View Full Version : Kit to re-magnetise speaker drivers
Has anyone in the UK come across the sale of kit to (re)magnetise drive units.
First of all you would need to remove the magnet from the speaker basket and cone/voice coil assembly, which will be difficult.
Second the gap flux (the strength of the magnetic field in the voice coil gap) is very large. Magnets are made by placing a magnetisable material into a large coil of wire and passing a heavy direct current through the coil. This requires specialist equipment, normally only possessed by magnet manufacturers, though some speaker manufacturers may have the necessary equipment.
The only manufacturer of magnets in the UK that I know of is Eclipse, https://www.eclipsemagnetics-shop.com/ , though at work we used rare-earth magnets made by a German company called Vacuumschmelze. https://vacuumschmelze.com/
Probably the best bet is to contact a speaker manufacturer to see if they could do the work, or if they don't do it themselves, to send your magnets away to be re-magnetised.
First of all you would need to remove the magnet from the speaker basket and cone/voice coil assembly, which will be difficult.
Second the gap flux (the strength of the magnetic field in the voice coil gap) is very large. Magnets are made by placing a magnetisable material into a large coil of wire and passing a heavy direct current through the coil. This requires specialist equipment, normally only possessed by magnet manufacturers, though some speaker manufacturers may have the necessary equipment.
The only manufacturer of magnets in the UK that I know of is Eclipse, https://www.eclipsemagnetics-shop.com/ , though at work we used rare-earth magnets made by a German company called Vacuumschmelze. https://vacuumschmelze.com/
Probably the best bet is to contact a speaker manufacturer to see if they could do the work, or if they don't do it themselves, to send your magnets away to be re-magnetised.
Thanks Barry. I have found it very difficult to find someone that will do it, so for that reason I thought of buying the kit and offering the service myself. From my enquiries it seems that if your magnetising coil chamber is big enough then you can place the whole driver in there without having to dismantle it. In reality it only needs to be deep enough to accommodate the driver magnet. The basket etc can stick out the top.
Do I have this right.
I'm not sure.
I would have thought the original magnet was made by magnetising the outer part first (the part external to the speaker), then the magnet is assembled with the centre coaxial pole piece to create the strong gap flux experienced by the voice coil.
If you re-magnetise by placing the whole magnet assembly into the magnetising coil, you will end up with a magnet having one pole at the speaker cone end (including the centre pole piece) and the other pole at the back of the magnet. There will be no (or very little) field in the voice coil gap.
I'm not sure.
I would have thought the original magnet was made by magnetising the outer part first (the part external to the speaker), then the magnet is assembled with the centre coaxial pole piece to create the strong gap flux experienced by the voice coil.
If you re-magnetise by placing the whole magnet assembly into the magnetising coil, you will end up with a magnet having one pole at the speaker cone end (including the centre pole piece) and the other pole at the back of the magnet. There will be no (or very little) field in the voice coil gap.
Thanks Barry. That seems a fundamental issue. I will do some googling. :)
Found this so far.
https://youtu.be/-VMJN_hoVbk
Have no idea of the size of the coil windings (or the number of turns), but judging by the size of the current carrying cables feeding it and the size of the power supply, a pretty heavy current is used. I couldn't see if the amount of current was indicated on the power supply.
Have no idea of the size of the coil windings (or the number of turns), but judging by the size of the current carrying cables feeding it and the size of the power supply, a pretty heavy current is used. I couldn't see if the amount of current was indicated on the power supply.
I have seen other magnetisers working on smaller drivers and I believe they charged the magnet at 600v. I am not looking to build the kit, just buy it :)
Chris, interesting idea of re-magnetising speakers. Not really come across this much before so did a bit of googling.
Interesting article about how speaker magnets lose their quality etc here (http://web.archive.org/web/20131012190423/http://www.vancouveraudiospeakerclinic.com/alnico.html).
Do you have any research which shows improvement that can be had after re-magnetising? Are you thinking of providing a service?
