 ].
> 
> ...


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.


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## MarkZ (Dec 5, 2005)

Aaron'z 2.5RS/WRX said:


> 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


Nope, there's no DC produced. A square wave (which is what a clipped signal is often approximated as) is still AC.


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## sqshoestring (Jun 19, 2007)

MarkZ said:


> Nope, there's no DC produced. A square wave (which is what a clipped signal is often approximated as) is still AC.


That might be the technical description, but it depends on where you set the time division on the scope. Square wave is still laying significantly more power on the voice coil.

I was going to say, a light bulb becomes a resistor at a certain level of current then limits the maximum allowed. Much different than a fuse that cuts a constant current level but often lets an overage pass for a short time.


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## thegeck (Sep 25, 2008)

You see light bulbs in xovers all the tmie in pro audio gear....its like active protection without effecting sound quality


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## MarkZ (Dec 5, 2005)

sqshoestring said:


> That might be the technical description, but it depends on where you set the time division on the scope. Square wave is still laying significantly more power on the voice coil.
> 
> I was going to say, a light bulb becomes a resistor at a certain level of current then limits the maximum allowed. Much different than a fuse that cuts a constant current level but often lets an overage pass for a short time.


Yeah. But it doesn't have much to do with clipping. The light bulb is to protect against overpowering the tweeter, clipped or unclipped. And unlike a fuse, it's reversible.

It's not a bad idea, assuming you can get a light bulb that exhibits similar power/time/duty cycle properties. I wonder how precisely they're designed in that regard.


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## thegeck (Sep 25, 2008)

double


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## hc_TK (Jan 18, 2006)

wouldnt you also dampen the peaks of the music? 
in other words, loose dynamics?


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## Stealthsounds (Feb 9, 2009)

i have never seen this before !!!!! its a great idea, can all crossovers be converted then ?

thanks


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## 14642 (May 19, 2008)

chad said:


> Peavey uses a unique scheme where there's a polyswitch and a light bulb in parallel that's in series with teh horn, the lamp does not engage till the polyswitch opens up


 
Hey Chad,
I don't know that I'd call that unique. Our car audio crossovers use this same kind of circuit, but instead of a light bulb, we use a resistor to attenuate the tweeter by a specific amount. 

The reason to use the bulb is that its resistance depends on temperature rather than strictly on the input current. The time that the current flows through the bulb increases the temperature. That means that huge high-frequency musical peaks don't contribute much to heating the bulb, so transients are, for the most part, unaffected. 

If the bulb is placed between the amplifier and the crossover filter, the terminal load of the crossover isn't affected by the increasing resistance of the bulb so the blub doesn't affect the crossover's response. However, placing the bulb there in the circuit means that it'll see low frequency transients as well, which may light the bulb and limit the current to the tweeter unnecessarily. Placing the bulb after the crossover provides the correct frequency response until the bulb lights, but at really high input power, the crossover changes and depending on the circuit, can sound really ugly, even though the tweeter won't blow.

This is why we use the polyswitch and the resistor, placed either before or after the filter, depending on it's effect on the frequency response.


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## Aaron'z 2.5RS/WRX (Oct 24, 2007)

89grand said:


> Actually, you don't have to be clipping and sending DC for the light bulb to protect the tweeters. Even with clean power, if there is too much, the bulb does it's job. The light bulb is matched to the tweeter and the protection you want based off it's rating. It's not just a random small bulb.





> Has nothing to do with DC and clipping isn't DC.
> 
> The light bulb has a positive temperature coefficient--that means it's resistance rises as it gets hotter. The harder you drive the speaker (through the bulb) the brighter the bulb gets, which prevents all that power from reaching the tweeter. Nice as a protection device and doesn't affect the sound as long as the bulb doesn't light.





> Nope, there's no DC produced. A square wave (which is what a clipped signal is often approximated as) is still AC.


 
Thank you, thank you and thank you, Sirs..

I just love correcting old, incorrect teaching/thinking...


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## ECM (Dec 23, 2007)

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.


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## sqshoestring (Jun 19, 2007)

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.


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## MarkZ (Dec 5, 2005)

hc_TK said:


> wouldnt you also dampen the peaks of the music?
> in other words, loose dynamics?


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?


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## imjustjason (Jun 26, 2006)

Threads like this are the exact reason that I joined this forum. There is no where else that I know if that this kind of knowledge can be gained as easily.

Thanks!


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## ChrisB (Jul 3, 2008)

chad said:


> Peavey uses a unique scheme where there's a polyswitch and a light bulb in parallel that's in series with teh horn, the lamp does not engage till the polyswitch opens up


lol, that is what I was getting ready to say. My old Peavey SP4Gs had light bulbs in the crossover.


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## chad (Jun 30, 2005)

06BLMUSTANGGT said:


> lol, that is what I was getting ready to say. My old Peavey SP4Gs had light bulbs in the crossover.


Yep and they sold an upgrade kit to do it for their other boxes.


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## hc_TK (Jan 18, 2006)

i would rather biamp the speaker..


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## mcsoul (Jul 23, 2008)

hc_TK said:


> i would rather biamp the speaker..


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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## Robb (Dec 5, 2008)

Hi guys, my kicker resolution crossovers have these bulbs in them, 2 bulbs in them.
I'm assuming 1 is for the tweeter and 1 is for the midrange ?
Is it still possible to blow a tweeter though with these crossovers ?

Also what should I have the tweeter attenuation set at ?


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## chad (Jun 30, 2005)

Probably both for the tweet, wired in parallel, depending on the bulb you have to add multiples. Community, back in the day, used to just use hand fulls of them all strapped together.


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