Effects of loss of magnetism?

[Flodstroem]

It was Speaker Repair Pros, they have Orange Country's old magnetizer. I have no idea the tool he tested with, he gave me the brand a while back, but I have since forgot.
If you do not have 3000 Oe, does it only partially recharge (like this, 16K vs 19K) or does it not charge at all?

Thanks

Yes you are right, if the magnetizing machine only partially could recharge the Alnico magnet it will only recharge the weakest domains in the magnet. The most resistant and long lives domains (those that will hold on the magnetism) will only partially or not at al be magnetized.

[ivica]

This may be better in a separate post, but thought I'd start here as most of you are knowledgeable about magnetism. I had a pair of LE85,s I sent in to be remagnetised. They measured below 5000 gauss before and came in a 16,000 gauss after. But, the original specs of an LE85 is 19,000.
Is the 3,000 loss is gauss due to the age of the magnet, the ability of the re-magnetizer to reach the 19,000 gauss or something else?

Thanks,

Eric M.

Hi Eric M,

I think that the used equipment for the re-magnetization was not strong enough or some problem about applied to the LE85.
May be the measurement instrument has to be calibrated before applied.....I just guess...having no experience, but I believe that JBL had used proper magnetic material for the LE85 production, if not QC would not pass the product initially. Lower magnetic field would produce bad HF response, and the driver sensitivity would be less then the proposed.

regards
ivica

[Eric M.]

What sort of issues could be with the LE85? Magnet and housing showed no damage or cracks? What else could be wrong with the LE85?

Thanks,
Eric

Hi Eric M,

I think that the used equipment for the re-magnetization was not strong enough or some problem about applied to the LE85.

regards
ivica

[Robh3606]

Frankly I don't understand why the flux density was down below 5K. With an alnico compression driver they can't by design demag with overdrive. Re read Gregs post. Just hook them up and have a listen. You can worry about this all day but the only way to see what's going on is by listening.

Rob

Greg's Post underlined important section


Here's the Scoop
Ferrite magnets do not demagnitize with time or drive. They are affected by temperature but that is reversible. They will return to normal when they return to room temperature. Ferrite is basically a lousy magnet material for speakers but it is cheap and readily available. JBL has done a ton of things within the magnetic circuit to make the material behave in a more stable manner. At 100 degrees F, a Ferrite motor will be down about 1.5 dB in level which means the midband of the woofer will be lower by that much and there will be increased output around the system resonance. The TS parameters will be completely different - as though the BL was reduced by about 18%.

Alnico magnets, by their nature are easy to demagnitize with drive. They will not change with time and their dependence on temperature is really small - maybe 1% at 100 deg.F. Alnico stability and resistance to back EMF is really good. This is why they make the best sounding magnetic structures. Unfortunatelly, given a big enough pulse of magnetic energy, they will demagnitize by up to 3 dB. The sensitivity to demagging is dependent on the specifics of the magnetic circuit and the length of the coil providing the field.

Underhung woofers (LE15 and such) midranges, tweeters and compression drivers do not have sufficient back EMF fields to push the operating point of the structure below the knee. They are essentially stable regardless of input signal.

The short-gap, long-coil speakers are the ones that have a problem. A 2231 can take a hit of up to 3 dB if a big enough hit of current takes place. 1.5 dB to 2 dB is more common. The effect does not get better or worse with time, it solely depends on how much current is driven through the coil. The more current, the more field. Once the field is bigger than a certain number, some amount of demagnitizing occurs. It is perminent (until externally recharged) and will only increase if a larger sustained current hit occurs.

Therefore, if you have a qualifying alnico woofer and you have played it loudly you have some damagging. You can have the unit recharged and it will be fine until you play it again. Exceed the critical level and it will start happening. If you never do, it won't ever demag. Most of these designs trace back to the 50's and 60's where 15 - 30 watt tube amps were the rule. They didn't have the current capability to hurt anything. With the advent of big solid state amps, the current levels went up and the problems started to surface.

