My sub cought on FIRE!!!

[Mark B]

Hey everyone,
Sub stopped working last week. I couldnt get the cone to move when I pushed on it. So I took it out of the box last night & found that the accordian looking part of the sub thats near the voice coil was burnt. Like an actual fire had turned it to ash. The wires run across this to get to the coil & it looks like they were the cause of the fire. Was I just running too much Juice? Or would something else cause this?

[Jeff_in_Atl]

Most speakers are burned out from distortion not being too loud. When the volume is turned up past the point where the amp can put out DC to the speaker it starts "clipping" and putting out AC to the speaker. This causes the windings to heat up and burn through. Although I've never heard of one catching on fire before. Usually it can be remedied by using a larger amp although a cheap amp can put out AC too. I would rather have a Kenwood 50 watt amp than some off brand 200 watt amp.

[raroZ28]

I would rather have a Kenwood 50 watt amp than some off brand 200 watt amp.

yup, my buddy has a 1000w rockford fosgate amp, another friend has a 2000w no brand amp from a trade center. probably paid like 200 for it with a set of subs and a box. The rockford pounds so much harder. sounds much better too.

[Mark B]

The amp was a JBL 500.1 Not the best.. But decent.

I did have it cranked up though.

The only thing that probably stoped the fire was the lack of oxygen in the box. I think I made the box around .5 cu./ft.

[Joe 1320]

Did you at least sink his battleship?

[MN6WS6]

"past the point where the amp can put out DC to the speaker it starts "clipping" and putting out AC to the speaker"

You have that backwards. Amps put out AC to the speaker all the time, DC is when it clips.

It sounds to me like the weakest link gave way. Sub locks up, amp continues throwing voltage at the sub, burn baby burn. You might want to check your amp and see if the motherboard has any heat damage. If it got hot enough to burn the suspension of the sub, it probably heated up the board as well.

[Jeff_in_Atl]

"past the point where the amp can put out DC to the speaker it starts "clipping" and putting out AC to the speaker"

You have that backwards. Amps put out AC to the speaker all the time, DC is when it clips.

I don't think so. I think I have it right.

[Kevin - Blown 95 TA]

Music is normally AC on a DC offset as it goes into the amp, then the AC is decoupled from the DC coming out of the amp. Otherwise the coil would quit moving back & forth pumping air and just be a static electromagnet heating up. When the input levels to the power amp get too high (like EQ turned up all the way), the transistors are swamped causing them to run in the saturation region then it clips off the peaks and puts out a continuous signal at a high level. This DC is bad because the amp & speaker drivers aren't mean't for contunuous high current and trying to dissipate that much current over a period of time heats them up - (this is true if the AC current is too high also from too big an amp or impedance mismatch), whereas during normal operation they can stand a few peaks because they get time to cool off in between. Think of it as duty cycle. Just like if you have too small injectors on your car and you overheat them by running a 100% duty cycle and then they burn a little bit and don't run right after that.. So, if the speaker drivers don't have heavy enough magnet wire, they can heat up, & burn off the insulation and short out. Then that's bad for the amp, but most of thosse have breakers or fuses protecting them.

[Jeff_in_Atl]

Music is normally AC on a DC offset as it goes into the amp, then the AC is decoupled from the DC coming out of the amp. Otherwise the coil would quit moving back & forth pumping air and just be a static electromagnet heating up. When the input levels to the power amp get too high (like EQ turned up all the way), the transistors are swamped causing them to run in the saturation region then it clips off the peaks and puts out a continuous signal at a high level. This DC is bad because the amp & speaker drivers aren't mean't for contunuous high current and trying to dissipate that much current over a period of time heats them up - (this is true if the AC current is too high also from too big an amp or impedance mismatch), whereas during normal operation they can stand a few peaks because they get time to cool off in between. Think of it as duty cycle. Just like if you have too small injectors on your car and you overheat them by running a 100% duty cycle and then they burn a little bit and don't run right after that.. So, if the speaker drivers don't have heavy enough magnet wire, they can heat up, & burn off the insulation and short out. Then that's bad for the amp, but most of thosse have breakers or fuses protecting them.

I have never heard anything like that before.

My understanding is that the radio puts out a DC current one way which moves the speaker one way then it switchs the current the other direction which moves the speaker the other direction. (Magnets work on DC not AC.) That is what makes the sound. When the amp starts clipping it puts out AC current which makes the speaker stop and vibrate at one position causing it to heat up and burn through.

http://electronics.howstuffworks.com/speaker5.htm

"the current going through the speaker moves one way and then reverses and flows the other way. This alternating current causes the polar orientation of the electromagnet to reverse itself many times a second. "

That is a bad way of saying it. The current alternates but it is a DC current and it switches at a changing rate. AC is a fixed rate of switching like 60 Hz. If you ran a a AC current into a speaker you would blow the speaker.

