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Old 15th April 2018, 05:02 PM   #1
Etocynned is offline Etocynned
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Default Magnet types

Ive often seen in discussions about good sounding drivers that Alnico magnets are prized.

Does Alnico have a sound or is this a nostalgia thing.

Looking at magnetic field strength Alnico has less strength than neodymium of almost any grade.

Why not use Neodymium?
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Old 15th April 2018, 05:44 PM   #2
Jonathan Bright is offline Jonathan Bright  Australia
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I am not 100% sure but I think people argue that over time Alnico retains its magnetic strength for longer. But you will have to wait till the gurus log on for a better answer.
Cheers, Jonathan
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Old 15th April 2018, 06:01 PM   #3
Scottmoose is offline Scottmoose  United Kingdom
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Actually AlNiCo is the most vulnerable of the three main groups of popular materials to demagnetisation.

A degree of nostalgia will play a part. Also marketing in recent years has played upon the perception of AlNiCo being superior, usually in unspecified ways. Note that there are different grades of all the magnet materials & a high spec. version of what might be perceived as an 'inferior' magnet type may in fact be technically superior to a low grade of a magnet type that is perceived as being 'superior'.

To that you can add the fact that it's not just about magnet material; the entire motor needs to be designed around it. AlNiCo magnets often need to be physically quite long for e.g., and this affects the physical shape, what you do with the VC gap & a host of other factors. It may ('may') overload more gracefully than some, or allow this. Neodymium magnets have their own peculiar traits & while very suitable for tweeters aren't always quite so suitable for high-power applications. Ferrite magnets are nominall weaker but a little more flexible in certain design terms. It's as much about what you do with the material as the material itself. That being said, AlNiCo does suit certain uses, and many drivers using it are very highly prized -take the Bell / Altec 755. The early A models (AlNiCo magnets) are very highly prized. The later ferrite models e.g. the C less so, even though on balance their EM damping & other design featurs didn't see wildly dramatic changes AFAIK.
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Old 15th April 2018, 06:29 PM   #4
weltersys is offline weltersys  United States
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Quote:
Originally Posted by Etocynned View Post
Ive often seen in discussions about good sounding drivers that Alnico magnets are prized.

Does Alnico have a sound or is this a nostalgia thing.

Looking at magnetic field strength Alnico has less strength than neodymium of almost any grade.

Why not use Neodymium?
Magnets don't have a "sound", but the strength to size and weight ratios result in different assemblies that have different Bl curves that do result in different characteristics.
Alnico was more expensive than ceramic (slab) magnets, so was generally used in loudspeakers that were a bit more "high end" than the run of the mill.
One could certainly be nostalgic for the better drivers made in Alnico's "glory days".

Neodymium by itself looses it's magnetism at relatively low temperatures, it was not until the mid 1980's that high temperature Nd2Fe14B formulations were introduced commercially in the automotive industry. Nd2Fe14B formulations soon were adopted by Electro-Voice for use in HF compression drivers and microphones. Neo was adopted by more and more companies which eventually brought prices down to the point where it eventually was adopted in "low cost" applications.

By the mid-1980s, the price of cobalt used in Alnico had gone up to the point where Alnico had fallen out of favor, replaced by larger, heavier ceramic magnets.
People still liked the slightly lighter weight of Alnico, but weren't willing to pay for it.

Alnico looses it's magnetism more than ceramic when subjected to gravitational shock, so was found to have long term problems on the road-if an Alnico driver looks "beat up", it may need to be remagnetized to perform like it originally did.

"Neodymium" as used in the most powerful magnets available also now uses an alloy of yttrium and dysprosium, those ingredients (like cobalt) have had massive price fluctuations, which has tended to keep the cheaper ceramic magnet structures around for applications where a premium price for weight reduction won't be considered "worth it".

Art

Last edited by weltersys; 15th April 2018 at 06:35 PM.
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Old 15th April 2018, 08:36 PM   #5
tomtt is offline tomtt
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No mention of distortion?

Would that make a difference?

( Actually, I dislike rhetorical questions )

A few clues, from people that have designed speakers -

Speaker Forum - Alnico verses ferrite verses neodymium - Ralph, August 24, 2004 at 17:37:22


Quote:
Ragnar Lian, co-founder of Scan-Speak, said that speakers with alnico magnets had 10x less distortion than ferrite and neodymium had about 20x less.
When a copper ring is added to a ferrite magnet, it can approach neodymium specs but only above a certain frequency, below which the ring becomes ineffective.

