| EWorkshop1708 |
I'm designing a circuit to shut off the amplifier if the Voice Coil becomes overheated, that way if I get too heavy on the volume control, my speaker is saved.
Some speakers have vents in them, so I plan to put a 10K thermistor inside one vent, and put it as close to the VC as possible without touching it. Then I'd glue the wires from the outside of the speaker to hold it in place.
I already vaporized a VC on a 350W/1200W subwoofer, and I don't want this to happen again! :hot:
I have a +350W RMS amplifier and it already uses LM339 Quad-Comparator to shut the AMP off when overheated, and I have 3 unused comparators left - for future uses. I want to use one of these to detect the Voice Coil temperature.
I'd use a voltage divider w/POT from regulated +5V as a voltage reference, and a resistor in series with the Thermistor, then feed that to one of the LM339 inputs.
Questions I have:
What is a safe temperature for Voice Coils? I want the temp to cut off below the glue-melting point - long before the copper melts.
Also, what resistance would an avg 10K thermistor read under the highest allowed VC temperature?
Thanks for your responses. |
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| just a guy |
| quote: | Questions I have:
What is a safe temperature for Voice Coils? I want the temp to cut off below the glue-melting point - long before the copper melts.
Also, what resistance would an avg 10K thermistor read under the highest allowed VC temperature? |
Can't help with that stuff, sorry, but maybe I might be of some service by asking a different question. Do you know why the last one burned?
For example, if you have a ported box and you blast it with 1200 watts at the port tuning frequency you can fry it very quickly due to the lack of cone motion. You can burn it with much less than the driver's rated power handling.
Maybe you know all this already, but it might be worth investigating the cause of the problem instead of adding a kill switch. Maybe you already have. |
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| AndrewT |
VC insulation becomes a problem long before copper melts.
But, the VC expands when heated. How far can it expand before it starts to rub on the pole? |
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| EWorkshop1708 |
It is in a 1.25cu ft sealed box, exactly what the speaker calls for in the Sony papers that come with the woofer.
The speaker had lots of excursion. I was clipping the amp a bit playing "war pigs" from Black Sabbath. I love playing hard rock with heavy drum and guitar notes.
Also, I need to measure the output of the amp, because this is the 2nd 350W RMS speaker (2 different types) that I've destroyed with this amp. I think I underestimated the real RMS power. It uses 5 pairs of MJL4281/4302 outputs with no SOA protection, and is fan cooled.
Transformer is a 9lb 55-0-55V 350VA toroid transformer (weighs as much as an Avel 500VA) equipment transformer, I used extra voltage preparing for sagging rails, but after building the amp, the rails didn't sag as much as I thought they would when driving 4 ohms. The transformer is never even warm under any load. I have a feeling the VA is not accurate on the toroid due to the weight. Unloaded, my rails are usually 75-0-75 or sometimes 77V on a high mains outlet.
The speaker didn't die right away, but over time from me cooking the coil with loud music. The speaker has vents, so I want to insert a thermistor on the new speaker when I get it, to keep track of the temp. |
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| EWorkshop1708 |
| quote: | Originally posted by AndrewT
VC insulation becomes a problem long before copper melts.
But, the VC expands when heated. How far can it expand before it starts to rub on the pole? |
Never had that rubbing problem with this speaker. The generic speaker before it developed that problem, which is why it got replaced. However, the Sony played problem free with no warning up to the point the coil blew.
I agree with you on the VC insulation, that's why I want the temp protect to cut off the amp if it gets close to that temp, that way the coil doesn't die over time. |
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| janneman |
Are you sure this is a 350W RMS speaker? Does it actually say RMS on the spec sheet? It may be 350W music power or something similar, with lower continuous power (that's how it is usually spec'd).
Jan Didden |
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| EWorkshop1708 |
350W RMS 1200W Max
Even says 1200W on the cone. However, I know better, and went by the RMS rating.
There is a higher priced version that's 380W RMS 1300W MAX, I may also consider, the speakers are nearly identical, except the 1300W one is black, and goes down to 18hz. |
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| EWorkshop1708 |
| Regardless of the speaker used, I want to implement voice coil protection, because all voice coils get hot, and deteriorate over time from playing too loud. |
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| AndrewT |
| and the magnets deteriorate as well. Alnico is much more resistant than ferrite. |
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| EWorkshop1708 |
Interesting, didn't know that.