BTW the company I work for has an abreviation of EMS which stands for ElectroMagnetic Sciences which was our first name.

[fastTa]

MN6WS6 and Kevin are right. Sinusoidal or alternating current (AC) is an electrical current whose EMF varies periodically in magnitude and direction. AC is just DC that cyclically changes direction and magnitude. Electromagnetic fields can be generated by either AC or DC. In the case of an audio speaker AC is what is normally supplied to the voicecoil or electromagnet.

Hook up an oscillscope to any car audio amplifier and you will and should see a nice clean sine wave (AC) if the amp is not experiencing clipping. When the amp does start to experience clipping, you will see the top and bottom of the wavforms go flat. This will normally induce a direct current which is never good for a speaker.

Usually when the amplifier clips, it is indicative of the amp's output's trying to generate a signal greater than that of the amp's supplying current will allow. Clipping can also be caused by the input signal being too high relative to the supply current of the amp. You'll see the cone push outwards if the polarity is normal, inwards if reverse. As little as 1V DC can damage a speaker. Start going over that for any length of time then throw in the effect with a direct coupled midrange and your woofer is toast.

[Kevin - Blown 95 TA]

That is a bad way of saying it. The current alternates but it is a DC current and it switches at a changing rate. AC is a fixed rate of switching like 60 Hz. If you ran a a AC current into a speaker you would blow the speaker.

BTW the company I work for has an abreviation of EMS which stands for ElectroMagnetic Sciences which was our first name.

I know what you are trying to say - You are getting caught up on definitions. If you alternate the current's polarity, then you have AC, but it depends on your ground reference. If you switch a DC source with an H bridge, say +150V, you can get a modified sine wave which is, again, AC - this is how your plug-in inverter works off your car battery, but it has a floating ground. AC can have a DC bias. AC can also vary in frequency, it doesn't have to be fixed at all, this is how transistors work in your amp. They put the varying frequency AC signal onto a DC voltage level set by a voltage divider to match the quiescent point of the transistor and this is the basis for all amplifiers.

You can put AC into a speaker just fine. Hook one up to a signal generator at low voltage and change the frequency - the speaker will change it's tone. You hook it up to the wall socket and it will blow up because infinite amps into 8 ohms is a crapload of current. The coils will be set up for certain frequencies, and other frequencies will not work. If you have a crossover, then you have to have all common grounds the same and grounded to the amp.

[Jeff_in_Atl]

OK I see what you are saying. I see how you say it is AC. I just don't look at it the same way. It's a "controlled" AC if you will. A friend of mine showed me a speaker and how to blow it on an O scope. You could see the current pulses go wacko when it starts the process of blowing up due to distortion. He took a clock radio and plugged it into a mid sized speaker and blew it up. Of coarse this was also about 25 years ago.

[Kevin - Blown 95 TA]

Sure, the AC is modified a lot. The radio broadcasting station has a big transmitter - amp and mixer - which combines the music signal (AC) with a high frequency carrier wave (also AC) onto a dipole antenna which sends the music out as an electromagnetic wave into the air. This station, say like Laser 102.9MHz for instance, and this modification of the carrier wave is called Frequency Modulation (FM). The carrier frequency is modified by the music signal. When it gets to your radio, the antenna picks up the weak signal, it is amplified, the carrier frequency is demodulated, and the varying AC music signal is processed and sent to the output stage and then the speakers.

[Jeff_in_Atl]

Sure, the AC is modified a lot. The radio broadcasting station has a big transmitter - amp and mixer - which combines the music signal (AC) with a high frequency carrier wave (also AC) onto a dipole antenna which sends the music out as an electromagnetic wave into the air. This station, say like Laser 102.9MHz for instance, and this modification of the carrier wave is called Frequency Modulation (FM). The carrier frequency is modified by the music signal. When it gets to your radio, the antenna picks up the weak signal, it is amplified, the carrier frequency is demodulated, and the varying AC music signal is processed and sent to the output stage and then the speakers.

I have a ham radio license.

[Kevin - Blown 95 TA]

I have a ham radio license.

That's a fun hobby. You get to talk to people all over the world. Had friends that were into it. I never had a ham radio, but I built a shortwave when I was a kid and I could pick up radio stations from places like Russia, Ecuadore, Peru, etc. It was fascinating.

Anyway, didn't mean to give you TMI. Just making examples of the use of AC in transmission. Of course, nowdays, there's digital from satellites, and digital media seeing widespread usage. Sure cleans up the signal.