A related subject are speakers with field coils (eletromagnets). I purchased an item from a guy who had a pair of Wolf von Langa field coil speakers. (https://wolfvonlanga.com/wvl-12639-son.html) Unfortunately it was not possible at that time to have a listen but from what I read the sq has a naturalness not generally found in 'normal' speakers.
Is it Alnico magnets you are thinking of (if not I wouldn’t bother). As for magnetising, I’ve seen two videos from manufacturers (ATC and Harbeth as I recall) and both showed the magnets being magnetised once the whole speaker (the drive unit that is) had been built.
My main interest is in re-magnetising Alnico drivers from the 1950/1960 era. Luckily most of these have relatively modest magnet physical size. I have no evidence to show how it might rejuvenate the sound, but wouldn’t it be nice to bring these 70 year old drivers back to full power.
Are you sure they have lost magnetism?
Are you sure they have lost magnetism?
The article sited in post #9 sums it up. The only way to be sure of listening to the drivers performance as the maker intended is to have the magnet re-energised. Expecting them to have the same magnet strength 70 years on is an act of extreme faith.
Any of you technically gifted people have a view on this bit of kit
http://www.magnetics.in/magnetiser.html
It’s not the type used by any audio manufacturers I know of - they all use a coil of wire into which the magnet is placed. In principle they are extremely simple.
dont say how much they are tho... a lot of folk with Alnico magnet speakers will beat there way to your door tho
I’m not so sure. The whole issue isn’t a new one and there already are companies that offer this service. In addition, whether it needs doing at all is hotly debated. Even an Alnico magnet won’t demagnetise unless struck repeatedly or heated heavily by the voicecoil - just using it won’t cause it to lose magnetic force (even after 70 years). In addition, to remagnetise it would need one to know what the original was or the sensitivity and magnetic damping if the speaker will be effected.
In addition, to remagnetise it would need one to know what the original was or the sensitivity and magnetic damping if the speaker will be effected.
As I understand it, the capacity/strength of the magnet in a given driver is determined by the chemical composition, size and design of the magnet assembly. Any re-magnetising equipment normally vastly over 'magnetises' (swamps) the material which can only magnetise up to its original specification. You cant overdo it.
Here is a typical Alnico magnet assembly on an 8" Saba Greencone. Its a tiddler :)
http://www.troelsgravesen.dk/greencones_files/greencone_2_1000pxw_intro.jpg
Here is a typical Alnico magnet assembly on a Saba Greencone. Its a tiddler :)
http://www.troelsgravesen.dk/greencones_files/greencone_2_1000pxw_intro.jpg
mine are similar but a bit bigger. 10"
As I understand it, the capacity/strength of the magnet in a given driver is determined by the chemical composition, size and design of the magnet assembly. Any re-magnetising equipment normally vastly over 'magnetises' (swamps) the material which can only magnetise up to its original specification. You cant overdo it.
Ah, yes, that makes sense.
What are the benefits of remag'ing a speaker?
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What you need to know is the stated gap flux of the speaker magnet you wish to re-magnetise. From that, you can calculate the total stored energy of the magnet, and this will give you some idea of the energy requirement of the re-magnetiser.
The device shown is a pulsed energy device having a maximum pulse energy of 10kJ (the manufacturer quotes this as 10,000Ws). This ought to be sufficient for small magnets, but I'm not sure if it is enough for large speaker magnets - but I'm no expert.
Unfortunately I can't open up and read the link cited in post #9, so cannot comment on the stability/longevity of Alnico magnets. Whilst Alnico magnets are more prone to demagnetisation than the newer rare-earth types, from my limited reading of the subject, AlNiCo(5) magnets only 'loose' < 3% of their field strength after 100,000 hours (approx. 11.5 years), dependant on operating conditions. So the condition of a 70 year-old Alnico speaker magnet is debatable.
Unfortunately I can't open up and read the link cited in post #9, so cannot comment on the stability/longevity of Alnico magnets.
I've downloaded the relevant page (it is on the internet archive) to a pdf and placed it here (https://1drv.ms/b/s!Arf68P30ulKQgqVKQvpYFN4a_w4DAg?e=npThtX).
