Recharging Alnico/Ferrite magnets

[Robh3606]

Hey Steve

That's the problem with EBay and 20+ year old drivers. Who knows where they have been with the Pro Drivers. At least the domestic stuff you may actually get a decent working driver set if you ignore the fact they should probably be reconned on the age factor alone. I am not gonna lie on that one I am prime offender of ignoring that simply because I couldn't afford to recone the drivers I have. Most of the drivers I have are ferrites for that reason. If you need to recone down the line it's one less thing to complicate matters trying to find a place to recharge the magnet. The main exception are my LE-14A's which have been refoamed. I plan on starting to building up a pair of L250Ti's is a month or so and want to get Le-14H-3 too use for this very reason. They work great as subs but I have no idea what to expect from them above that range. I certainly would not use them the way they are in an application like the L250ti without a recone and recharge which is frankly out of the question cost wise.

Rob

[Ducatista47]

I have searched the posts and read what I could find, and hope I have not missed anything. What I need to ask, and I hope I'm not being too thick here:

I have aquired two 2245H baskets that have been through many years of ignorant disco torture in French Canada. They had 2245's instead of 2242's, so they used them...and used them...and used them. They appear to have been blown up and burned out, reconed with waffle surrounds and blown up again; perhaps several times, I suppose. The last fried parts are still in place. VC's read open and rub a lot, cones & domes are abused, surrounds are torn, only the spiders seem to have survived.

My question is: are you saying the motor is going to be in good working condition after all this? No demag issues or heat damage of any kind? If there ever will be a case for ferrite remag, is this it? Inquiring ignorant Peoria mind wants to know.

Clark in Peoria

[QwertyAccess]

Only my opinion and theory in any case, but i believe after that kind of abuse the ferrite magnet could definately use a recharge, alcino is supposed to be really easily demaginitized "permanetly" compared to ferrite magnets under high power, but if enough abuse is given to a ferrite magnet over a longer period of time i would imagine it would have lost some power atleast, i cannot imagine it to be at the same levels as it was new in that case.

[franz]

Hi there!

Although I am for sure not among the most knowlegebale members here, I guess its time to "de-mystify" the subject a bit.
Every single loudspeaker manufacturer has to have a magnetizer to get their ferrites charged! I don´t think they are assembled in charged condition for obvious reasons.
So it´s nonsense to say that there are only 3 or 4 places in the US and Europe together that can handle it.

I once had a K 130 recharged at a Magnet manufacturer in Dortmund/Germany and I know there are at least some other places in Germany alone that can do it. Some (of the better) recone shops have their own devices or know a place where to do it. BTW the process was succesfull in almost restoring the original Q values.
The question is if their charging coils are big enough to accomodate a large magnet and the joules they can give it are sufficient. Even more a problem might be to find the right guys who are willing to do it and handle the shipping, billing, etc. involved. Charging itself only takes a few seconds and seems to be far from complicated.

IMHO the simplest way to detect a low gauss magnet is to take a close look at the free air impedance curve. If a woofer with a decent magnet and normal B values of around 1 T / 10000 Gauss indicates a max impedance of only 10 -20 Ohms that´s a clear sign that either the coil has shorted windings or the magnet is low. "Normal Q" woofers I encountered had Z max values of 60 - 200 Ohms depending on field strength and Q es of course. The best way detecting a problem obviously would be to compare a speaker in question to a good sample impedance-wise. Also spl in midband is usually some dB lower for a decharged magnet.

Bill Hanuschak (GPA) once told me that the only way to effectively demagnetize a ferrite magnet is to put in the oven and heat it beyond the curie-point which is > 250 °C AFAIK. So its unlikely to find a weak ferrite magnet while alnicos definitely are prone to it when pushed hard.

Hope that helps.

greetz

Franz

[QwertyAccess]

mmm baked Ferite magnets, sounds good...

[jbl_man_uk]

Hi All,

a man to remember, ...certainly as important like James B. Lansing.
Carl Friedrich Gauss 1777 - 1855,... born in Braunschweig / Germany.
HP

BTW: he looks a little ugly hearing some want to recharge Ferrites

Yes,someone even named a speaker manufacturer after him in Hollywood...shame they are no longer around!

[Rolf]

Fear not Guido... It may depend on the equipment that a specific shop has, but OCS does them as complete speakers.


Widget

So does Seas in Norway.

[4313B]

How would you know if a driver had lost 10% of it gap strength?? What do you have to compare it to??

You measure it. It's a transducer. If you have no way to measure it then have your reconer measure it for you and recharge it if needed. For me personally, it is SOP to recharge alnico magnets. $15 at Great Plains. We've discussed this before. It isn't anything to devote an entire thread to. Just do it.

[Robh3606]

You measure it. It's a transducer.

Hello Giskard

Well that makes sense. What would you look for in the measurements?? How much reduction in gap flux density is acceptable?? I just grabbed 10% off the top of my head. When a beat up old warhorse comes in how much are they discharged?? With say 15%-20% give you clearly measurable/audible differences??

Rob

[4313B]

What would you look for in the measurements?

Reasonable consistency between driver pairs.

As for the rest of your questions one can work through the formulas to determine the effects of motor strength. (A quick source of some formulas - http://www.birotechnology.com/articles/mass.html)

For those less inclined towards math I think one may be able to do simulations using BB6P.

[Ian Mackenzie]

My recollection is that loss of flux will at least effect compressions drivers in the extreme Hf area with loss of output. Another reason why devices like alnico 2405's vary from one that another in hf output unless new as new.


Kinda down grade Ebays collector prices on vintage drivers been advertised as mint!

[Mr. Widget]

Kinda down grade Ebays collector prices on vintage drivers been advertised as mint!

Unfortunately true... there are those coveted "red wax seals intact" drivers that actually are good, but it isn't worth the gamble. It is better to buy a rust free beater, remag it and pop in a new diaphragm. Barring physical damage, you should be as good as new.

...and well yeah that isn't cheap, but neither is a new driver!


Widget

[magger]

Thank you all very much for your inputs regarding this specific topic. It is greatly appreciated! Now I have a better idea of how I am going to move on with my vintage JBL gear

[gtimbers]

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 2235 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.

[4313B]

Sweet!
Thanks Greg.