Just caught my speaker on fire!

[philiparcario]

Re: Just caught my speaker on fire!

don't be cheap don't be stupid. 25w resistor is not right a pair of 10 watts is not right get these 50watters!



http://www.newark.com/vishay-dale/rh...w-1/dp/41K9167


I use 6 6ohm ones in parallel gives me a 300watt 1 ohm resistor I run a 500 watt sub amp into my dayton rss390 no matter how loud or how long or how deep the bass is my resistor circuit remains at room temp!

[markk]

Re: Just caught my speaker on fire!

In a typical two way with a 7", the bulk of the power is dissapated in the woofer. Given that a 2 way based on a 7" woofer will likely over excurt at 10-20 watts, the 10-20% of power that is dissapated in in the tweeter network means that even if the tweeter network sees 5-10 watts, it's hard to imagine a scenario where a resistor in the tweeter leg of this two way dissapates more than 5 watts.

Things might be different with a large two way or three way.



Look halfway down on one of Art Ludwig's page

http://www.silcom.com/~aludwig/EARS....nd_loud_sounds

The average power in the midrange network can be 20-60% of the power dissapated. In a three way playing loudly, say 50 watts is being pumped into the unit(essentially all of it turned into heat), then a possible worst case would have 30 watts are going into the midrange leg. So it's pretty easy to imagine that 5-15 watts would dissapate as heat in the resistor. It would be operating close to maximum, and, heat transfer would be limited if the resistor is glued onto wood, with one side bonded closely to the wood, and the other sides covered a bit with hot glue.

This is definitely an arguement in favor of an active setup in a 3 way or high power setup.

[Pete00t]

Re: Just caught my speaker on fire!

Did the burn start on the positive side or the negative?

[curt_c]

Re: Just caught my speaker on fire!

This is definitely an arguement in favor of an active setup in a 3 way or high power setup.

I'm not sure its so much an argument in favor of active as it is an argument in favor of proper component sizing and implementation. -Not that I don't recognize the positive attributes of an active design.

At the risk of being obtuse and redundant, I'll list some suggestions on getting the most power dissipation from a power resistor without becoming a fire hazard.

*Resistors will be cooled by convection. Mount them where they will have plenty of airflow potential. -Near the port vent area is not a bad idea.

*If you use the Aluminum housed Vishay/Dale or Arcol and you do not use a heat sink, be sure to derate their power capabilities accordingly. - About 30% of their rated power would be a good ballpark figure. More accurate derating specs can be found here:
http://www.mouser.com/catalog/specsheets/rhnh.pdf Obviously mounting them on a heat sink is a wise choice, but heat sinks are expensive. As an alternative, surplus CPU heat sinks are available for $5 or so, and can be modified to mount the resistors. -A dab of heat sink grease or similar is mandatory for optimum heat transfer.

*The ceramic sand cast resistors such as PE's are obviously not designed for heat sinking, but some transfer can be gained by rigidly mounting them to an aluminum plate or heat sink. Don't use glue or cable ties, but mechanically sandwich them between two plates of aluminum screwed together. -The overlap becomes the heat sink 'fins', so don't scrimp on the size too much. -Aluminum flat stock, channel, or square tubing are available at your local hardware store. The other concern here is to make sure the resistor leads are insulated from the aluminum.

*Heated air rises. Mount the resistor or heat sink so that any fins are orientated vertically and are unobstructed by other materials.

*As tempting as it looks, using the woofer magnet assembly or basket for a heat sink is probably not all that good of an idea. For one thing, magnet materials are not particularly good at transferring heat.

*If the crossover is mounted in a small sealed area, such as a base, temperatures can become an issue as well. Add some ventilation holes, or use an aluminum plate to transfer the heat from the inside out...

C

[Pete00t]

Re: Just caught my speaker on fire!

I love a mystery. Curt I dont think its your design. I do wonder if he went glue heavy on the the resister. Insulating it with the glue. Perhaps thats even silicon. If its exterior even worse. Causing a heat build up. My 2 cents . I dont have facts

[[email protected]]

Re: Just caught my speaker on fire!

im in the process of a 3-way build, and this problem had not accured to me. im glad you shared this with us. good tips curt. an aluminum heat sink is a very simple fix. i hope those with tatorials and this aspect.