Thanks Edward
Unfortunately there is a screen navigation pop-up at the top of each page, covering an important part of the text which discusses both the contribution to the demagnetisation by an audio signal, and the re-magnetising capability of the pulse magnetiser. But what I can read supports what I understand of the problem (though I hadn't realised that older speakers use a much larger diameter voice coil than modern speaker designs). The typical gap flux of a modern speaker is ~ 12,000 Oe, so the quoted magnetising field of 20,000 Oe ought to be sufficient to provide saturated re-magnetisation.
Thanks Edward
Unfortunately there is a screen navigation pop-up at the top of each page, covering an important part of the text which discusses both the contribution to the demagnetisation by an audio signal, and the re-magnetising capability of the pulse magnetiser. But what I can read supports what I understand of the problem (though I hadn't realised that older speakers use a much larger diameter voice coil than modern speaker designs). The typical gap flux of a modern speaker is ~ 12,000 Oe, so the quoted magnetising field of 20,000 Oe ought to be sufficient to provide saturated re-magnetisation.
Oh I see what you mean. Pasted below is the text. The relevant website no longer exists and the business closed down so I'm claiming 'fair usage rights'.
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There are five primary mechanisms working in concert to demagnetize Alnico magnets - fortunately only a couple of them are likely to affect loudspeakers...
1. Age:
The flux in all magnets diminishes slowly over time, due to thermal fluctuation of the magnetic moment. All grades (3, 5, 5G, 7, & 8) of Alnico suffers from this problem more than ceramic ferrite magnets, and what we call "permanent magnets" are not quite permanent after all. The lower the permanence coefficient of the magnetic circuit, the faster the magnetic domains return to their random equilibrium. In typical Alnico 5 or 5DG magnets, the flux drops very slowly, at 2~3% per decade, for broadband sensitivity loss of only 1dB over 40-years, although losses at high frequencies may be more severe. Sumitomo Special Metals Co, of Osaka, Japan, have documented this phenomenon in considerable technical detail, but the research is not available on-line.
2. Thermal losses:
There are two distinct forms of temperature related flux loss: reversible, where the flux recovers to its original level as the temperature returns to normal; and irreversible, where the flux never recovers. Good thermal design in the magnetic structure minimizes reversible losses, and (mercifully) the temperatures that endanger alnico alloys, the Curie point, are far higher than those observed in loudspeakers, at 520 degrees Celcius. In loudspeakers, thermal losses in magnetic energy can be neglected.
3. Pulse loss:
Alnico speakers are intrinsically susceptible to modulation of the flux by the AC field from the voice coil. When operating normally, this momentary (AC) flux is superimposed over the permanent (DC) magnetic field. During normal listening, there the magnetic flux is dynamically modulated, causing third harmonic distortion. This non-linearity is a major contributor to the 'Alnico Sound', and is often welcomed by musicians and Alnico speaker lovers. Unfortunately, as the input power rises, the strength of the modulating field may exceed the intrinsic coercivity of the Alnico, partially degaussing the magnet. A single high power 'pop' can reduce sensititivity by 2dB, and repeated pulses, such as loud cracks from a faulty connection will cause additional (smaller) losses. Alnico drivers become noticeably quieter, especially at the top of their range. A speakers' sensitivity to discharge is proportional to the aspect ratio of the Alnico slug, the length of voice coil winding, and amount of available amplifier power. Virtually all Alnico woofers that have been enjoyed loudly show some flux loss, and those with short slugs and long coils suffer most. Most such designs date from the 1950s and 60s, when 35-watts was considered high-power, and amplifiers lacked the ability to endanger magnets. The problem effects virtually all Alnico drivers that have been used with modern, high power amplifiers.
