Overheating Question ... I think?

[mlbarlow]

OK - So, I was out at a gig recently, and about 40 minutes into the gig, sound stopped coming out of my amp ... I'll spare the details, but I tried to think through all the possible issues, and deduced the problem was the head, and not some other part of my setup.

I took it to a repair guy, he thoroughly checked it out, and told me there was nothing wrong, other than my state of being 50 bucks poorer.

I had it out at a couple of other gigs ... all was well.

Next gig, after about 40 minutes of playing, my sound just kind of faded away ... I could still hear myself, though faintly. My full volume returned after a minute or so. Happened one or two other times. The input light was registering an adequate signal entering the head.

I took it back to the repair guy. He can't get it to fail, but for that matter, I can't get it to fail unless I'm at a gig, playing at a moderately loud volume for 40 minutes straight. The repair guy thinks it might have something to do with my cabs - that
I'm not using the correct impedance, or that they are wired improperly.

I've got two Avatar cabs - both four ohms. My head is rated down to 2. In both instances in which I've had problems, I've only used one of my cabs. I have never had a problem with this setup prior to these recent events, and I've had the same gear for a couple of years.

So, could it really be my cab? It hasn't endured any drop or anything like that, and no one has messed with the wiring ... I'm at wits end here.

[David1234]

It's interesting that you had the problem with just one cab, and not with 2.

Still, here's a possibility: legend has it that many neo speakers lose magnetic strength when they overheat, so you might actually have a speaker heating problem. When you use one cab, maybe you push it harder than when you have 2 cabs. Was it a warm stage when you had the trouble?

Maybe you need to test with your bass feeding back at loud volume (low notes, using distortion ... that's fun ... and turn the treble down and the bass up. The distortion evens out your volume for you). If you can make one cab go quiet, immediately switch to the other cab and see if it's still fine. If so, go back to the quiet cab (which will have cooled a little) and see how it is, and how long it takes to go quiet again.

Just guessing, by the way ...

[lowfreq33]

What kind of amp is it?

Is it in a rack? If so, What kind?

Are you using proper speaker cables?

Did the tech test it with your cabs?

[mlbarlow]

Thanks for the ideas, guys.

It's a Kustom 1200HD.

It's in a 2-space SKB rack.

The wires I'm using seem fine ...

And no, the tech hasn't tried it with my cabs, though that would make sense.

[craig.p]

You could be getting what's known in the auto-repair business as a "wall job." Ask your tech:

a) what impedance dummy load he's using;
b) what frequency/frequencies he's using from his audio generator;
c) what voltage drop he's seeing across the load when the test is in progress.

That oughta rattle a few apples off the tree.

[Zooberwerx]

Well, sounds thermal....unit heats up, stuff expands, contact is lost. How would a cold / bad solder joint react in this type of situation?

Riis

[SPQR]

Originally Posted by David1234

legend has it that many neo speakers lose magnetic strength when they overheat, so you might actually have a speaker heating problem.

This is no legend, but it is not a temporary issue. Neo magnets have a curie temperature that is much lower than ceramic. The curie temperature is the point where the internal energy is high enough to allow the magnetic domains (little groups of crystalline structures) to unalign and become randomly oriented. So when neo gets hot, magnet no work - until it is remagnetized. This doesn't sound like the OP's problem.

And if the speakers heating up was the issue it wouldn't take 40 minutes. This would either happen in a few minutes or the speakers would break after 40 min.


This sounds like a thermal limiter in the amp that is protecting you from damaging it.

[Implosion]

Originally Posted by SPQR

This is no legend, but it is not a temporary issue. Neo magnets have a curie temperature that is much lower than ceramic. The curie temperature is the point where the internal energy is high enough to allow the magnetic domains (little groups of crystalline structures) to unalign and become randomly oriented. So when neo gets hot, magnet no work - until it is remagnetized. This doesn't sound like the OP's problem.

And if the speakers heating up was the issue it wouldn't take 40 minutes. This would either happen in a few minutes or the speakers would break after 40 min.


This sounds like a thermal limiter in the amp that is protecting you from damaging it.

And the curie temperature for neodymium is over 300 Celsius / ~ 550 Fahrenheit (if my memory serves me correct) so I seriously doubt that this would be the problem.

[Petebass]

Originally Posted by Implosion

And the curie temperature for neodymium is over 300 Celsius / ~ 550 Fahrenheit (if my memory serves me correct) so I seriously doubt that this would be the problem.

Correct. So unless your gig is on Mars, this is not going to be the cause of your problem.

That particular amp does not appear to have a cooling fan. It looks like cooling is via the vents running down the side of the chassis. By putting it into a closed 2RU rack case, you've inhibited it's ability to dispel hot air from those vents.

Testing my theory is as simple as removing it from the rack case at the next gig and see if it shuts down after 40 mins. If I'm right, it's time to grab a drill and turn the side of your SKB into Swiss cheese. I've done this many times before and the trick is not to drill into handles of anything that might ruin the structural integrity of the SKB rack.

