The demag has nothing to do with clipping. It's the intensity of the back EMF field that causes the problem. The higher power the amp the more powerful the EMF field can be. Read Gregs post that is already linked.
DC and heat are not the issue.
Rob
The demag has nothing to do with clipping. It's the intensity of the back EMF field that causes the problem. The higher power the amp the more powerful the EMF field can be. Read Gregs post that is already linked.
DC and heat are not the issue.
Rob
"I could be arguing in my spare time"
I think this an over exaggerated issue.
I have been working in the PA business all my life and had aprox 150 K140/E140 and 100 K120/E120 drivers in our systems. First driven by Crown DC300 and later Carver PM1.5. They where abused in power terms and we never discovered problems with the alnico types.
You should ear the jbl 4731x , I am hitting 117db 12 ft away. 100 db is loud ? Maybe for old farts , lol .Originally Posted by rdgrimes
In your setting with multiple boxes and drivers in the dozens yeah it would be. At home definately more noticeable with single driver pairs. It's a fact it happens. Only the K's count the E's would be fine.I think this an over exaggerated issue.
Rob
"I could be arguing in my spare time"
" loudspeaker handbook "chapter 4-principles of magnetics"
The only way your gonna hurt an Alnico is dropping it. Driving a pair of L-300s with the Mcintosh MC402 is going to destroy your ears before it has any effect on the magnets of the Alnico.
If this was a problem we should have noticed it with our stage monitors. A 2 or 3db drop in output level is very noticable, certainly if you have two side by side with one E140 and the other with a K140. Most of them had been reconed twice, the surrounds where tearing......In your setting with multiple boxes and drivers in the dozens yeah it would be. At home definately more noticeable with single driver pairs. It's a fact it happens. Only the K's count the E's would be fine.
Rob
I would have to say your ears will be well and truly done in before an ALNICO magnet starts to demagnetise. Also, the driver voice coil will be having serious issues as well.
Allan.
Hi otenmsa,
If we take JBL nominal driver impedance value to be 8 Ohms, and for 2231A driver max.nominal power to be 100W, that would give us that effective voltage value to be 28.23V, or about 40V maximal voltage amplitude. On the other side lowest applicable frequency of such amplitude would be about 25Hz (in order not to overcome Xmax).
From some simulation program minimal impedance of the driver 2231A in L300-box would be about 6.7 Ohms around 150Hz), what would reduce the maximal voltage about 10%, So I believe if You limit the voltage to 36V max value and use frequency range over 25Hz, your driver would operate under JBL declared operating conditions, and demagnetization would not have to become a dangerous threat.
But "hard" limiting the power amplifier voltage to the mentioned value,( if such limitation have happened ) can destroy compression on UHF drivers, so a kind of "soft" limiting has to be applied.
I have seen one K120 with the reduced magnet "strength" ( about 2~3 dB lower output), and one Altec VHF driver that had reduced sensitivity too, so demagnetization can happen to AlNiCo drivers.
Regards
Ivica
I think the question is a good one and probably best answered by an transducer engineer. As mentioned, its not a heat-related problem as the Curie temperature for Alnico 5 is far beyond the operating temperatures geneated by a JBL woofer in a typical residential sound system.
My understanding (limited) is the problem stems from the overall size of the motor system and the whether the coil assembly is underhung or overhung. For example, the LE15a with its relatively deep motor and underhung voice coil is not generally susceptible to de-magging. However, the overhung/short magnet 123a could loose 3-6 dB (maybe more)when driven by a Phase Linear 400 and playing the Sheffield Drum record. Put a powerful enough amp on a pair of L100s and watch what happens. (Happened to me in the 80s').
So the question remains, how to determine the amount of safe power that can be used with a given JBL woofer with an Alnico magnet. Take the example of the L100. What could be considered safe? 100 watts, 125 watts (unclipped).
I finally got one of the soft clipping lights on my Nak PA7 (around 200WPC) to flicker the other day driving my L300s. That was loud! Keeping it at that level for any amount of time would have been crazy.
some old info I once read
Hi,
You are right, temperature is not the main reason for AlNiCo 'demagnetization'. An "inverse magnetic field" ( produced by current pulse) can partially demagnetize the magnet. As seen for Alnico Hc is less then Ceramic, not to mention Samarium or Neodymium.
For Alnico-V Hc is about Three TIMES LESS then Alnico-8. I have added some graphs.
As driver Re and Le are almost constant, only the value of the applied peak voltage and its duration can define the current peak value...
Regards Ivica
many thanks for sharing that old JBL's info but the second graph doesn't concern alnico demag issue but only shows the failure mode of the 1" comp dia. vs the material and surround design of the dia. along the time at different power levels applied
and as mentioned above the material (alnico) isn't the only parameter that affects the degauss issue but the magnet circuit design itself and voice coil geometry also do (Eddy's currents) ...
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