Speaker magnet recharger schmtc ?

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#2
Caps and Thyristors is about it. Most that I remember charged the cap bank up to several hundred volts and then dump it into the coil. The coil needs careful design as it needs a magnetic circuit that concentrates the flux into the magnet structure. Typically that is an outer pot open at the top and a large center post that the magnet sets on. The coil is wound around the center part of the post. You want to maximize ampere turns so the coil resistance should be rougly comparable to the capacitor source resistance (ESR). The magnetic circuit needs to be thick enough not to saturate. Typically the steel in the woofer circuit will saturate so the magnetic field can blow through it and magnetize the magnet.

Have fun.
David
 
#4
Here is the basic idea.... as said before it is the matter of huge caps and thyristors. I worked with such a machine some time ago, but the size of the caps inside was really scary (if I remember correctly altogether some 10.000 uF at 450 Volts). So be VERY careful if you decide to build one, this thing can kill you in one shot. You will need high gate sensitivity 800V thyristors with at least 200A continous/2000A peak current, and about the coil..... well again if I remember correctly it was made about some 4-5 AWG copper wire (4.5-5.2 mm) epoxy potted (if it is not, the current "shot" can easily deform the coil...). Good luck, and really be careful with this thing!
The schematic is only for getting the idea, you will need larger values to get it working for speaker ferrite ((or NEO)) magnets (1.2-2.4T).

Greetings: Andrew
 

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#5
my friend is having some JBLs and Altecs needing some juice

besides that , I have some TT330N14K ( he said he's having some even bigger ones ) and 12 to 16 bigbadass caps ( by memory 1mF/450V , but need to check exact value and quantity )

though , it would be nice to implement voltage reg (including input doubling autoformer) and maybe even shot-timer ....
 

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#6
Hopefully you are NOT planning on regaussing Alnico motors??
In a word : those Do Not lose flux over time.. well... unless 'time' is 2,000 years...true.
Those only need magging /demagging to work on them.
IF they are monster sized mag castings. Not many JBLs were.. being v frugal with alnico content..
Ferrites do time out as do Neos. But it's passably long enough, although Heat kills Neo dead .. pretty well instantly.
Weber Speakers has an Alnico suitable Gauss gizmo.. Ask them?
 
#8
Cal Weldon said:
I remember a concern with Alnico vs. Ferrite in that the magnetizer has to be switched 180º going from one to the other or you will be demagging one of the types. This was an issue one of our members here had after sending his units to a well known speaker place.
Click to expand...

Typically Alnico is inside the core pole and ferrite is outside, so for the same polarity you need to reverse the magnetizer. No problem with doing it wrong as long as you are fully saturating. We also had one with a very adjustable magnetizing strength. We used it once to demag when the demagnetizer was out. Just set it low and in reverse for the current polarity. Gave it a quick shot and tested magnetism with a screw driver on the side. Flipped it back and forth until we lucked onto zero magnetism.
 
#9
Bare said:
Hopefully you are NOT planning on regaussing Alnico motors??
In a word : those Do Not lose flux over time.. well... unless 'time' is 2,000 years...true.
Those only need magging /demagging to work on them.
IF they are monster sized mag castings. Not many JBLs were.. being v frugal with alnico content..
Ferrites do time out as do Neos. But it's passably long enough, although Heat kills Neo dead .. pretty well instantly.
Weber Speakers has an Alnico suitable Gauss gizmo.. Ask them?
Click to expand...

Alnico won't lose on its own but it is easily demagged by high power amplifiers through the voice coil. When the old JBLs were designed they typically used 20 Watt amplifiers. Give it a couple of blasts with a 300 Watt amp and you can knock it down by several dB. This is the only likely reason why some think Alnico and Ferrite sound different.
 
#10
some more clever chaps then ZM wrote about those issues :

Recharging Alnico/Ferrite magnets - Page 2

Here's the Scoop

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.
Click to expand...
 
#11
speaker dave said:
Alnico won't lose on its own but it is easily demagged by high power amplifiers through the voice coil. When the old JBLs were designed they typically used 20 Watt amplifiers. Give it a couple of blasts with a 300 Watt amp and you can knock it down by several dB. This is the only likely reason why some think Alnico and Ferrite sound different.
Click to expand...

No...Published materials data claim otherwise.
Alnico/cobalt's Domains (magnetic bits) DO deflect in the presence of powerful fields.
However Alnico domains...Rebound... from those deflections pretty well instantly and 100% .. returning to their initial and stable level.
A magnetic spring, in effect.
That is One of the attributes that make them special.
Ferrites don't do such, producing a potentially (IF ALL else is equal ..rarely) sharper instananeous bass impact. That! is the immediately apparent difference in sound.
Ferrite's Main and arguably Only advantage is it's pennies for pound costs.
Ferrite Does lose flux with age. Proven and measured... long ago.
Motors (rotational types🙂 Are measurably reduced in performance simply by the passage of time, sans Any actual usage/operation taking place.
IF considering a Remag Ferrite is the One that actually requires it NOT Alnico.

Likely hundreds of the Naive have been taken advantage of by self serving types.

LOTS of info available on Magnetic Materials Properties..
All one does need do is Search, then read it.
Little need to perpetuate Urban myths. 🙂
 
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#12
Bare said:
Likely hundreds of the Naive have been taken advantage of by self serving types.

LOTS of info available on Magnetic Materials Properties..
All one does need do is Search, then read it.
Little need to perpetuate Urban myths. 🙂
Click to expand...

So when, as a JBL engineer, I measured the drop in output after high drive I guess I was just hallucinating? Or did I have a self serving motive?
 
#15
bear said:
Dunno, but those little horseshoe alnico magnets always went flat after a while...
_-_-
Click to expand...

That gets you into the realm of "magnet keepers" and operating points. If magnets and their operating points are pushed well below "the knee" then there are irreversable losses. Another example is Sturmey Archer Dynohubs. Take one apart and it instantly loses all magnetism.

http://en.wikipedia.org/wiki/Magnet_keeper

http://www.brightsparkmagnetos.com/... armature out of my rotating-coil magneto.htm
 
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