recharge Alnico V mags

[robertbartsch]

I'm considering having the mags recharged in a pair of 075 tweets.

I was told by my JBL re-coner (Erik Wayne from Pomptom Plains NJ) that it requires a de-mag and re-mag and that he does not provide this service.

Who does this and how much should I expect to pay?

[Robh3606]

Have a read

http://www.audioheritage.org/vbullet...ead.php?t=4031

Rob

[robertbartsch]

Ok - thanks I have read the magnet info here a few times already.

My drivers were purchased from Fleabay and are +30 years old. The frams are blown and I would like everything to be OK before I install the new units.

[JoMoCo]

GPA (Great Plains Audio) are the only ones I am aware of at this time, but I would also like to know if any other reputable speaker outfit closer to home (the Northwest) does a good job of recharging. Anyone?...

[toddalin]

Have a read

http://www.audioheritage.org/vbullet...ead.php?t=4031

Rob

There is one simple difference between the Alnico and ferrous drivers that has the potential to affect the sound that is never discussed/disclosed in those papers.

The Alnico magnet is located inside of the voice coil whereas the ferrous magnet lies to the outside. This has other ramifications. For example, on the ferrous woofer, the magnet is located directly adjacent to the voice coil, whereas with the alnico woofer, it is next to the bobbin.

[edgewound]

There is one simple difference between the Alnico and ferrous drivers that has the potential to affect the sound that is never discussed/disclosed in those papers.

The Alnico magnet is located inside of the voice coil whereas the ferrous magnet lies to the outside. This has other ramifications. For example, on the ferrous woofer, the magnet is located directly adjacent to the voice coil, whereas with the alnico woofer, it is next to the bobbin.

What ramifications does this detail have?

The difference in sound between the alnico and ferrite JBL's has everything to do with the refinements of the flux field symmetry that was researched by the necessity of changing over to ferrite due to cobalt scarcity at the time.

In other words....the ferrite drivers were not only cheaper to manufacture, they were more accurate too....and heavier, by a quite a bit. An advantage of the alnico pot structure with the magnet in the center is it's self-shielding with much less stray field for CRT's, and magnetic media. The added weight can actually be a plus for home speakers due to the increased mass that the cone assembly pushes against....less re-coil.

The only effect the bobbin would have is if it is made of metal....like aluminum. This creates eddy currents in the gap that can cancel out high frequencies and add distortion.

[edgewound]

I'm considering having the mags recharged in a pair of 075 tweets.

I was told by my JBL re-coner (Erik Wayne from Pomptom Plains NJ) that it requires a de-mag and re-mag and that he does not provide this service.

Who does this and how much should I expect to pay?

If he has a gauss meter, he can check the flux density in the gap and determine if it really needs a recharge or not.

[Robh3606]

If he has a gauss meter, he can check the flux density in the gap and determine if it really needs a recharge or not.

Yes and there is an excellent good chance they are fine as is.

Rob

[toddalin]

What ramifications does this detail have?

The difference in sound between the alnico and ferrite JBL's has everything to do with the refinements of the flux field symmetry that was researched by the necessity of changing over to ferrite due to cobalt scarcity at the time.

In other words....the ferrite drivers were not only cheaper to manufacture, they were more accurate too....and heavier, by a quite a bit. An advantage of the alnico pot structure with the magnet in the center is it's self-shielding with much less stray field for CRT's, and magnetic media. The added weight can actually be a plus for home speakers due to the increased mass that the cone assembly pushes against....less re-coil.

The only effect the bobbin would have is if it is made of metal....like aluminum. This creates eddy currents in the gap that can cancel out high frequencies and add distortion.

So then, why weren't they made like this in the first place, and why did JBL go to so much trouble to do it as they did in the first place instead of how the competition did it?

Are you saying that the competition was ahead of the curve at the time and JBL was full of beans?

Could be a world of minute details associated with the rearrangement of the magnet structure, some of which may not be measureable using test equipment.

Show me the "white papers."

[Robh3606]

So then, why weren't they made like this in the first place, and why did JBL go to so much trouble to do it as they did in the first place instead of how the competition did it?

You have been on this site for how many years?? You don't understand why JBL originally made them using Alnico?? Your joking right just breaking Edges chops??


Rob

[hjames]

Circa 2004, with a 2007 followup ...

The Great Alnico / Ferrite Debate

http://audioheritage.org/vbulletin/s...ead.php?t=4031

[toddalin]

You have been on this site for how many years?? You don't understand why JBL originally made them using Alnico?? Your joking right just breaking Edges chops??

Rob

I'm not saying the Alnico (or ferrite) sounds "better" or "worse," but only that it can sound different and that the geometry of the magnet structure can contribute to that difference.

In other words there is more to it than just the amount of gauss in the gap, which seemed to be goal and focus of the noted articles.

And we are just talking about the magnetic assembly. More recent cone materials, suspension, compliance, etc. are irrelevent to this discussion as they can easily be updated, retaining the original magnetic structure.

And if it sounds different, that can contribute to one's enjoyment/displeasure.

[Ian Mackenzie]

I think we have had several rants on Alnico versus ferrite and here is another point of view below. (no alliance with AE)

A motor is only as good as you wanna make it and there are good and bad examples of both varieties. In general you get what you pay for.

It would seem that in certain designs the Alnico is more prone to de magnetisation from high power use and I have heard this from Pro Shops about the Alnic Tad woofers. If you pull apart an Alinico driver the wrong way it will need re gaussing.

http://www.aespeakers.com/Lambda001-1.php

Lower the electrical conductivity of the pole piece. This typically means using extremely expensive materials or sacrificing more Bl. One group of materials of interest would be the iron/cobalt alloys such as Hiperco 27 from Carpenter. The problem is the extremely high expense where the raw material for a single pole piece would be upwards of $500 on a large driver. Another option would be a powdered iron. While this is electrically non-conductive, it is also not highly magnetically permeable. You could expect to lose 20% of the flux in the gap due to the lower permeability.
(2) Permanently mount a highly conductive layer that is not magnetic between the VC and the pole. This cannot be a moving piece like an aluminum VC former. It needs to be a rigidly mounted layer, much thicker than the typical VC former.

[edgewound]

Show me the "white papers."

Todd....

JBL Tech Notes Volume 1, Number 9.

"Distortion and Power Compression in Low Frequency Transducers".

I have it, but don't have time to copy it and upload it.

The Alnico-Ferrite story is there.

[toddalin]

The only effect the bobbin would have is if it is made of metal....like aluminum. This creates eddy currents in the gap that can cancel out high frequencies and add distortion.

Not the only effect if you really think about it. For example, as the voice coil heats up, the heat is transfered to the magnetic assembly. Put that bobbin in the way between the coil and magnet and you have an effective insulator to that heat transfer.

Like I said, many small details that may or may not be measureable.