AlNiCo demagnetisation

[badman]

Anyone have extensive experience with this phenomenon? For those who don't know what I'm talking about, AlNiCo is known to lose some of it's magnetic strength when subjected to a sufficiently large voicecoil current. This loss is supposedly as much as 3dB, though 1.5 dB seems much more common.

A few questions/thoughts:

Is this loss always a bad thing? I'd think that you'd lower the sensitivity of the driver but increase Qes. You might also see some top end issues, but how might they manifest themselves?

How much is Qes affected by a loss of charge? One might assume that a 3dB sensitivity loss would correspond to a doubling of Qes, and less with lower loss levels

Are there any other effects I'm not considering?

 

[Tubamark]

Extensive experience - not really. But I can verify that it is real. I have a pair of 1964 Bozak Alnico Woofers that were given to me. The Qts on these is supposed to be somewhere around .5, but mine clock in at 1.45 Qts!

As the speaker otherwise seems to be operating fine, I can only assume the Alnico has lost its pep.
Since I don't know what the original Qes is supposed to be, I can't verify that a changed Qes is what caused the Qts I have, but its a reasonable assumption.

-- Mark

 

[badman]

Upon running some tests last night, I found that the vintage JBLs I was testing had not suffered much of this effect. They were within +/- 10% of manufacturer spec on Qts, and well matched between the 2 sets (+/- 2% per model %).

But who knows how much abuse they've seen. Wonder if the cast iron return circuit does anything to reduce their susceptibility to demagnetization.

Still have to test the 122A, which is a monster AlniCo motor.

 

[pinkmouse]

That high end favorite, the TAD4001 is renowned for this. They age rapidly and unpredictably, which amongst other issues is why the PA industry stopped using them in the eighties.

As for alnico problems under load, Tom Danley posted on this a while back, a search should find it.

 

[Robh3606]

"Upon running some tests last night, I found that the vintage JBLs I was testing had not suffered much of this effect. They were within +/- 10% of manufacturer spec on Qts, and well matched between the 2 sets (+/- 2% per model %)."

Which ones were you looking at?? Short underhung coils like the LE-15A/K-145 cannot generate enough of a field to demag the drivers. The over hung coils like the 2231A or 122A would be the type that this could occur. I use a bunch of older JBL alnico's and they sound fine and tune up just the way they are supposed too.

With these older drivers there is no way to know what if any abuse they have seen. I think most of the consumer woofers are usually in good shape. Figure a 2231A would be one of the more likely to get demaged woofers. If it does occur there will be some rolloff up top with the demag. Depnding on how high you push the driver it may or may not be an issue.

Rob

 

[badman]

123As and 2213s. Same driver except 2213 has a larger diameter spider and a different frame.

Once I refoam the 122As I'll run them as well. I'll also test some LE5-2s.