Sounds like another good reason to be able to monitor the temp of the motor assy. |
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| Eva |
The best indicator of voice coil temperature is the own DC resistance of the voice coil, it increases with temperature (see thermal coefficients of copper).
A method for measuring DCR on the fly would consist in adding a low amplitude (100mV) low frequency (1Hz or DC) component to the signal and then measuring the resulting voice coil current through a small shunt (like .050 ohms). The current sense signal should have to be amplified and well filtered to attenuate audio components below the noise floor so that only the test component remains.
Then VCA could be employed to attenuate input signal when required so that voice coil is kept below a given temperature.
BTW: Your amplifier is probably capable of producing over 600W with 4 ohm load. You may not have noticed that you were dealing with so much power because these speakers have low efficiency. On the other hand, these speakers are intended to be powered with junk car-audio amplifiers that advertise 1200W but produce 300W at best. |
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| Wayne Parham |
You almost never fuse the voice coil wire, or the leads going to it. A very high current pulse would do that, but the most common failure mode is long-term heat buildup causing the voice coil adhesive to fail. The coil unwinds and begins to run in the gap, eventually causing failure. The following link may be of some help:
You can always test the voice coil resistance, even in circuit while in use. Of course, that requires a small series resistance to measure across, which will lower the damping factor. Another thing you can do is to fasten a temperature sensor to the pole piece. |
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| EWorkshop1708 |
| quote: | Originally posted by Wayne Parham
You almost never fuse the voice coil wire, or the leads going to it. A very high current pulse would do that, but the most common failure mode is long-term heat buildup causing the voice coil adhesive to fail. The coil unwinds and begins to run in the gap, eventually causing failure. The following link may be of some help:
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NICE heat exchanger for the speaker! That's the ONLY heatsinking I've ever seen on a speaker, other than some that just use aluminum dustcaps. Very impressive! :cool: Also I read the article, and you mention the pole-piece temps, and I know steel does not transfer heat very well, so I can see why the pole is much hotter than the magnet. In one example of yours, the pole piece was 195 degrees F! :hot: Maybe a safe limit could be around 180 degrees?
Now when this coil blew, it shorted to 0.9 ohms, but it also has loose open turns that rub inside even though the coil doesn't measure as open. My guess is that when it shorted, (the adhesive and enamel melting) the high current melted together some of the turns of wire, which caused the bright flash of light. It sounds like the type of failure you mention.
So you say that just measuring the pole-piece temp with a thermistor may be enough.... I may try that, but I was hoping to get the thermistor closer to the coil itself, by using the side vents (you can see the coil) |
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| EWorkshop1708 |
| quote: | Originally posted by Eva
BTW: Your amplifier is probably capable of producing over 600W with 4 ohm load. You may not have noticed that you were dealing with so much power because these speakers have low efficiency. |
I have to agree. The generic 12" speaker before the Sony was much louder at lower volumes, but didn't sound NEAR as good. When I got the Sony, I had to turn up the volume on the amp to match the same volume overall, but the lower notes (20-40hz) were more solid and much more clear to make up for it.
I'm also going to put a clip-indicator LED on the amp as well to avoid future abuse, but I still want to have the Voice-Coil protection capability as well, because I hate burning up my money by burning up speakers. I've pondered going up to a 15 inch, but I'm not sure that a 15's VC is really any better. |
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| EWorkshop1708 |
Here's the schematic for the Voice Coil temp protection.
I based it off of a safe temp of approx 166 degrees F and the specs of the Thermistor. The circuit is nearly identical to the one inside the Amplifier that protects the outputs/heatsink from overheating, just set to a different temp. |
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| judtoff |
I just wanted to add I've owned that sub before. And with a "150Wrms" amp I was able to get the coils stinky(Although I wouldn't suggest running them ported to anyone) . Those subs are extremely over rated, and inefficient. IMHO they are garbage. You can find much better sounding, looking, power handling, and more efficient for the money.
Look into a sub you can use ported in a small box tuned low. I got much more output and bandwidth with a tangband W8 740 8" sub with a 12" passive radiator in a .8cuft box than the sony in a 1cuft sealed box. With the same power.
And the tangband costs less ;). And it handled the power better. Or maybe it was because I had to turn it up much less to match my mains ;).
You're amp would probably be much too much power for a single 8" tang band sub, but maybe 2 of them in series would be more suitable. |
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| woody |
You may want to look on the JBL PRO webpage they have an
article on power handeling and the danger of UNDER powering
speakers. |
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| 95Honda |
You'll never get a motor hot enough to de-magnatize ferrite/ceramic
If you are "Underpowering" you would never blow a speaker...