For example: Low frequency drivers with 4-inch voice coils (JBL D120, D130, D140, LE15, 2205, 2215, 2231, Gauss, or TAD drivers) can lose 2~3dB, becoming 'dull' or 'slow' sounding, unless they have been grossly abused, in which case they more and sound dreadful. Drivers with short Alnico slugs and 3-inch voice coils, (JBL 2213, 123A, etc) are effected more severely, so typically show 3dB sensitivity loss, while small alnico woofers (JBL 116A, 125A, 127A etc) are commonly 4dB down after quite modest use. Conversely, speakers with higher coercivity ( taller Alnico slugs) and short voice coils, such as Tannoy dual concentrics, Altec and EV woofers, rarely suffer unless they have been badly abused. Users report that Altec 416, 515, and 604 drivers become 'mushy' after heavy use, but most of their loss of definition is due to fatigued cones. Remagnetization can restore the magnet, but cannot address cone fatigue.
Most Alnico compression drivers (JBL LE85, LE175, 375, 2410, 2420, 2440, 2441, Altec 288, 290, 802, 804, 806, 808, Emilar EA-175, TAD 2001, 4001, Goto Unit, etc) have tall magnets, short voice coils, and modest power inputs, so do not suffer degaussing. Those compression drivers with low aspect ratio magnets, especially if used <500Hz (Vitavox S2, University ID series, Klipsch K-55-V, Electrovoice 1823M etc) can suffer flux loss, and will benefit from remagnetization.
4. Shock:
Knocking or hammering on an Alnico magnet causes a sudden, dramatic reduction in magnetic energy as the shockwave knocks the closely aligned magnetic domains from their parallel pattern into a more random pattern. Shocks during shipping (from accidental drops) can cause a reduction in sensitivity. Mercifully, beating expensive loudspeaker magnets with hammers is not a common pastime. The solution is remagnetization, and, providing nothing else was broken, original performance is restored.
5. Disassembly:
If an Alnico magnet structure is opened, such as to re-centre a shifted pole-piece, repair a broken throat, or remove a bug-screen, the magnetic field collapses instantly. While this type of damage is not common, ill-advised repairs create very unwelcome problems. If a compression driver is dropped, aside from the possible 'shock' loss (see above) the magnet may shift inside the structure, breaking the throat molding. In these circumstances, the output may drop 20dB, as the magnet is seriously discharged. If there are other issues such as shifted pole, broken throat, corrosion, or loose internal parts, we can discharge the magnet, repair the problem, then remagnetize the driver, back to factory spec.
THE SOLUTION
Recharging an Alnico magnet can be performed without driver disassembly, or leaving any marks. Vancouver Audio Speaker Clinic use an industrial capacitor-discharge magnetizer that can deliver 440 volts at 13,000 amps into the water cooled charging-fixture that completely surrounds the magnet. The 10ms pulse generates over 20,000 Oersteds, completely saturating any magnet structure, returning it to its absolute maximum flux. In refining our recharging process (using our own drivers) we measured (before and after) flux density, sensitivity and frequency response.
We charge a flat rate fee for recharging Alnico drivers with magnet structures up to 190mm (7.5") diameter. If there is no cone or diaphragm in the driver (allowing access to the gap) we measure and record the flux density before and after recharging. Our probe is 0.85mm (0.034") thick, so fits all but the narrowest gaps. Narow gap drivers incl; JBL 'bullet' variants (075, 076, 077, 2402, 2403, 2404 and 2405, and Beyma tweeters. We can remagnetize these drivers, but measurement is not practical. We recharge woofers, midranges extended-ranges and full-range units, incl: Lowther, Fostex, and Celestion guitar speakers. There are some drivers that rarely need recharging, except if they are accidentally damaged, incl: JBL LE85, LE175, 2410, 2420, 2421, 2425, 2460, 2461, 2470, 375, 376, 2440 & 2441, Altec 288, 290 & 291, 802, 804, 806 & 808 as well as Emilar, Renkus Heinz, Coral, Goto Unit, B&C;, RCF and Yamaha models.
The good news for terminally careless audiophile is that there is no limit to how many times an Alnico magnet can be recharged. There is an additional (modest) labour charge if the magnet also needs to be recentred and/or reglued.
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