alnico units by JBL

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<snip>

Not a thermal thing but more to do with its resistance to demagnetization related to its operating point. It was a bit random because you don't know where the signal would end up with your short blast, positive or negative.

I distinctly remember both Greg's approach, and seeing the curve difference. I thought this was a well known phenomenon and that people offered remagnetizing services? <snip>

I use JBL alnico drivers in my system and can confirm all of the above from direct experience. Alnico woofers are quite likely to partially demagnetize if driven really hard.. Not so likely with well treated vintage home hifi gear, but if you are planning on buying and refurbishing JBL pro drivers you should plan for the contingency that they need to be remagged.

I have a set of 2402J magnets (annular ring horn tweeters) which are almost completely demagnetized. I have a pair of much older 075s with the original magnets which are fine. I prefer the alnico 075 to my later ferrite 2402s, but that may have something to do with the diaphragm vintage (all are OEM JBL)

Mids are 2440 again alnico and the magnets are fine (I was told they had been remagged before I got them.)

I've also had 2420s, 2405, D130/D140 all alnico..

The biggest issue with these drivers is that most aftermarket replacement diaphragms are not very good quality, and I've found I much prefer aluminum to titanium, and OEM to anything else I can afford.. Other aspects IMO of the driver design whether woofer, or horn driver probably play a bigger role in performance than the type of magnets used.

Incidentally there are analogs in the pro line for most if not all of the early alnico home drivers and they generally are a little to much less expensive to acquire.

Radian does make a decent diaphragm for the 2440 at about 1/3 the cost of OEM, but it is not an exact equivalent.

I like the JBL Pro stuff if that was not clear..
 
Why would Greg Timbers not just start with less flux density rather than trying to beat it down with a random process?
What was the impact of the short blast ending on a positive or negative cycle ?

Why he was doing it I certainly can't recall. He either wanted a higher Q driver for an experiment or just as likely was farting around and showing me the effect.

What we are talking about is much like AC demagnetization of tape. You must run back and forth over numerous cycles and then have the field fade away to guarantee that it ends up in a neutral demagnetized state. You can't stop at zero either, due to hysteresis, so it is impossible to predict what the remance will be. You would have to get to the coersive level in the opposite direction to reduce to zero, and that is hard to control.

If you give the unit an AC blast then the end point is undefined. Your final magnetization would be a crap shoot, hence several trials to get to the right point.

David S.
 
Why he was doing it I certainly can't recall. He either wanted a higher Q driver for an experiment or just as likely was farting around and showing me the effect.

If you give the unit an AC blast then the end point is undefined. Your final magnetization would be a crap shoot, hence several trials to get to the right point.
David,

Thanks for the explanation and the references.
The references would explain why I had not experienced the demagnitization in Alnico units I used:

"Alnico was chosen because of its stable operating point. This material is insensitive to temperature changes and back-EMF from the coil. JBL has overcome the tendency of alnico to demagnetize at high power levels by utilizing a massive shorting ring at the base of the motor assembly."

By the way, Crown introduced the DC-300 in 1967, and the M600 and M2000 in 1972. Can't find any reference to a D600.

Like they say, if you remember the 1960's, you weren't there ;).

Art
 

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Confusing my Crowns with my Nikons. It was an M600. I think the M2000 was a couple of them bridged?

The "back EMF from the coil" would be a reference to flux modulaton distortion, as cured by the aluminum ring. Alnico is inherently imune to that and apparently better with regard to Curie effects (hot).

Hard enough remembering the 70s!

David
 
Good reference

I think there is some misinformation on this subject so I would like to clarify one point, and have a declaration from a professional;)
That any audible difference between equivalent Alnico and ferrite drivers is due to the different magnetic properties of the materials seems reasonable but is actually incorrect.
A ferrite PM can have essentially identical magnetic properties to the Alnico it replaces if it is dimensioned correctly (different from the Alnico of course). The historical performance difference was due to the conductivity difference.
The conductive Alnico provided an Faraday flux stabilizer by default. For a ferrite driver that had to be added separately as an aluminium or copper loop.
When a low quality speaker company just replaced the Alnico with ferrite then it would have had more distortion, hence the popular belief.
For JBL and other companies that added Faraday loops then the ferrite drivers are comparable with the Alnico. Minor differences in temperature behaviour and improvements over time in other areas of course.

Is that a correct analysis?


Best wishes
David
 
For Mr Fahey, its a reference to those who "studied chemistry" in the late 60s, primarily in the San Francisco flower power areas.

David S.
Oh, we had many of those too. ;)

Unfortunately, *many* braindead or full body dead by now :(
No kidding.