Seriously, turn down the volume or buy a speaker that can handle more power...
It's simple... If you have a an amplifier that puts out 350 watts, buy a speaker that has a continous rating of 700 watts. That way if your amplifier was driven into full clipping and could (theorically) produce 2X its rated output at something like 50% distortion the speaker would survive....
Otherwise, use less power...
For the sake of argument, don't go on about clipping and cone movement, etc... I did a full test, read it here...
http://forum.carstereos.org/clippin...41.html?t=47441
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| Wayne Parham |
Woofers are generally cooled by forced air convection. The voice coil gap is vented, and cone motion works like a lossy pump. Like a two-cycle engine exhaust, this kind of system can be tuned. If you tune it to work best at deep bass frequencies, then the vents don't work well at high frequencies. At high frequencies, the cooling vents stall. When power is presented to a woofer at high frequencies, the cone doesn't move enough to provide effective venting. That's why higher frequencies tend to be harder on a woofer than low frequencies, where the vents are able to cool the voice coil better.
Wicking heat from the pole piece with a cooling plug helps remedy this situation a great deal, by the way. Whether the cooling vent is stalled because of high frequency or acoustic loading (via horn or other mechanism), heat can be removed from the core with a cooling plug. In fact, I'd say cooling plugs are more effective than vents in any situation where cone motion is small.
High power horn loaded speakers are improved with cooling plugs because horn loading reduces excursion. Even though efficiency is higher than direct radiators, there is still an abundance of heat. Since excursion is reduced, venting is stalled and that makes cooling plugs attractive.
Also, wide band woofers, used at low frequencies through the midrange, can benefit from having both cooling mechanisms, venting for low frequencies and cooling plugs for higher frequencies. Two-way speakers with a woofer that covers a wide band can be improved by having cooling plugs and voice coil gap vents. |
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| DigitalJunkie |
| quote: | | If you are "Underpowering" you would never blow a speaker... |
I'm not sure that's true.
I had a friend that blew some 160Wrms speakers with a 50Wrms amp.
He pushed it into clipping,and the DC fried one of his speakers,simple as that. Clipping is bad,speakers do not like DC. |
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| 95Honda |
| quote: | Originally posted by DigitalJunkie
I'm not sure that's true.
I had a friend that blew some 160Wrms speakers with a 50Wrms amp.
He pushed it into clipping,and the DC fried one of his speakers,simple as that. Clipping is bad,speakers do not like DC. |
It wasn't a 160Wrms speaker then...
Really, read this-
http://forum.carstereos.org/clippin...41.html?t=47441 |
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| Corax |
Wayne Parham mentioned a good idea.
However, if I'm not mistaken, EWorkshop1708 stated that he is using the Sony in the recommended 1.25cu ft sealed box. If you apply for a longer time an average of i.e. just 200 watt to this speaker the vented voice coil might exchange heat through the hole in the magnet, but the heat is trapped because of the closed box. Can you imagine how hot it'll get inside that box?! :dead:
I guess the box is made up of wood which is not known for excellent heat transfer capabilities. Therefore it would be just necessary to measure the inside temperature of the enclosure and turn down the volume continuously with raising temperature or just shut it off to let it cool down.
The temperature sensor can be placed i.e. at the far most top position of the enclosure.
For a fellow of mine I 'invented' the forced closed box cooling with Peltier cells/elements running at 12V in a car. Depending on the size (and the current requirement) of those Peltier cells/elements you might need a bunch of them to get rid of the power (and therefore heat) you want.
After these 'modifications' he never fried a voicecoil of his sub anymore. It stays always 'cool' - at least inside the box.
At the outside, with the huge heatsinks, the temperature is not always in the range to call it nice and comfortable. So be aware that the heatsinks might get really hot (of course depending on size, convection abilities in the trunk, etc.). The Peltier cells/elements will generate a max. difference of 60° centigrade between the two plates, which should be considered when planning such an active cooling system. ;)
For PC cooling purposes I've even seen someone stacking two of 'em to increase the temp difference between the most outer plates. |
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| Wayne Parham |
I think solid state heat pumps are a great idea. Put one on the pole piece and use it to wick heat out of the motor core. That's the most important thing - getting the heat away from the immediate proximity of the voice coil. It is an added complexity, but it is still a good idea.
