Need advice on replacement Air Core Inductor for crossover...

[dominic54321]

My crossover fried this weekend in a monitor speaker for my bands PA system. The woofer and tweeter seem to be okay with a battery test. The only thing I noticed was a fried Air Core INductor. I need to replace it with an 0.24mH 18awg Inductor but can't find the exact match, could someone point me to a good match. Should I get one that's slightly larger or smaller? Oh, the speaker cab is rated for 175rms. The inductor that fried was for the tweeter. Any help would be appreciated..thanks!

[dominic54321]

Also, what would cause something like this? Amp clipping? Bad capacitor? The amp does have a limiter which was 'on' at the time with very very minor clipping every once in a great while.

[wcriley]

Parts express has this:
http://www.parts-express.com/pe/psho...number=266-310
The wire's a little heavier, but the inductance is the same.

They also have .25mH in 18 guage which might be close enough. It's also cheaper. Someone who knows more than me will need to chime in as to how much difference it would make to the Xover frequency.
http://www.parts-express.com/pe/psho...=255-402&DID=7

[christw]

How powerful is the amp you were running into it?

[dominic54321]

Originally Posted by christw

How powerful is the amp you were running into it?

550watts at 4 ohm. I had 2 8ohm monitors rated at 175RMS each with the power amp one turned up half way at that point with very very minimal clipping every once in a while. I really think this club had bad voltage/voltage drop going on. We've used them for years, with the same setup run at full, and everything ran perfectly.

[billfitzmaurice]

If the coil is part of the tweeter high-pass it's virtually impossible to blow, and if you somehow manged to do so it would have also blown the rest of the filter components and the tweeter. So it's probably the woofer low pass, and 18 ga is only good for maybe 100 watts.

[christw]

Originally Posted by dominic54321

550watts at 4 ohm. I had 2 8ohm monitors rated at 175RMS each with the power amp one turned up half way at that point with very very minimal clipping every once in a while. I really think this club had bad voltage/voltage drop going on. We've used them for years, with the same setup run at full, and everything ran perfectly.

Your amp is running 275 watts RMS per cab and then some if you're driving it to clipping. Where the knob was is irrelevant. If it was clipping, it was reaching past it's rated output. You need to either better attenuate your moniters volume, use a smaller amp, or get moniters with better power handling. Fixing the crossover will just be a fire extinguisher against a house fire. It might work for a little bit but it won't fix the real problem that you're overpowering the cabs.

[Jerrold Tiers]

I don't think that is such a bad overpowering...... the peak to average ratio on most PA is low, I agree, but usually 10 dB or so, unless it sounds pretty squashed. The limiters on the amps might squash it a tad more.

A 500 watt/4 ohm amp is 250 W in an 8 ohm speaker rated 175W...... not a problem for most, although the tweeter may be the most likely element to suffer. Speakers are not "calibrated" that well, they don't reliably fail at a specific power in the general case.

The most likely scenario, assuming the tweeter OPENED, is that the tweeter did that FIRST, removing the load from the filter. Then the unloaded tuned circuit (filter) would be a very low impedance to ground at or near its crossover frequency.... The coil would get a lot of current whenever the signal had energy in that range, and very possibly would burn up. The capacitor might have some damage too, maybe not, or you would have mentioned it.

if the tweeter SHORTED, the coil could have been fried by small amounts of DC, or by the LF that would be passing through it (it blocks only HF). The coil would have an impedance of only 3 ohms at around 2 kHz. Not much at all around 100 Hz.

[dominic54321]

Thanks for all your suggestions guys. I don't think it's a bad power ratio either, considering we've been using this setup for years with NO problems. I suspect that the club had voltage drop, the amp was getting VERY hot, and would clip at the same level it usually would be running fine. That's why I had the amps attenuators dialed back to half.

[billfitzmaurice]

Originally Posted by dominic54321

I suspect that the club had voltage drop, the amp was getting VERY hot, and would clip at the same level it usually would be running fine. That's why I had the amps attenuators dialed back to half.

Even in the most severe case that would not toast a coil.

[dominic54321]

So, before I pull the trigger and buy a replacement Air Core Inductor, I'll obviously be testing the capacitors. The Air Core Inductor is 0.24mH, 18awg, DCR is unknown. Would it be ok to replace it with a .25mH 18awg DCR=.22 like this one....
http://www.parts-express.com/pe/psho...=255-214&DID=7
It's rated for 300watts.

