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Thread: WEak alnico drivers, symptoms?

  1. #1
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    WEak alnico drivers, symptoms?

    Hello guys, I have read a lot of this and other forums and made some research but the question remains: what are the symptoms of a weak alnico driver in both woofers and HF drivers?
    Till now I understand that the HF driver will lose some high frequency capacity. How much? I can measure but don't have a reliable reference of a good driver (806A/511 combo) and therefore can't be sure.
    The recharge of the drivers costs a bit when shipping is involved.
    The question is important to me and many others I'm sure.
    Chris

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    Dang. Amateur speakerdave's Avatar
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    GT has said that with the advent of large solid state amps JBL began to notice a loss of about 1 dB sensitivity in the woofers with 4" voice coils. Woofers with smaller voice coils were much more sensitive. I'm not aware of any authoritative comments about compression drivers, but any Alnico device whose coil has been blown could have received a voltage jolt that would cause some loss. When having them reconed or rediaphragmed it is more or less standard procedure to have them regaussed.

    On the other hand, one of our most prolific posters (and buyers of used drivers) has a magnetic field tester and has said he's never run across a driver that needed recharging.

    So if you are using a system that sounds ok, I would suggest you stop worrying about it. If you are building a new system to an exacting standard, especially if it is for someone else, you want Alnico drivers that have been both reconed and regaussed. In the case of drivers whose original software (cone, etc) is no longer available, their overall condition is probably a reliable indicator. Woofers can be regaussed with the cone in place.

    David

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    Senior Member tom1356's Avatar
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    Quote Originally Posted by speakerdave View Post
    .... Woofers can be regaussed with the cone in place.

    David
    Can coaxial drivers like the 604 be regaussed with the woofer cone and tweeter diaphragm in place?

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    Dang. Amateur speakerdave's Avatar
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    I would think so, but ask Great Plains Audio, to be sure. They would be the ones I would have work on a 604. Speaker frame/motor assemblies are made before the magnets are charged.

    David

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    Administrator Robh3606's Avatar
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    Let's be more specific.

    Thanks everybody for the answers. From the last answer I conclude that there will eventually be a loss in efficiency. Makes sense.
    What about the measurment of one of my 806 on a 511 horn. Is that the expected FR ? It drops quite fast in the highs arround - 10dB at 9 kHz.
    Chris
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    Senior Member grumpy's Avatar
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    807-Z (symbiotik)

    ... on 811b. measured at horn face between vanes, & then backing off
    to a similar distance (~.5m) ... uniform drop in amplitude (so I show
    only one plot). 1KHz+ as I had an active crossover in place. -grumpy
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    Senior Member grumpy's Avatar
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    FWIW,
    I'd be happy to run other plots, I have 511b's, but no other Altec drivers.
    The 807's came free with the horns . -grumpy

    or K77's in K-horns, if that will float anyone's boat...

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    similar result

    Thanks grumpy, that's exactly what I was looking for. Your result is very similar to mine: a 10 dB drop from 1 to 9-9.5 kHz.
    If somebody else doesn't show other result I must conclude that it is the nature of the beast. Is that what can be called the vintage sound ?
    Chris

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    RIP 2011 Zilch's Avatar
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    Without compensation, that looks like "Theater" sound to me, the "X" curve.

    http://www.jblpro.com/pub/cinema/cinedsgn.pdf

    See also Fig. 5 here:

    http://www.jblpro.com/pub/technote/tn_v3n03.pdf
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    Senior Member grumpy's Avatar
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    Plot was indeed with no compensation, and should have been well within the
    "beam" ... direct vert center, and either side of the center vane (both sides measured
    identically, as one would expect). I can check out another 807-z and look at
    a more extended response (lower and w/511) later. -grumpy

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    Theater sound

    Thanks Zilch for your valuable post. The Altec drivers were created for theatres and therefore aimed at the voice reproduction.
    My measured driver approaches the X cinema curve. I must then conclude that we are very far from the modern drivers HF extension and that for HI-Fi musical use the HF shall be or boosted and/or completed with a tweeter crossed at 3-4 kHz.
    This seems strange to me since many members report a good sound from two way Altec systems!?
    Some others FR measurements available from other members?
    Chris

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    RIP 2011 Zilch's Avatar
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    Quote Originally Posted by krzys View Post
    Thanks Zilch for your valuable post. The Altec drivers were created for theatres and therefore aimed at the voice reproduction.
    My measured driver approaches the X cinema curve. I must then conclude that we are very far from the modern drivers HF extension and that for HI-Fi musical use the HF shall be or boosted and/or completed with a tweeter crossed at 3-4 kHz.
    This seems strange to me since many members report a good sound from two way Altec systems!?
    Some others FR measurements available from other members?
    Chris
    We are working on this in another thread, actually, and that's why I jumped in here. Read on through the end from here:

    http://audioheritage.org/vbulletin/s...t=14690&page=6

    There does not seem to be a lot of quantitative information on Altec performance available, surprisingly, but more is forthcoming. I recall one member asserting that VOTT did not translate well for home listing for the opposite reason, namely, that they were overly bright, intended to be used behind the screen, and EQ'd by it as shown in the second link I provided.

    That doesn't appear to be the case, however. Model 19 provided the compensation you suggest is necessary, but even there, it seems insufficient; the requisite sensitivity differential between woofer and mid/high compression driver not being available to accomplish the task with those components.

    Another major element in this is the nature of the horns used. All compression drivers require compensation above mass breakpoint. JBL exponential horns do it "automatically" by narrowing the beamwidth with increasing frequency. Altec 511 and 811 are apparently radial (rotated exponential profile) horns requiring some compensation. Constant directivity horns, which includes MantaRay (presumably; I've never worked with one,) require full compensation for the compression driver rolloff to achieve flat frequency response.

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    Senior Member Tom Brennan's Avatar
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    Quote Originally Posted by krzys View Post
    This seems strange to me since many members report a good sound from two way Altec systems!
    Well I think it's fair to say that some of us like many Altecs despite the high frequency response not because of it. The Nineteens are much better though and I would guess that the other "Teens" are also.

    Makes me think about building Jeff Markwart's compensated crossovers for my 605As which look like they'll be sticking around. Right now I'm simply using them with the treble knobs on the old PAS-3 turned up.

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    Administrator Mr. Widget's Avatar
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    Quote Originally Posted by Zilch View Post
    Altec 511 and 811 are apparently radial (rotated exponential profile) horns requiring some compensation.
    ...or the correct driver. Take a look at this curve of the 511 horn with a TAD 2001 driver on it. This is a curve taken of cyclotronguy's system that is pictured in the EV Tweeter thread. The time windowing or sampling frequency reducing the usability of this plot on the lower mids on down, but as you can see, that 1" Be diaphragm does make it all the way out to 19KHz with no EQ or compensation and is still quite usable to 23KHz.

    Quote Originally Posted by Tom Brennan View Post
    Well I think it's fair to say that some of us like many Altecs despite the high frequency response not because of it.
    That is an interesting observation, you might give the little TADs a go and see if they improve your experience.


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