[SPQR]

300C isnt that hot for a speaker coil, it only takes a few hundred watts. It is possible under "normal working conditions" to demagnetize a neo motor - but most manufacturers SEEM to have been able to manage heat fairly well and avoid the problem. Personally, this remains to be seen.

[Petebass]

Originally Posted by SPQR

300C isnt that hot for a speaker coil, it only takes a few hundred watts.

300c at the coil isn't possible but even if it was, you'd been to get the coil even hotter than that to get the magnet up to 300c. Talk to any large PA company and they'll tell you speakers sometimes need replacing during concerts. Yes they get hot to touch, but not hot enough to burn you. Skin burns at much less than 300c.

[BassmanPaul]

It's a good idea to leave a blank space above a power amp to aid in cooling. To test out whether the rack is causing the problem, take the amp out of the rack and try to replicate the problem. If you cannot, get a larger case.

Paul

[mlbarlow]

This is just a shot in the dark, but could the electricity at the venue have anything to do with this? Both times it's been at the same venue, but that's been the only place I've gigged for a while.

Anyway, I'm trying to get it back from the tech right now ... he lives about an hour away, and is hard to contact. So, once I get it back, I can try some more diagnostics.

[SPQR]

Originally Posted by Petebass

300c at the coil isn't possible but even if it was, you'd been to get the coil even hotter than that to get the magnet up to 300c. Talk to any large PA company and they'll tell you speakers sometimes need replacing during concerts. Yes they get hot to touch, but not hot enough to burn you. Skin burns at much less than 300c.

I hate to stray off topic, but this is still somewhat related.

What you have is anecdotal evidence. Just because you know a magnet structure (which is composed of much more than just a coil or magnetic material) doesn't get to 300C does not mean the coil can not be at 300C or above. The thermal mass of steel in the motor is a MUCH larger mass than the coil.

Look at is this way, lets assume a speaker can have 3dB of power compression at "full power" (actually "full voltage"). 3dB would mean half the power, which would mean for a given voltage we would have twice the dc resistance. The thermal coefficient of copper and aluminum is 0.0039 K^-1 given by the following equation:

R(absolute temp) = R(ambient temp)*(1+a*delta(T)), delta(T) being the change in temperature from ambient

So, for every 2.5 degrees C we get a 1% increase in resistance (2.5*0.0039 is about 0.01). So if we had 3dB of power compression, which is certainly likely, our resistance would have doubled and the temperature would have gone up by 250C. Now if the magnet structure is somewhere around 100C - this would become part of the ambient temperature and the coil would be at 350C.


I dont need to ask any large PA company, large PA companies ask my company's technical services dept.

[Petebass]

Yes but you're talking about the temp of the voice coil, and that will heat up equally for ferrite and neo speakers. Neo doesn't lose it's magnetism until the magnet, not the coil, reaches aprox 300c. How hot would the coil have to get before the magnet reaches 300c?

Not possible!

[SPQR]

Originally Posted by Petebass

300c at the coil isn't possible

Originally Posted by Petebass

Yes but you're talking about the temp of the voice coil

Winner: Me.

Originally Posted by Petebass

Not possible!

Now I'm tired of playing your game.

[Petebass]

I'm not playing games. I'm just pointing out that for neos to lose their magnetism, the MAGNET has to hit 300c. You're going on about the temp of the voice coil hitting 300c. They're two different things.

[B-string]

Question? What is the failure temp of the varnish on the voice coil windings, the combustion temp of the paper cone and what temp are the glues used rated for? Interested is the reason for the questions. Ceramic magnets fails at what temp.?
What is the heat transfer co-effecient for amb. air at any given airflow (the voice coil is not in direct contact with the stucture if it is still operational).

[SPQR]

Originally Posted by Petebass

I'm not playing games. I'm just pointing out that for neos to lose their magnetism, the MAGNET has to hit 300c. You're going on about the temp of the voice coil hitting 300c. They're two different things.

You don't seem to want to believe what i'm saying so i'm done convincing. To be clear though, I dont mean to say its very likely the entire magnet will get to curie temperature and it will stop working completely. It is possible for an inside-slug Neo motor to have just the outer edge reach a high enough temperature to be affected.

Then we can talk about the change in coercivity as temperature goes up, but you know about that right? You hang B-H curves of materials on your bedroom walls.

Originally Posted by B-string

Question? What is the failure temp of the varnish on the voice coil windings, the combustion temp of the paper cone and what temp are the glues used rated for? Interested is the reason for the questions. Ceramic magnets fails at what temp.?
What is the heat transfer co-effecient for amb. air at any given airflow (the voice coil is not in direct contact with the stucture if it is still operational).

Most of the properties are highly dependant on what your working with. Coil insulations come in all sorts of ratings depending on the manufacturer. Cone combustion would be heavily dependant on damping material present and there are many people doping cones/spiders/coils with fireproofing chemicals.

These are really good questions, but they are pretty wide open as far as answers go.

[BassmanPaul]

I would imagine that if a voice coil reached 300˚ that the cone would be on fire!

Paul