ReMagnetizing Question

[thevott]

The thing I don't like about ferrite is that metalic objects stick all over the outside! Anyway, IMHO, my best sounding HF drivers are my ferrite 902s (probably due to proper flux density). Its also nice to know that they're not demagnetizing every time I crank up the system.

[joyspring]

As an aside, I found this new article at Don McR's Audio Heritage site:

(JBL) David Smith Recalls

The relevant bits are in the last paragraph:

The SFG story was that the ferrite magnets structures were designed, as you know, because of the rising cobalt prices. Ferrite structures with the same parameters of gap dimensions and flux density were made up but when listened to they sounded different. Further measurements showed much higher second harmonic distortion. The flux modulation ring dealt with the 2nd Harmonic and the undercutting also helped the low frequency distortion due to the symmetry. Mark Gander's AES paper shows typical curves. The potential demagnetization of Alnico was also noticed at the time. I distinctly recall one of the engineers having fun with modifying the parameters of Alnico woofers at will during development: "I think the Q is a little low on this one. Maybe a 2 dB drop in sensitivity would be about right. Lets give it full output for a second from the Crown DC600....Brrrapp. Curve it again and, yes, that's about right."

<font size="2" face="Verdana, Helvetica, sans-serif">From experience, Alnico 604s are notorious for severe demagnetisation, sometime exhibiting upwards of 6 dB difference between left and right pairs over extended usage!

Regarding distortion, recall that Altec 604/416/515 voice coils are underhung (voice coil is shorter than the magnetic gap) and exhibit NO DIFFERENCE in distortion between Alnico or Ferrite magnetic structures.

JBL (and EV, et al) have all since mitigated ferrite magnetic structure flux non-linearity, to the point of far surpassing Alnico so far as second- and third-harmonic distortion is concerned.

BobR

[RonSSS]

I hope to have the 8C's done locally with parts from GPA.
For the alnico 8B's, I'll send them to Bill. Even though I bought these new in 1977, I agree that remagging would be wise. And Bill is the man!
Ron

[jcm]

Concerning alnico vs ferrite: I can detect quite a difference in sound between a 604-H an the K. The alnico sounds softer in the treble regions. But maybe it's a question of magnet age.
jcm

[jcm]

Concerning alnico vs ferrite: I can detect quite a difference in sound between a 604-H an the K. The alnico sounds softer in the treble regions. But maybe it's a question of magnet age.
jcm

[Altec19]

An Alnico woofer or compression driver in need of remagnetization tends to have reduced amplitude output in the upper part of the range that it's designed to produce. They also sound "mushy" for lack of a better term. Maybe the ferrite 604-K is working as designed and the Alnico 604-H is in need of remagnetization???

[RacerXtreme]

Is it best to have a driver remagnetized before or after it's reconed - or does it even matter?

[bfish]

Remagging can be done at anytime, alone, or when reconing. No dissassembly required. Just because it's an old Alnico, doesn't necessarily mean it needs done, though. Weak mags produce weak sound. If you've got a suspect, swap cabs with its mate and recheck, to see if it's really weak, or just a room mode. Bill includes remagging on all Alnicos he recones, and the last I had zapped were just $15 each for just remagging. I live close enough I just made an appt., dropped off about a dozen when he opened, and picked them up at closing time the same day. The appt's the key, might not get it done if you just showed up. With a pickup loaded with hot magnets, the trip home was interesting (keep your distance from kids on bikes!)

[bfish]

Sorry about the frowny-face, he sneaked in there on me somehow!