Alnico Magnet Question

[tekno.mage]

Or does your Alnico lose it's flux on the bedpost overnight? <grins>

More to the point, in the process of repairing a pair of loudspeakers I had to disassemble the Alnico magnet assy of one of them - and the magnet parts got left apart for longer than intended (2 - 3 weeks). On re-assembly the unit with the magnet that was taken apart is now around 3dB - 5dB less sensitive than it's friend (which may not have been identical - I had no chance to hear them before repair).

Units in question are a pair of HPD315s. And yes - the sensitivity loss affects both drive units of the dual-concentric - tested independantly - ie no xover. Only change apart from magnet take-apart was replacement of rotten surrounds. Obvious things like voice coil impedances all check out ok. Crude measurement of freq response with sig genny, scope & sound meter suspended a meter above up-facing driver gives curves of same shape, but one between 3 & 5 db less than other.

Question 1 : was three weeks long enough for the disassembled magnet to fade away to this audible extent? The Alnico ring was kept safe in a cardboard box, away from other magnets, while the other parts were cleaned up.

Question 2 : is this amount of difference in sensitivity between two units of same manufacturer, model, year & month of manufacture (but not sequential serial nos or matched drivers) within manufacturer's tolerences - for Tannoy in 1976?

Question 3. If I've messed the magnet, how do I remagnetise (or where in UK?) and how would I measure flux density of the two magnets anyhow?

Hoping someone can help (feel free to snigger - I know, I took it to bits and now it don't work right...) Hmmm I *have* checked all the obvious like bits fitted right way round, wiring correct etc etc. Free air resonance of LF new surround is about right. Less sensitive speaker still *sounds* like a Tannoy DC - just a bit quieter than the other one.)

Cheers,
tekno.mage

[AndrewT]

Hi,
I think I remember something like this in school physics. It is why you store magnets with that little plate across the poles.
Also the magnets lose quite a bit due to heating.
Try contacting Tannoy up in Coatbridge, Scotland. They may offer a service to rezapp alnico.
Let us know the result if you find a solution.
regards Andrew T.

[tekno.mage]

Hi Andrew,

Hmm. It didn't get heated at all (anything but - it was quarantined in the hall after I saw what it did to my Mac screen from 18" away!) but it didn't have any plates across the ends when it was apart (they were being cleaned along with the "pepperpot" which was one of the reasons to take it all apart).

I must admit I've always kept spare magnets stuck to metal things like filing cabinets - to keep them away from sensitive things like recorded magnetic media (disks, tapes etc) and to prevent them attracting each other so hard I can't get them apart!

I didn't do the 'stick on metal furniture' thing alnico ring as a friend mentioned he'd heard that sticking magnets to metal furniture drains the magnetism....

I'll certainly keep people posted as to progress on this one, and also the blow-by-blow photos of the driver repairs and dis-assembly, plus measured results when done.

I was hoping to find a loudspeaker repair shop down here in Hampshire (rather than Scotland!) who might be able to re-magnetise should it be necessary. I don't think it is that hard to do, providing you have the relevant (and power-hungry) magnetization machine on hand. I used to have a huge mag-tape de-gausser (for bulk erasing big computer tape reels several at a time), but it's long gone and probably wouldn't have been suitable. I've read that it's possible to re-magnetise with the driver fully assembled - which would be nice <grin>

I suppose one way to get an idea of how strong the two magnets are is to measure how far away they have to be from the Mac screen before I see an effect..... But I doubt that will be a sensitive enough test - and the screen may not like it much!

[Ouroboros]

I've seen a magnetizing jig in use. Basically a big bank of capacitors get discharged into an air-cored coil, with the magnet to be magnetized positioned in the coil. Then the switch is pulled and the whole jig does a jump! Not the sort of thing you can do yourself, but I bet it won't be too difficult to find a place to do it. Better do both magnets to ensure that the speakers are still matched.

[phase_accurate]

I once heard that Alnico magnets don't like it if the magnet assemblies are disassembled by shifting the magnets or polepieces sideways. But I don't know the reason for that.

New drivers are indeed magnetised after complete assembly usually. So Tannoy should be able to remagnetise the assembled driver if they still have the matching equipment for your oldie.

Regards

Charles

[tekno.mage]

Originally posted by Ouroboros
I've seen a magnetizing jig in use. Basically a big bank of capacitors get discharged into an air-cored coil, with the magnet to be magnetized positioned in the coil. Then the switch is pulled and the whole jig does a jump! Not the sort of thing you can do yourself, but I bet it won't be too difficult to find a place to do it. Better do both magnets to ensure that the speakers are still matched.

That makes sense - I plan on looking up the local speaker recone companies to see if one of them will oblige. But if all else fails, I'll talk to my friend who builds Tesla coils - he's already got huge capacitor banks, large coils, big power supplies etc, so maybe it's something we'd be able to put together if all else fails!

Comments from people on the Tannoy specific forum I read agree that I managed to lose about 30% flux by taking the alnico apart - and yes, I'd get both units remagnetised so they'd both (hopefully) then perform the same.

[AndrewT]

Hi,
the magnet will get warm when in normal use and hot when driven really hard.
That could be due to two sources;
1. radiation from the hot coil and
2. I think the magnet has a self heating effect when the speaker coil is being driven. Please correct me if i'm wrong.
regards Andrew T.

[planet10]

Originally posted by Ouroboros
I've seen a magnetizing jig in use. Basically a big bank of capacitors get discharged into an air-cored coil, with the magnet to be magnetized positioned in the coil. Then the switch is pulled and the whole jig does a jump!

These things store a scary amount of energy (ie can vaporize nails)... not something you do casually.

dave

[capslock]

If I remember correctly, AlNiCo stays magnetized at room temperture even if the return circuit is removed. Some grades of AlNiCo are pretty temperature sensitive, though.

Ferrite will lose some of its magnetization the instant the return system is removed.

See

www.magnetsales.com

for a primer on magnetization.

[AndrewT]

Hi,
I have looked up that magnetsales link (a good site Thanks) and alnico is the most resistant to hi gh temp loss of all the magnetic materials. I think you can discount hi temps.
the ref also quotes a variety of alnico types. I'm going to start a new thread on Tannoy alnico.
regards Andrew T.