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

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?

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

Click to expand...

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.

edit: beat me to it lol

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

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

Click to expand...

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

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

Click to expand...

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.

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

Click to expand...

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.

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.

MB Quart uses this same set-up

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

But yeah he had to being pushing some serious Power LOL

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

AceX said:

MB Quart uses this same set-up

Click to expand...

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.

Click to expand...

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.

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

Click to expand...

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

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.

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?

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

Click to expand...

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

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 ].

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

Click to expand...

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.

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

Click to expand...

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

MarkZ said:

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

Click to expand...

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.

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