Thanks for the addendum to the "Urban Dictionary".
Always interesting :)

EDIT: almost forgot; just yesterday, by sheer chance, I was reading pages at a JBL Forum.
I'm researching everywhere to build a larger Magnetizer and wanted to see theirs.
Very interesting:
http://audioheritage.org/html/history/history.htm
 
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Sorry to derail this topic a bit more, I just remembered this from John Watkinson's "Putting the Science Back into Loudspeakers":

"Traditional loudspeakers use ferrite magnets for economy. However, ferrite is an insulator and so there is nothing to stop the magnetic field moving within the magnet due to the Newtonian reaction to the coil drive force. In magnetic materials the magnetic field can only move by the motion of domain walls and this is a non-linear process. The result in a conductive magnet is flux modulation and Barkhausen noise. The flux modulation and noise make the transfer function of the transducer non-linear and result in intermodulation.
The author did not initially believe the results of mathematical estimates of the magnitude of the problem, which showed that ferrite magnets cannot reach the 16-bit resolution of CD.
Consequently two designs of tweeter were built, identical except for the magnet. The one with the neodymium magnet has higher resolution, approaching that of an electrostatic transducer"
 
For JBL and other companies that added Faraday loops then the ferrite drivers are comparable with the Alnico.

question...did they continue with this 'practice' :confused:

JBL have published a bit about variations on where to place the Faraday turn.
The newer Dual Coil structures are clever, eliminate the cause of the problem entirely and therefore have no turn.
What is odd is that the JBL tech. note about the DC structures totally misses the point.
Was it written by a trainee? seems unlikely to be deliberate disinformation.

Best wishes
David
 
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Sorry to derail this topic a bit more, I just remembered this from John Watkinson's "Putting the Science Back into Loudspeakers":

"Traditional loudspeakers use ferrite magnets for economy. However, ferrite is an insulator and so there is nothing to stop the magnetic field moving within the magnet due to the Newtonian reaction to the coil drive force. In magnetic materials the magnetic field can only move by the motion of domain walls and this is a non-linear process. The result in a conductive magnet is flux modulation and Barkhausen noise. The flux modulation and noise make the transfer function of the transducer non-linear and result in intermodulation.
The author did not initially believe the results of mathematical estimates of the magnitude of the problem, which showed that ferrite magnets cannot reach the 16-bit resolution of CD.
Consequently two designs of tweeter were built, identical except for the magnet. The one with the neodymium magnet has higher resolution, approaching that of an electrostatic transducer"

This was one of the pieces of misinformation I had in mind in my first post.
In particular the comment on Barkhausen noise seems incorrect. I can find no support for this at all despite enquiries.
I would appreciate any professional's comment on this too.


Best wishes
David
 
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For tweeters some use amorphous. I have these in the Raal Lazy ribbons.
I have Alnico in my JBL2482's (Phenolic diaphragm).
For bass I use 2 x Eminence Kappa PRO LFII's and 2x Kappa 15A's for mid bass. Nothing wrong with the Kappa use of Ferrite IMHO. I only use the LFII's to 100Hz and the 15A's to 330Hz though;)

Sorry to derail this topic a bit more, I just remembered this from John Watkinson's "Putting the Science Back into Loudspeakers":

"Traditional loudspeakers use ferrite magnets for economy. However, ferrite is an insulator and so there is nothing to stop the magnetic field moving within the magnet due to the Newtonian reaction to the coil drive force. In magnetic materials the magnetic field can only move by the motion of domain walls and this is a non-linear process. The result in a conductive magnet is flux modulation and Barkhausen noise. The flux modulation and noise make the transfer function of the transducer non-linear and result in intermodulation.
The author did not initially believe the results of mathematical estimates of the magnitude of the problem, which showed that ferrite magnets cannot reach the 16-bit resolution of CD.
Consequently two designs of tweeter were built, identical except for the magnet. The one with the neodymium magnet has higher resolution, approaching that of an electrostatic transducer"
 
According to GT overpowering of Alnico magnets happens when back emf created by high current in the voice coil pushes the field "below the knee" from which it does not bounce back. Overhung voicecoils driven hard by a high current amp can create this condition. Underhung coils in woofers and the voice coils in compression drivers and horn tweeters do not. Smaller woofers with overhung 3-inch voice coils like the LE10A family are particularly susceptible.

The JBL 1500AL family; 2220/130A,B,C (barely), LE8T, LE15/2215, and 1400ND (neo) are underhung. Altec 406, 414, 416, 515B are underhung. JBL 2230, 2231, 2205 Alnico's are overhung. Some of the special measures in the 1500AL were taken to deal with the swayback character of Alnico woofer FR, not demagnetization.
 
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