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Thread: Demagnetized Alnico horn driver

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    Demagnetized Alnico horn driver

    Since the topic of Alnico speaker magnets and their susceptibility to demagnetization comes up from time to time, I thought I'd post this story.

    A client of mine has a Leslie 122 that's never sounded right to me; the balance between horn and woofer was tilted very much towards the woofer, making it sound dull and bassy. Nothing I tried to restore the highs worked. Another Hammond/Leslie tech suggested that I try swapping in another known-good Jensen V-21 Alnico magnet horn driver, which I did. The difference was night and day; the speaker screamed like it should. So, the question was: what was wrong with the other driver? I'd had it apart and run every test I could think of on it such as DCR and impedance of the voice coil, etc... and the voice coil was in good shape and properly aligned both vertically and horizontally.

    I brought the weak horn driver back to the shop (again) and, not having a gaussmeter, tried to think up with a way to test the magnet strength. I ended up putting a compass on the edge of wooden table and, holding the driver in my hand with the pole pointed towards the compass, I moved it in perpendicular to the earth's magnetic field to see how close I would need to get the driver to the compass to achieve full 90 degree deflection of the compass needle towards the driver. I did this with a couple of known-good drivers and then tried the weak one. I had to hold the weak one almost twice as close to the compass to get the same deflection. It's not very quantitative, but it seems to confirm the source of the problem.

    Additional clues are that 1) while all other components in this Leslie are from 1971, the horn is from 1975, and 2) this Leslie was sold to my client by a guy with a large collection of Leslies and Leslie parts who's been know to dispose of components that don't work right by slipping them into organs and speakers that he typically sells to people who are buying their first "real" Hammond Organ or Leslie speaker. I've read that if an Alnico magnet is stored near a large ceramic magnet, like one attached to a woofer, with a field pointing in the opposite direction, the Alnico magnet can gradually get demagnetized -- or it can happen suddenly via a sharp blow.

    I ran this by the experts on a Hammond/Leslie list, and I'm told that while a demagnetized horn driver is uncommon, it's definitely been observed before. This is my first direct observation of an Alnico magnet speaker driver that's lost sensitivity due to demagnetization.

    Weber says they can zap it back to full strength.

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    Old Timer J M Fahey's Avatar
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    I had to hold the weak one almost twice as close to the compass to get the same deflection. It's not very quantitative, but
    Ah !!!, but .. it is !!
    Assuming *everything else* is the same, and knowing magnetic field (same as sound pressure) decreases with the square of the distance, your 2:1 distance variation implies a 4:1 magnetic strength loss.
    Congratulations.
    As to the Alnico strength loss, if by any chance the magnet was pulled from the frame and then remounted (misaligned / chock full of iron filings?) , you have a huge drop, consistent with what you observed.
    If you get it remagnetized, it's as good as new.

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    JM,

    Considering that this driver is known to have come from someone with all sorts of stuff stored wherever it happens to land, I suspect that it was improperly stored too close to another magnet with an opposing field direction. It's on its way to get remagged.

    The information on this site is the same as many others concerning how Alnico magnets can get demagnetized:

    What types of magnets are there?

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    To wrap up this thread, I got the magnet regaussed by Weber, but it didn't solve the problem. Using the same method I used before, the magnet in this driver, though stronger now, still isn't as strong as the magnet in two other Jensen V-21s, nor does it put out the same SPL with the same signal applied.

    The only conclusion I can draw is that, for some reason, the Alnico magnet used to build this driver was made of an inferior alloy. As I'm sure the pickup makers on the list know, there are several different Alnico alloys with different maximum field strengths. Perhaps someone got the mixture wrong. I'm also told by the best Hammond/Leslie techs I know that they're not entirely surprised. One of them has seen people go to great lengths to try to improve the sound of a Leslie that simply had a dud horn driver, and I would now include myself in that group.