Most of the qualifying 4" motors will loose 1 - 1.5 dB unless they are pummeled. Some of the older 3" with really short magnets, like the 2213A and 123A will typically be around 3 dB down. They go really easily. The old Decade woofers (116A and 127A) only had to see an amplifier in the room and they got really nervous. FYI, the new 1500Al used in the S9800 can take continued pulses of 5000 watts and loose no more than 1%. The test can only be done a few times before the coil is destroyed, but the magnetic assembly is totally stable.

[Flodstroem]

What sort of issues could be with the LE85? Magnet and housing showed no damage or cracks? What else could be wrong with the LE85?

Thanks,
Eric

Eric, my suggestion is that you search for someone that really could measure the magnetic field strength in your LE85 to about ± 2-3% accuracy (a measurement with a transversal measuring probe). If it really is a magnetic issue, then you need to fully recharge them. After that they should be like JBL specs. Also, you need to recharge both of them for not to get unbalanced efficiency.

[Eric M.]

Thanks.

Yes, both were done and both were taken apart, thus the need for regaussing.
It sounds like it's either the test tool or the remag machine that is lacking in strength.

These were sold to someone with the knowledge that they needed to be remagnatized. They went from me, to Speaker Repair Pros to the customer. Speaker Repair Pros reported the before and after measurements and I was curious why the measurement was lower than the JBL spec.

I never got a chance to listen to them after regaussing. I listened before they went out. Other than a loss of efficiency as compared to other LE85's I have, these sounded fine.

[ivica]

What sort of issues could be with the LE85? Magnet and housing showed no damage or cracks? What else could be wrong with the LE85?

Thanks,
Eric

Hi Eric M,

May be that re-magnetization was not done properly.
I believe that ma be the used equipment was not 'strong enough' or the fixture of the LE85 was not properly arranged. I have no other 'idea' what what would be the reason.

regards
ivica

[Eric M.]

I think I'll need to ask the power of the machine used.
What is the proper arrangement for a driver in a remagnetizer? I'll ask him about that as well.

Thanks,

Eric

Hi Eric M,

May be that re-magnetization was not done properly.
I believe that ma be the used equipment was not 'strong enough' or the fixture of the LE85 was not properly arranged. I have no other 'idea' what what would be the reason.

regards
ivica

[Flodstroem]

recharging speaker magnets is not as simple as one might think.
there is a huge iron mass to magnetize and in the middle is the Alnico magnet that is surrounded of plenty of iron.
Thats why one need lot of power for to do it properly

I have built my own magnetizer years ago and I use it randomly when JBL speaker come to my possession.

Here are a few pics of the magnetizer Im using, or the parts that composes the machine
Data: 36,000 uF@900Volts (adjustable between 50-900V) = approx 14.6 kJ (kilo Joules)
Tha huge SCR is for 1400V/1600Amps, 20kAmps peak. The magnetizing coil was constructed using rectangular magnet wire of 8x3 millimeter

You could se a 2440 placed in center of coil

The power supply/source is a Lumina Power CCPF that could charge the capacitor bank with 1500 J (Ws)

[Mr. Widget]

Very cool Dr. Frankenstein!


Widget

[Eric M.]

Flodstroem, that is incredibly cool! About a year ago I posted questions about building a unit like yours. No one responded. Would you shave the schematic and construction details? I'd love to build one.
What do you use for testing gauss after? I could not find a meter that had a small enough probe to fit the voice coil gaps of anything less than a LE15a and didn't cost as much as a small car!
What did you coat the coil in after winding?

Seriously, I want to build one.

Thanks,

Eric

recharging speaker magnets is not as simple as one might think.
there is a huge iron mass to magnetize and in the middle is the Alnico magnet that is surrounded of plenty of iron.
Thats why one need lot of power for to do it properly

I have built my own magnetizer years ago and I use it randomly when JBL speaker come to my possession.

