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
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
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.
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.
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!
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!
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.
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
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
Ouroboros said:
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.
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.
Ferrite will lose some of its magnetization the instant the return system is removed.
See
www.magnetsales.com
for a primer on magnetization.
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.
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.
I doubt the magnetism changed significantly between the two in the space of a few weeks. They were probably like that to begin with. As others have mentioned, getting both of them remagnetized is highly recommended. Depending on the design, the magnets can sometimes be remagnetized without any disassembly. Sometimes they have to be partially or entirely disassembled.
Hi, bad luck with your magnet. The same thing happened to me once and I made some research regarding Alnico magnets. If you break the magnet circuit it will always lose about 50%. You will need to assemble it and have it remagnetized. Alnico magnets are always assembled and then magnetized.
Check with local magnet suppliers, they usually have a magnetization rigg and may be able to help you. I have some Altec driver that I have remagnetized and it worked OK.
Best regads: Ulf
Check with local magnet suppliers, they usually have a magnetization rigg and may be able to help you. I have some Altec driver that I have remagnetized and it worked OK.
Best regads: Ulf
Damn. This happened to me too yesterday. I took apart a rusty JBL 2470 for cleaning. Looks good but plays 6 dB lower than it should.
Where should I send it for magnetization?
Read about magnet load lines, e.g. here: DYNE Analytics • View topic - Magnetic Load Lines
Alnicos have a very "curvy" so-called intrinsic curve. As soon as the magnet is put under load (that means the magnetic flux "sees" some resistance, for example an air gap) the magnet gets partly demagnetized (depending on the load). It won't demagnetize any further as long as the load isn't increased, but you'll have to magnetize it again to bring it back (and it will demagnetize itself at once if the load is still there). Therefore magnetized Alnicos should not be disassembled.
Neodyms or Ceramics have a nearly rectangular curve. They won't get partly demagnetized under load, but if you push them too far they're nearly completely demagnetized.
Alnicos have a very "curvy" so-called intrinsic curve. As soon as the magnet is put under load (that means the magnetic flux "sees" some resistance, for example an air gap) the magnet gets partly demagnetized (depending on the load). It won't demagnetize any further as long as the load isn't increased, but you'll have to magnetize it again to bring it back (and it will demagnetize itself at once if the load is still there). Therefore magnetized Alnicos should not be disassembled.
Neodyms or Ceramics have a nearly rectangular curve. They won't get partly demagnetized under load, but if you push them too far they're nearly completely demagnetized.
Yes, I have learnt this through stupidity. Now the question is where to magnetize my JBL 2470 in Northern or Central Europe.
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