    But, since I have a good diaphragm, faceplate, etc... what one of my friends is going to do is to send me another magnet structure from a driver with a blown voice coil. I'll end up with a good driver, but it's not how I expected the story to end. I was confident that the existing magnet could be recharged to optimal field strength. The guy I dealt with at Weber says he zapped it twice to make sure it was properly charged.

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    Lifetime Member Enzo's Avatar
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    If the chemical makeup is "wrong," then the rest of the batch ought to be wrong as well, and there would be a lot of Leslies with the same problem.

    You regaussed it and it remained weaker than normal. Perhaps there is a crack within the magnet, or some other structural defect like a void that prevents a full strength field. Personally, I'd think that before alloys.

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    Quote Originally Posted by Enzo View Post
    If the chemical makeup is "wrong," then the rest of the batch ought to be wrong as well, and there would be a lot of Leslies with the same problem.
    Other Leslie techs I've talked to are saying that that might actually be the case. When I first did the restoration of this 122, almost everyone said it sounded great. (It lives in a jazz club.) One guy, however, said that he thought it was lacking top end--and got into an argument with the organ's owner who thought he was being a jerk. At first, I thought it was just a matter of the room it was in, but gradually, in comparison with various other Leslies I've been working on, I realized that he had a point. It didn't sound bad, per se, but it didn't sound quite right. It only became really clear when, at the suggestion of one of the most experienced Leslie techs I know, I swapped a known-good horn driver into the cabinet for comparison. This tech maintains that he knows of several cases where people have worn themselves out, repeatedly going through the electronics trying to figure out why their organ or Leslie doesn't sound right, only to discover, in the end, that it's the result of some sort of manufacturing inconsistency or defect in the horn driver.

    Quote Originally Posted by Enzo View Post
    You regaussed it and it remained weaker than normal. Perhaps there is a crack within the magnet, or some other structural defect like a void that prevents a full strength field. Personally, I'd think that before alloys.
    I'd be happy to box it up and send it to you for an autopsy :-)

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    Lifetime Member Enzo's Avatar
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    Sadly, I lack the necessary X-ray machine.

    I don't doubt at all that you are correct that it is weak, and while I have been fooled before, it seems like a wrong formulation would have shown up in a ton of them.

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    The end result is that you've done all you can, and now you need to take the pragmatic approach of replacing the driver. Unless your friend wants to spend the $$$ for metallurgical analysis, with no practical outcome I might add, this is the best you can do.

    You talked about the voice coil being in good shape. You don't really know that. The magnet wire du jour was coated with organic-based varnish, which tends to dry out and crystallize with time (plenty of vintage speakers, pickups, transformers and inductors have died this way). You might have a shorted winding or two, while still reading good a DVM. This is actually a more rational explanation than "Perhaps someone got the mixture wrong". This just doesn't wash with my personal logic. Though it is not impossible that Jensen was shipped a batch of "inferior" magnet slugs, I highly doubt that this is the case.

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    Quote Originally Posted by jrfrond View Post
    The end result is that you've done all you can, and now you need to take the pragmatic approach of replacing the driver. Unless your friend wants to spend the $$$ for metallurgical analysis, with no practical outcome I might add, this is the best you can do.
    Errr....well, yes, of course. I just wanted to post about how it worked out. Theoretical stuff interests me. I always keep a few good V-21s in stock; I'm not desperate to fix this one. One of my spares is in the Leslie at the moment, and that may end up being its permanent home.

    Quote Originally Posted by jrfrond View Post
    You talked about the voice coil being in good shape. You don't really know that. The magnet wire du jour was coated with organic-based varnish, which tends to dry out and crystallize with time (plenty of vintage speakers, pickups, transformers and inductors have died this way). You might have a shorted winding or two, while still reading good a DVM. This is actually a more rational explanation than "Perhaps someone got the mixture wrong". This just doesn't wash with my personal logic. Though it is not impossible that Jensen was shipped a batch of "inferior" magnet slugs, I highly doubt that this is the case.
    The voice coil reads fine via every test I can give it, but one of my fellow Leslie techs has already dropped a spare magnet structure in the mail to me. That way, we can test the voice coil vs. magnet theories.

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