[dominic54321]

Originally Posted by billfitzmaurice

Even in the most severe case that would not toast a coil.

Thanks Bill. I respect your comments. I've been reading your posts for years.
Now that I think about it, when I was first setting up that night, one of the 1/4" cables was knocked half way out of the monitor from someone tripping over it. The power amp was on. After a few seconds of realizing it, I quickly plugged it right back in. Do you think that could have been the culprit. The speaker didn't act up until about an hour later.

[billfitzmaurice]

Originally Posted by dominic54321

Thanks Bill. I respect your comments. I've been reading your posts for years.
Now that I think about it, when I was first setting up that night, one of the 1/4" cables was knocked half way out of the monitor from someone tripping over it. The power amp was on. After a few seconds of realizing it, I quickly plugged it right back in. Do you think that could have been the culprit. The speaker didn't act up until about an hour later.

Impossible to say. Without seeing a schematic of the crossover one can't even say if it's in the highpass or lowpass section. But I have seen pics of coils in some Fender cabs that were badly toasted. They were low-pass coils for the woofers, and seriously under-gauged.

While you're in there get rid of the 1/4", replace them with Speakon.

[dominic54321]

Originally Posted by billfitzmaurice

While you're in there get rid of the 1/4", replace them with Speakon.

Yeah, I was thinking the same thing! ;-)

[Jerrold Tiers]

The 1/4" problem likely had nothing to do with it.

You said that the inductor "was for the tweeter"..... how do you know that?

You also implied that the coil was overheated/burned, normally "fried" would be meaning that, but ....... Are you sure the coil is actually bad?

Reasn I ask is that there are some reasons for using an air-core vs an iron core, largely that they don't "saturate", meaning they don't change inductance as current increases. That's usually more important for the woofer.

And , if the speaker is 8 ohms, and the coil happens to be in the woofer and NOT the tweeter circuit as you originally said, then resistance may be an issue.

The resistance of the replacement coil you mentioned, IF in series with woofer, would lower the "damping" factor to about 35, if you are into that questionable spec.

Assuming most of the 175W is woofer power handling, then the mentioned replacement coil could see almost 5 watts of dissipation (power loss) at max input. Depending on coil physical size, that could be an issue, although generally average current is much lower than max. You are, however admittedly hitting the speaker with some extra power..

[dominic54321]

Originally Posted by Jerrold Tiers

The 1/4" problem likely had nothing to do with it.

You said that the inductor "was for the tweeter"..... how do you know that?

You also implied that the coil was overheated/burned, normally "fried" would be meaning that, but ....... Are you sure the coil is actually bad?

Reasn I ask is that there are some reasons for using an air-core vs an iron core, largely that they don't "saturate", meaning they don't change inductance as current increases. That's usually more important for the woofer.

And , if the speaker is 8 ohms, and the coil happens to be in the woofer and NOT the tweeter circuit as you originally said, then resistance may be an issue.

The resistance of the replacement coil you mentioned, IF in series with woofer, would lower the "damping" factor to about 35, if you are into that questionable spec.

Assuming most of the 175W is woofer power handling, then the mentioned replacement coil could see almost 5 watts of dissipation (power loss) at max input. Depending on coil physical size, that could be an issue, although generally average current is much lower than max. You are, however admittedly hitting the speaker with some extra power..

Hmmm, there are two inductors. I followed each on the board. The inductor that melted is for the tweeter, the other for the woofer. The inductor is housed on a plastic spool, it's not just the wire. The plastic is what melted, but so bad that it is rendered unusable because it melted in and around the windings making it impossible to pull off without messing up the windings.

Both the woofer and tweeter are 8 ohm.

It says on the circuit board...0.24MH 1mm(or 18awg) but doesn't state the DCR value. If I knew the DCR value, I could try to replace it with one that was close. Most 0.24mH 18awg inductors that I have seen so far are around .19 to .24 DCR. I'm hoping the one I get doesn't change the specs too drastically. thanks

[billfitzmaurice]

Originally Posted by dominic54321

Hmmm, there are two inductors. I followed each on the board. The inductor that melted is for the tweeter, the other for the woofer.

As I already noted the only way enough current could pass through the shunt coil in the HP filter to melt anything is if something else was blown as well.