Passive Crossover Catching on Fire!

[Paul Marossy]

Sorry, I put this in this subforum because I know it will be viewed more and I'm hoping to get an answer on this...

A Carvin speaker at the church I attend had the passive crossover actually catch on fire in the middle of a church service a couple of weeks ago. These speakers are supposed to be able to handle 400 watts continous/800 watts peak. We don't ever get close to even the continuous rating. Anyhow, the sound guy pulled the speaker down, pulled out the passive xover PCB, and found that the inductor had actually melted off of the PCB and (caught on fire). Does anyone know what could cause such a thing?! Is it possible that an ultrasonic feedback occurred somehow and made things on that PCB overheat, or would there be some other cause?

[R.G.]

Your church really rocks if you WERE close to continuous rating! 

A plausible cause is that the power amp feeding the speaker may have gone into self oscillation and burned it out.

Check out the power amp very carefully before hooking it all up for church again. Especially check it for DC offset voltage on the output and with an oscilloscope for high frequency oscillation.

If the power amp has MOSFET power transistors this last is not trivial. I have a 100MHz rated Tektronix scope that I use for testing. I once had a power amp that was oscillating at 300MHz and I could not see it on the scope. A 20MHz scope doesn't have a chance. You almost need an RF power meter to find things like that, and even then the RF meter has to have good high frequency response for an RF meter.

[Noplasticrobots]

Have it exorcised.

[blanik]

maybe the cross over was reversed? 
lol
R.

[Paul Marossy]

Thanks RG, I figured it was some kind of ultrasonic oscillation thing, but I wasn't sure of exactly where to look.

[Dragonfly]

...and here *i* was, wondering what the schematic for that "effect" was....

the old "burning crossover in the church" trick, eh ?

[Dragonfly]

Your church really rocks if you WERE close to continuous rating! 

Especially check it for DC offset voltage on the output ...

that was my first thought.....ive seen it happen a couple times in the past....

[Paul Marossy]

The sound guy also thought that it might be DC on the output, but wouldn't you be able to hear something funny going on if that were the case?

[R.G.]

Maybe - but maybe not.

If the amp is running into a low resistance load that it can still supply without burning up and only putting out a little signal, then it might not be running into the current or voltage limits and might sound OK.

Audio power amps are funny. If you have one with marginal stability, it can be set off into "singing" by the wrong speaker cable. Or it may only oscillate at certain output levels.

[Paul Marossy]

Thanks RG, I'll relay that to the sound guy. 

[boogietube]

I once used a shielded cable for mains and fried the power amp- according to a sound guy friend-this was a very bad thing to do.lol

[idlechatterbox]

QUOTE:
"Have it exorcised"


that should work, as long as one remembers that you never put out an electrical fire with water, even HOLY water....


on a more serious note, two thoughts:
1. you guys are talking about this phenonomenon as though it's not that rare. Is it? I always thought that you had to have serious current in order to combust a PCB, and that you'd simply blow a fuse or fry a reasonably thin wire before that ever happened. Just curious.

2. your church setup cranks 400w?
things have changed since I was in the choir... 

[Paul Marossy]

your church setup cranks 400w?
things have changed since I was in the choir...

No, we're probably half that - if even that.