Motor core heat is caused by magnetic losses and by voice coil radiation, and if it isn't removed, it just builds up inside the motor. The magnet surrounding the pole is a good container for heat, like a thermos bottle. So wicking heat out of the core is a good idea, whether by cooling plug, heat pump or any other mechanism.
I used to think heat would build up in the box, and it does to some degree. But what I found is that is a secondary problem, and it's a distant second, to be honest. Once you get the heat out of the core, you're way ahead of the thermal runaway problem. If you pump heat out of the core into a sealed box, the air temperature will rise, but not to the blistering levels present in the core. Naturally, the smaller the box, the higher the air temperature can rise because there is less air volume and internal surface area to dissipate heat.
Of course, if you remove heat from the core only to the inside of a sealed box, the cooling vents are less effective because the local ambient air temperature becomes higher. In some sense, that robs Peter to pay Paul, but it is better to remove the heat from the motor core than to leave it there. But it is best to remove the heat to outside ambient air. An easy way to do this is with a cooling plate mounted on the outside of the box. In my 12Pi hornsub, I use aluminum plates that provide access to the woofers for service. It is physically connected to the cooling plug, so it doubles as a cooling plate.
An external cooling plate works like a heat sink. Large surface area is a good thing, so grooves can be cut into the plate if desired. In my case, grooves weren't needed, as the access panels had more than adequate surface area without fins or grooves. However it is fashioned, a cooling plate is a good way to remove heat to the outside. The heat is transferred from the motor core by conduction through a cooling plug or by solid state heat pump or any other mechanism. |
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| nigelwright7557 |
| quote: | Originally posted by EWorkshop1708
Regardless of the speaker used, I want to implement voice coil protection, because all voice coils get hot, and deteriorate over time from playing too loud. |
If your speakers are frying then its simply too much power into the speaker. The answer is to buy higher powered speakers.
I run a disco and havent ever fried a speaker because I use an amp that matches the speakers.
The only time I have fried speakers is when the amp has gone faulty and put DC into the speakers.
It might be worth having a look at the DC on the output of your amp in case your amp has gone faulty and is outputting DC. |
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| Wayne Parham |
| quote: | Originally posted by nigelwright7557
If your speakers are frying then its simply too much power into the speaker. The answer is to buy higher powered speakers. |
I would agree with you generally. If you're regularly burning voice coils, then you're pushing your speakers too hard. Either dial it back a bit, or increase capabilities. An easy way to do that is to buy speakers made to handle more power.
The thing is, heat is the problem so getting the heat out is the answer. The way speaker manufacturers increase power handling is with improved thermal management.
Increasing efficiency is the best thing to do to start with, as an efficient loudspeaker converts more electrical energy into acoustic energy, leaving less to be wasted as heat. But even the most efficient speakers turn at least half their power into heat, so even they can be improved with good thermal management. |
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| DigitalJunkie |
| quote: | | Normalized power (AKA amp at full clipping). This shouldn't have been much of a surprise to anyone knowing that a square wave with the same Pk-Pk voltage as a sine will have almost double the power. As you can see on the spreadsheet and graphs, power handling severely dropped when the square wave was applied in this test. Time to failure was cut by a factor of 4 for most drivers. |
There ya go.. Clipping = Square wave. |
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| 95Honda |
| I think you missed the point, look closely at the normalized power tests. There has never been debate that a square wave of the same voltage amplitude has more power than a sine, but that isn't even what we are talking about... |
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| The golden mean |
I think the fundamental problem is lack of understanding that the physical laws determines how loud and for how long you can play. Heat and heat transfer is an issue and most closed boxes are filled with some insulation material (where wool and mineral based materials have different heat transfer properties).
The problem: to play loud you have to use equipment that is up to the task. Also be aware of the risks of damaging your hearing, here there is a rather simple connection between SPL and exposure time. Taking drugs as alcohol make things worse. If you have been exposed to high SPLīs please let your hearing rest for up to a couple of days. Even moderate levels exposed to continuously can cause damages. I have learnt this the hard way i.e. running my big motor bike without silencer in a garage or shooting as a kid, without any hearing protection. I have tinnitus and have to take great care not to make things worse. OK, I can play loud for a shorter period but have to take the risk seriously.
Some weeks ago I disregarded my own experiences ( I had fun playing loud and for to long) so now I have to rest more than normally.
