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Old 29th January 2013, 05:09 PM   #21
kevinkr is offline kevinkr  United States
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Originally Posted by speaker dave View Post
<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..
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Old 29th January 2013, 05:18 PM   #22
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Originally Posted by weltersys View Post


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.
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Old 29th January 2013, 05:44 PM   #23
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Originally Posted by speaker dave View Post
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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Old 29th January 2013, 05:54 PM   #24
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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
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Old 29th January 2013, 06:09 PM   #25
Giarsun is offline Giarsun  France
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The Great Alnico / Ferrite Debate
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Old 29th January 2013, 06:20 PM   #26
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Confusing my Crowns with my Nikons. It was an M600. I think the M2000 was a couple of them bridged?
You got to see the "good stuff".
I never saw a M600/M2000 amplifier, just tons and tons of DC300s and the smaller versions.

Bridged sounds plausible, looks to be as big as a Macintosh 2300.
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Old 29th January 2013, 06:48 PM   #27
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Good reference, thanks!
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Old 1st February 2013, 05:02 AM   #28
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Originally Posted by speaker dave View Post
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
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Old 1st February 2013, 06:26 AM   #29
tinitus is offline tinitus  Europe
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Originally Posted by Dave Zan View Post
Is that a correct analysis?
from what I know about it, yes, 100%, spot on, short and precise as can be

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'
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Old 1st February 2013, 08:57 AM   #30
JMFahey is offline JMFahey  Argentina
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Quote:
Like they say, if you remember the 1960's, you weren't there .
I beg your pardon?
Who says so?
I started studying *Engineering* in the 60's , plus designing, building and selling Musical Instrument Electronics .... which I do until the present day
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