[airtime]

Re: Just caught my speaker on fire!

Check the speaker drivers as well. See if they still have changed from factory specs. If you are cranking that much power you should consider exterior mounting for ventilation (and inspection?).


Again, check that amp. A warm cap??? I have seen amps let DC pass when the volume goes up. Under rated xover parts typically don't just melt! The cone coil just blows.

charles

[Pete Schumacher]

Re: Just caught my speaker on fire!

The average power in the midrange network can be 20-60% of the power dissapated. In a three way playing loudly, say 50 watts is being pumped into the unit(essentially all of it turned into heat), then a possible worst case would have 30 watts are going into the midrange leg. So it's pretty easy to imagine that 5-15 watts would dissapate as heat in the resistor. It would be operating close to maximum, and, heat transfer would be limited if the resistor is glued onto wood, with one side bonded closely to the wood, and the other sides covered a bit with hot glue.

This is definitely an arguement in favor of an active setup in a 3 way or high power setup.

Keep in mind that in this particular case, the shunt resistor is taking in as much power as the midrange driver.

[Dean1000]

Re: Just caught my speaker on fire!

Please use extreme caution if you play Talking Heads "Burning Down the House" :D

[markk]

Re: Just caught my speaker on fire!

Keep in mind that in this particular case, the shunt resistor is taking in as much power as the midrange driver.

Exactly.

I'm not saying anything bad about the ministatements in particular, just about the power handling issues with three ways in general.

I mean, think for a minute-you buy your emotiva xpa-2 and pump 500 watts into 4 ohms. The xpa-2 is massively heatsinked, with multiple output devices, to handle the heat generated. Now where is that 500 watts going? At most, drivers are 1-2% efficient at converting the electrical energy into acoustic energy. So 490 watts of your xpa-2's output has to be dissapated as heat. That can really only be in two places-the voice coil and the resistors. (ignoring the small contribution of other resistive elements, i.e. the R of the inductors etc...)

Now you're not really putting 490 watts into your VC and resistors...but you get the drift. In a three way, especially a low impedance/4 ohm 3 way, if played loudly, care needs to be chosen in regards to heat dissapation.

Active speakers do get around this limitation quite well. See john k's article on hybrid designs. You don't even need multichannel amplification to address this.



You have to read the article to fully appreciate the graph. It would be even more pertinent for a three way (the graph is for a 2 way).

I'm not against resistors and passive designs. But they can waste more heat than is commonly realized and this should be incorporated into the design (If part of the design target is higher average spl levels...)

[davepellegrene]

Re: Just caught my speaker on fire!

Just to clarify for a few that are not sure. I had the peg board OX mounted with a 1/4" space between the board and the MDF cabinet side. The resistor was mounted tight to the board with two zip ties. Hot glue was applied over the zip ties covering about half of the resistor. Then two inches of foam was applied directly over the ox. I was feeding them from my Yamaha amp using low level outputs. to my GX3 that is rated for 450 watts at 4 ohms. The GX3 was set two notches down from full, roughly 90%. The Yamaha amp at the time of the smoke was around -5dbs. Not sure how that plays out with the low level output, but at -5dBs on the hi level outputs there would have slight audible signs of clipping. I am always careful to back the volume down if I hear the slightest bit of distortion or clipping.
Something I just found out from my daughter, who was sitting in the sweet spot the hole time :eek: said she heard, what sounded like a driver bottoming out a couple times in the last few minutes. Thought it was ok since I didn't do anything about it. I was in and out on the deck several times cooking steaks so I didn't hear it. So obviously I was driving the whole system just a bit to hard. :o

So I guess the real fix here would be a second set of Mini's and another amp. :D I'm sure my wife will agree. :rolleyes:

Here is the foam that was on top of the XO.




Dave

[Brewski]

Re: Just caught my speaker on fire!

Dave How close was the foam on top of the XO to the resistor? I'm wondering if the heat built up there and caused the foam to start to melt and possibly drip on to the resistor.