My suggestion is that you make some loudspeakers that is up to the task as said. Volt loudspeakers have some drivers that are cooled in an unusual way;radial. Use an amp so it doesnīt clip as others have pointed to.
https://secure.wilmslow-audio.co.uk...4e61e21da7d0c33
I use Volt RV3143:s in two subs weighting more than 200 lbs each and although they can play loud , the extreme purity of the bass notes makes it unnecessary to always play loud. Hot voice coils are not the way I want it...And there seems to be no reason to monitor them; no failure in about eight years.Turning the volume up doesnīt change the character of the sound and this is as I perceive it a very good trait.
I mean; does the sound "boomy" muddy exaggerated unnatural bass coming from some cars passing by, windows down, make me happy? Get better bass and enjoy it without any technically problem. ;) My point of view only! :cool: |
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| tade |
I agree with those recommending a new woofer. Find the nicest highest efficiency woofer that fits your needs and invest.
I am using a pair of Dayton RS 12" in two 2 cubic foot sealed boxes. They are in the corner and stacked so that the cone movement opposes. One fires up and the other down. This made such a huge improvement in vibration it is difficult to describe. I also have the motors mounted on the outside. Essentially the top box is being supported by resting on the frames of both woofers. It is extremely solid. Not only does it look really cool, but it helps with cooling and won't eat up the box volume. Hook up is really simple as these woofers have spring binding posts.
I am running the pair of four ohm woofers in series with 250 watts; 89db/watt. I haven't pushed the amp to complaint yet and let me say that the matrix is awe inspiring. As for drums that kick, that is all located in the midbass. These subs would eat 400 watts apiece when crossed over at 300hz (it was the amp complaining). Now that I have midbass horns running 105db/watt above 70hz, the subs just love it. I think the biggest problem with the low efficiency of that sub was that you were getting low, but you just couldn't play the "high" stuff with authority.
I hope I didn't write too much : )
Good luck and over engineer!
http://www.partsexpress.com/pe/show...tnumber=295-464 |
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| ChocoHolic |
...as far as I know most standards for power rating of speakers do employ something like a noise signal of which the power is taken more or less correct, but then employ a ON/OFF duty cycle (i.e. 1 minute ON / 3 minutes OFF).
This is fitting to unbelievable 400Wrms ratings with 60mm...70mm voice coils.
Imagine a 60W light bulb, it is bigger than normal voice coils - has to dissipate just 60W - and becomes quite hot.
Even vented coil systems cannot completely ignore obvoiusly giant amounts of heat...
Have a look to some measurements:
http://www.diyaudio.com/forums/show...5880&highlight=
Measuring the voice coil temp is a good thing if you drive the system continuously high. Of course a direct measurement is more accurrate than a indicrect, but also indirect can be helpful if you know the relation between the VC and the measured spot.
Furtheron you do not need to shut down completely, you can reduce the input signal softly with a VCA or OTA or LED+LDR or.... |
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| The golden mean |
There are many ways to obtain good sound. So please accept my subjective view. I like clear and clean bass that disappears fast as is intended in the music. Thus I donīt like systems where you have to crank the volume up to enjoy and to experience some dynamics. I have lived with such speakers-amps but now itīs quite different. Listening late at night can be amazing as so little is "lost" compared to high volume listening. And very little background noise and probably cleaner mains add to the amusement. Dynamics I said...amazing!
High volume listening can be a thrill when I have slept enough and neighbors are away.
My way to better bass was: taking care of psycho acoustic knowledge; -maybe. The combination of active filters for the Volt 12" in vented enclosures and 8" Thiel&Partner (Accutone) in closed boxes took some time to get right, but it was clearly worth it. Hope this isnīt to rave about it but nowadays I donīt tend to get impressed while visiting audio fairs.
Room tuning and acoustics is something to learn and it pays dividends to do it.
I still think; if you burn voice coils something is clearly wrong. You donīt have the equipment for sustained high loudness listening.
So I agree to what ChocoHolic wrote. Heat can be a big issue. But I still think itīs up to getting better equipment.
To monitor or not, I say not...Very hot voice coils kill dynamics IMHO. ;) |
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| The golden mean |
FWW here is a link to a commercial site and the subject is partly about the negative effects of very hot voice coils. The solution is said to be the use of a servo system. In my opinion servo systems have their drawbacks as well. One may say that itīs always a matter of correcting faults that have already occurred.
http://rythmikaudio.com/memory.html
Sorry; there seems to be a intermittent problem to access this page (although I read it just minutes ago without any problem).
In case you encounter a problem with the above link maybe it is possible to find it at:
http://rythmikaudio.com/products.html
(look for Memory Effects and a full page document)
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