Is Demagnetization in Alnico Drivers Ever a Real Problem?

[nola]

Is Demagnetization in Alnico Drivers Ever a Real Problem?

Klipsch user here and their older Heritage gear (Klipschorns, LaScalas, Heresy's) all use Alnico drivers. Most of mine are 20-35 yrs old now. I think older Altec-Lansing drivers were also alnicos. An old article found via a search for the Altec 511 horn and Voice of the Theatre speakers (VOTT) mentioned that demagnetization could be a reason for some speakers to sound bad.
A wiki search for alnico confirms this: "This means alnico can produce a strong magnetic flux in closed magnetic circuit, but has relatively small resistance against demagnetization."

Has anyone ever tested the old alnicos? I am not even sure how to test them. If they were out of spec, how would you re-magnetize them to factory spec?

Hope I posted this to the correct forum. Thx

 

[Dan ʕ•ᴥ•ʔ]

Yes, they can discharge over time and can discharge immediately if dropped (shock). I've run across several over the years that had weakened and needed to be recharged.

 

[nola]

Q:

1 - How do you know if the magnet is weak or demagnetized? Is there some type of "meter" to check it with?

2 - If "bad", how and where or by whom can you get back into spec?

Thx

 

[Dan ʕ•ᴥ•ʔ]

Well the easiest test is to lay something that's known to be magnetic, like say a screwdriver blade, near the alnico slug and see if it "grabs" the blade. If the magnet is weak, it will barely hold onto it.

Great Plains Audio is one source that provides recharging services.

 

[nola]

Thanx
I have not opened the woofer doors for the Khorns, nor LaScalas, but the mids and tweeters have strong magnets by that criteria. So do all of the drivers in the Heresy Is and IIs that I own. I checked these easily while doing recaps. Ohms across the drivers also told me that a tweeter on the H II was blown. I called GPA re Qs about drivers for Altec 511 horns.
BTW how do you load an avatar here?

 

[Tom Brennan]

It's said by astute people that weak magnets will manifest themselves sonically as weak midrange in a woofer and weak highs in a treble driver.

There's a great deal of discussion on this subject on the forum at the Lansing Heritage site, do a search there.

www.Audioheritage.org

 

[Fred Sanford]

Here's one bit that I'd copied & pasted from the Lansing site:

je

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 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.

 

[toddalin]

Q:

1 - How do you know if the magnet is weak or demagnetized? Is there some type of "meter" to check it with?

2 - If "bad", how and where or by whom can you get back into spec?

Thx

While some report that AlNiCo magnets can lose as much as 1/2% per year in magnetic flux just due to age, many refute this and say this is not so. However, it is known that large blasts of power can weaken AlNiCo magnets.

There is a way to check this. Purchase "Woofer Tester" from Parts Express and measure the BL value. A weak magnet will lower this value.

I had three JBLs AlNiCo woofers (L,C,R) remagnetized and the BL before (the one I checked) was ~18.5 and after was ~22.5. Specs call for a BL of >20. (I've also had all of my compression drivers and tweeters remaged, just "in case.")

Was the "after" difference notable? Not that I could tell off hand and I would have needed to do a direct A/B comparison to tell.

Orange County Speaker can do the remag for you. They don't do a before/after check. They just put it in the "flux capacitor" and hit it with "1.21 gigawatts" of flux to saturate the magnet and call it good.

 

[basplin]

From what I’ve read (including on GPA’s website) tube amplifiers ~30WPC and below do not have the capability to demagnetize speakers, and it’s only high wattage speakers that cause this. Is this true? Around what level of current hit will cause demagnetization?

 

[GordonW]

We once had an Altec 288 (painted purple, labeled Sunn, BTW) that was down close to 6dB, compared to its mate. Swapping the diaphragm, showed that the problem was in the magnet assembly.

Sent the offending weak driver to Great Plains Audio, and got it re-magnetized. It came back within 1/2dB of the other one.

I assume that driver was dropped- HARD- at some point.

Regards,
Gordon.