Take it easy
Jay

[Pete Schumacher]

Re: Just caught my speaker on fire!

Exactly.

I'm not saying anything bad about the ministatements in particular, just about the power handling issues with three ways in general.

If care is taken with component selection, matching driver sensitivity mainly, then padding will be reduced to a minimum, and power will be mostly dropped in the VC, not in the passive XO.

My Byzantium 3-way is crossed at 300 and 2000, 2nd order. I've got the Hafler 500 feeding a 2 Ohm resistor which is in series with the LP inductor and HP capacitor. There's a zobel, an inductor, and currently, a 12 Ohm resistor, across the mid terminals. I had Ozzie blasting for 20 minutes yesterday, at levels I personally would not listen to for long considering the spectral distribution (LOTS of mid energy). I've no doubt the peaks were nearing 150W, with RMS levels around 20W. It was LOUD! I touched both the shunt resistor and series resistor on the mid. The shunt was barely above room temperature. The series resistor was only slightly warm to the touch. Tweeter padding resistor was the same temperature as the board it was mounted to.

I have no doubt that the Statement mid XO could be re-worked with a shunt inductor to help control the low end where resonance occurs, which would allow for a larger shunt resistor and consequently reduced power consumption. Of course, that raises cost due to the addition of a large value inductor. And since both watts, and resistors, are cheap, it makes sense to double up on the parts to keep temperatures down and accomplish the task at hand.

As Dave is finding out, keeping the parts free of obstruction to air flow will likely be all that is needed to keep this from happening again in the future.

[davepellegrene]

Re: Just caught my speaker on fire!

Dave How close was the foam on top of the XO to the resistor? I'm wondering if the heat built up there and caused the foam to start to melt and possibly drip on to the resistor.

Take it easy
Jay

Jay,
If I had to guess what happened I would say the resistor was so well insulated from the foam and glue that it just got hot to the point that it melted or burned everything around it. I don't think the foam would catch the board on fire I think the resistor did that. I would say the resistor to the board is what did the damage. Even the cabinet behind the board is chard. This was my first build so I really had no idea they could get that hot. All the builds I have done since I put as much power to them as I could with the XO's out of the cabinet and nothing ever got hot. They were all smaller two ways and the woofer was the first to go. My daughters three way Nightmare's I may have to address but she only has a 35 watt Haffler. When I tested them they would almost take all my GX3 had but only on base light material. The woofer is crossed at 1800 Hz so I have it probably doing a little to much. I will probably rework them down the road when I have a little more knowledge and make OX's are right.
All my other builds I have pulled the foam out of and replaced with rigid fiberglass which I think is much safer. The only other build I am worried a little about is my Continuums because the XO's are packed in fiberglass pretty tightly. I use them more for when I want to listen while sipping a glass of wine not rocken out with a case of beer anyway. :D So they should be fine.

I've had the Mini's for about two years and were hooked up to my Yamaha amp that is rated at 100 watts per channel and it would start to clip first. I think the Mini's are just one of those speakers that you could feed them with a fire hose and they would take it. Just when you mix that with a noob you might burn down the house.
I have been thinking about making a board that is vented. Maybe like cutting grooves with the table saw so it looks like heat sinks then flip it on the other side turn it 90 degrees then groove it that way. That will leave square holes were the grooves over lap. Then I thought about hanging them 90 degrees from the back of the cabinet kind of how a wine glass slides into a wine rack, but with a support top and bottom with a simple groove to slide them into. This would give them plenty of air flow all the way around them and be easy to access and mount. Isn't that how the boards inside a computer tower are mounted?

Dave

[Pete Schumacher]

Re: Just caught my speaker on fire!

Isn't that how the boards inside a computer tower are mounted?

Dave

Sadly, computer towers rely on forced air cooling, and the boards are mounted horizontally, the worst possible way for natural convection to help. Your idea, with the board sticking out into the space, vertically aligned, is a quite valid approach.

I don't think you need to go to that extreme though. Simply getting the resistors off the board a quarter inch and an unobstructed air flow should be all you need to do.