Sorry, I have not noticed this group.
But here I am, still alive. Of economical reasons I have changed from loudspeakers to medical actuators, but the same physics are ruling.
I do not know the reason for previous question, but I guess it is caused by an old JBL slogan "the alnico sound". The sonic difference alnico-ferrite is caused by more parameters.


The physical shape of the normal ferrite magnet system is close to whorst case, causing many distortion mechanisms. Also ferrite magnets are relative "soft". When you put a current through the voice coil, it acts at an additional magnet, modulating the main magnet.

Alnico have a sharp break point, at the right side magnet are "stiff", and on the wrong side, it is demagnetized. This was the main problem when we had the first generations og high power audio amplifiers (Dynaco Mark 3 and Stereo 120). Either it was OK, or the speaker was significantly and permanetly degraded .
For ferrite, sound was normal more or less bad, but speaker was not demagnetized.
I have made tests and simulations on 3 differend magnet systems having same linear movement and same Bxl, locking the coil in mid position, applying 10 amp current, and observing force in + and - direction. Result can vary, but the metioned numbers is representative. Of course, this is static properities, and dynamic properities may be different. I have not had time for further investigatemens, but I am an okd guy, and there shall also be lot of work and fun for younger people, come along !

- Ragnar Lian

Recharging Alnico/Ferrite magnets - Page 2




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.

- Greg Timbers
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Last edited by tomtt; 15th April 2018 at 08:40 PM.
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Old 15th April 2018, 09:00 PM   #6
Etocynned is offline Etocynned
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Actually in the chart of materials on the website i saw alnico was almost a vertical line where neodymium was a 40° slope.
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Old 15th April 2018, 09:35 PM   #7
JMFahey is offline JMFahey  Argentina
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Well, I commercially design and manufacture speakers.

1) the voice coil floats inside a perpendicular (to current path) magnetic field and does not know or care what made it (Alnico/Neodymium/Ferrite or Field coil) , only its intensity.
So if it floats inside a, say, 10000 Gauss (1T) field, no matter *what* created it, voice coil will react the same.

2) being perpendicular , voice coil *interacts* with it, and moves, thatīs the basic principle behind electric motors, but does not "modulate" the externally supplied magnetic field.
If it did, it would kill it in a short time.

2)
Quote:
Also ferrite magnets are relative "soft".
Magnetic "hardness" is called "Coercivity" and is basically a measure of how hard it is to realign magnetic domains (at the molecular level) which allow permanent magnetization and demagnetization.
Plain steel in various grades has lowest coercitivity, so itīs easy to magnetize even for very low magnetic fields, ... and instantly loses it when external field is removed.

Next is Alnico.

Then Ferrite, some 4 times as hard as Alnico.

Then Neo and similar formulations, so hard that it can not be magnetized in iron core magnetizers (core saturates way below necessary field strength) so you need air core coils driven by thousands Ampere Capacitive Discharge supplies.

I should know, I own three conventional magnetizers and am building the fourth one, specifically to make NEO speakers.

3) As of "Alnico distorting 10/20X less than Ceramic" is, at best, poetic licence or ignoring a few Physics facts.

Truth is that Alnico magnetic structures had been developed for decades, while Ferrites were "new kid on the block", Engineers were NOT familiar with them, and first magnetic structures using them were crude, just 2 flat disks and a piece of cold rolled round steel bar as polepiece ... but later they refined their act considerably, specially making symmetrical fields (plain ones are strongly asymmetrical) and adding conductive rings close to the voice coils.

Alnico, being metallic (actually itīs just one special kind of steel alloy) acts as one by itself, while ceramic is ... uh .... "ceramic" and so a high grade insulator, you have to add a shorting ring (copper or aluminum) on purpose.

I attribute the demagnetization of Alnico precisely to its softness: besides current through the voice coil which is actually perpendicular to its field, so only some fringe field might act on the main one, anything that makes main field move along the magnetizing curve is bad, because itīs a lossy path, and it will never return to former point but to a lower one.
Every time and the effect is accumulative.

So when people carelessly let one Alnico magnet stick to another (Alnico or Ceramic), put it way too near to a transformer or choke or electric motor and similar abuse, it will always lose at least a little ... along the years itīs a mess.

Now if you let Alnico unmolested (say in a Cinema Sound system) for decades, it can hold its own very well.

Removing a magnetic system plate for recentering (when it was knocked hard and shifted, pinching the voice coil in the process) is DEADLY.

A Ceramic can lose 20/30% and still remain usable (although losing "snap"); an Alnico will lose 60/70% and become mush, so only Speaker Shops able to do that are those having their own Magnetizer (or access to one, say, in a nearby Factory).
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