Here are a few pics of the magnetizer Im using, or the parts that composes the machine
Data: 36,000 uF@900Volts (adjustable between 50-900V) = approx 14.6 kJ (kilo Joules)
Tha huge SCR is for 1400V/1600Amps, 20kAmps peak. The magnetizing coil was constructed using rectangular magnet wire of 8x3 millimeter

You could se a 2440 placed in center of coil

The power supply/source is a Lumina Power CCPF that could charge the capacitor bank with 1500 J (Ws)

[Flodstroem]

Very cool Dr. Frankenstein!


Widget

Flodstroem, that is incredibly cool! About a year ago I posted questions about building a unit like yours. No one responded. Would you shave the schematic and construction details? I'd love to build one.
What do you use for testing gauss after? I could not find a meter that had a small enough probe to fit the voice coil gaps of anything less than a LE15a and didn't cost as much as a small car!
What did you coat the coil in after winding?

Seriously, I want to build one.

Thanks,

Eric

Thanks both of you.
Eric, you have got a PM.
The measuring equipment is a F.W.Bell Gausmeter. Unfortunately, my own goes only to 20 kG but its OK for most of the driver magnets. But, not for the 244X series, those have a magnetic strength larger than 20 kG.
The magnetizing coil is baked in fiber glass and epoxy. Its vacuum impregnated after using fiber glass between layers and between windings. Also a there is fiber glass wound round the outer circumference.

[Eric M.]

Is the probe for the FW Bell 5070 thin enough to fit most voice coil gaps? That was the issue I had looking for gauss meters. Finding one that fit voice coil gaps. Most are too thick.
Will the 5070 fit the narrower gaps of 077 and 075 or the narrow VC gap of something like the D123?

Thanks,

Eric

The measuring equipment is a F.W.Bell Gausmeter. Unfortunately, my own goes only to 20 kG but its OK for most of the driver magnets. But, not for the 244X series, those have a magnetic strength larger than 20 kG.

[Flodstroem]

Is the probe for the FW Bell 5070 thin enough to fit most voice coil gaps? That was the issue I had looking for gauss meters. Finding one that fit voice coil gaps. Most are too thick.
Will the 5070 fit the narrower gaps of 077 and 075 or the narrow VC gap of something like the D123?

Thanks,

Eric

Eric, the probe used in the pics are 0.95 mm (approx 0.038") thick and it works with all the JBL woofers and midrange I have reconed or refoamed so far. It also works with 24xx series of CDs as you also can see in the picture. If it works on the 077 and other smaller HF drivers I really dont know. They have a much smaller radius so even if the magnetic gap is 1 mm or more the short radius will prevent the probe to fit.
FW Bell has a gauss meter model #5080 and that one could be used for measuring magnetic fields up to 30 kG/3T. It looks same as the #5070 but has other probes. I dont think they are any different in size than those used for #5070.
The #5080 is perfect for al Alnico drivers and other brands that I know of.
But I dont think FW Bell stil produce them, they are obsolete by now I guess.

[Eric M.]

Thanks,

I'll keep an eye out for the 5070 or 5080 or really any probe that is .038" or less. That's a big help just having that measurement as a starting point.
I've been so tempted to just file down the probe on my meter! I'm guessing that's not such a good idea .... is it?

Thanks,

Eric

Eric, the probe used in the pics are 0.95 mm (approx 0.038") thick and it works with all the JBL woofers and midrange I have reconed or refoamed so far. It also works with 24xx series of CDs as you also can see in the picture. If it works on the 077 and other smaller HF drivers I really dont know. They have a much smaller radius so even if the magnetic gap is 1 mm or more the short radius will prevent the probe to fit.
FW Bell has a gauss meter model #5080 and that one could be used for measuring magnetic fields up to 30 kG/3T. It looks same as the #5070 but has other probes. I dont think they are any different in size than those used for #5070.
The #5080 is perfect for al Alnico drivers and other brands that I know of.
But I dont think FW Bell stil produce them, they are obsolete by now I guess.