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Thread: Measuring compression drivers. Questions.

  1. #31
    Administrator Robh3606's Avatar
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    Is there a way to sweep these drivers without having to put the back cap on?
    You are supposed to do a sweep open as part of the install before the cover is on. There is no reason you can't screw on test leads where the regular leads would go. The frequency sweep range and voltage on the PDF Aural Test

    Rob

    A.
    PREPARATION
    1. Check unit for damage (cracked throat, shifted top plate, etc.) Remove front
    cover and old diaphragm assembly. On ring radiators, remove inner cone and
    outer horns and unsolder old diaphragm.
    2. Using gap gauge, check gap for size and alignment.
    3. On 2-inch throat drivers, tap on top phasing plug. The unit should make a solid
    sound. A “hollow” or “clicking” sound indicates a loose phasing plug.
    B.
    INSTALLATION
    1. Fold piece of masking tape lengthwise around cardboard digger with adhesive
    side exposed. Insert in gap and rote to clean out debris. Continue cleaning until
    tape is clean when withdrawn. Inspect visually with magnifying light.
    2. Inspect new diaphragm for damage. Line up holes in diaphragm to mounting
    pins on the driver. Carefully lower assembly into place. Replace diaphragm
    mounting screws. On ring radiators, solder leads to terminals.
    3. On drivers, hook unit up to oscillator set at correct input voltage indicated on the
    appropriate technical manual and sweep according to technical manual. Listen for
    buzzing. Use small mallet to tap lightly on side of diaphragm frame until it is centered
    in gap and buzzing ceases. Tighten down diaphragm mounting screws.
    4. Reconnect terminal leads onto diaphragm assembly and replace cover. On ring
    radiators, replace inner and outer horn.
    5. Perform final sound test.
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  2. #32
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    Quote Originally Posted by kelossus View Post
    Bloody hell I have no idea whats going on.

    Noticed one of the diaphragms buzzing this afternoon. Measured the driver and its back to shit again? WTF!!!!

    Can anyone advise what exactly the tolerance markings on the top plate mean? My compression driver top plate has +2 +2 +2 +1 and my Diaphragm is a -2. My diaphragm comfortably clears the phase plug and further shimming completely ruins the already shit measurements. I feel like the coil might be rubbing somewhere in the gap. Is there a way to sweep these drivers without having to put the back cap on?
    It's been mentioned ( & it makes sense to me ) that those numbers indicate just how much ( likely in microns or some very small unit ) that quadrant of the phase plug is out of spec. to the exact design measurement.

    Typically ( from what I've seen ), most of these numbers ( on the alnico 4" drivers ) are in the positive zone.

    Consider the numbers as a guide suggesting where ( in the quadrant ) you might put an extra shim.

    Regarding your latest distortion.

    Since you found metal within a slot of a phase plug that strongly suggests to me that the VC gaps may also still need a better cleaning.

    These metallic bits will migrate about ( which somewhat helps explain the sudden onset of your drivers buzzy symptoms ).


  3. #33
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    Thank you very much for the responses guys. I attacked it this morning after with a fresh mind and had a slight improvement.

    I cleaned the gap again, shuffled the shims and I found only one quadrant needs to be shimmed. Once shimmed I installed 3 of the mounting screws and ran a 500hz-1200hz sweep. Edgewound on an earlier thread specified 3.5v for the 16ohm phragms but I didn't take it over 2.5v, 3.5v would have been insanely loud. Couldn't hear any fouling so I assembled it all back together. I am now an expert at removing/installing the 2440 driver from the 4350 cabinet by myself though I hope to never do it again.


    These sweeps aren't as good as my initial ones but the 6% THD from yesterdays sweep is now gone. If I can get another brand new 2441 Diaphragm I will send these interstate to an expert and get them fitted and recharged. Sadly in Australia there a few who can service these JBL drivers and even fewer that would back their installation up with measurements.

    BLUE TRACE = LEFT 2441, the one I had had issues with. RED TRACE = RIGHT 2441

    WITH 2308 ATTACHED, 1 foot away from speaker

    Name:  LEFT RIGHT SWEEP NO 2308.jpg
Views: 116
Size:  60.1 KB

    WITHOUT 2308, 1 foot away from speaker
    Name:  LEFT RIGHT WITH 2308.jpg
Views: 117
Size:  59.5 KB

  4. #34
    Member sebackman's Avatar
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    Hi there,

    Sorry to be a party pooper but how do you measure and with what gear. Unless you can make certain your gear is calibrated and you are measuring both drivers with exactely the same position there is no knowing if the difference is real or just the result of poor placement. And pretty sweeps pointless if you are not using a echo free chamber or a MLS pulse. Sweeps are usefull in controlled environments but less so outside.

    Are your filters identical?

    Are you measuring on horn or just the naked drivers?

    kind regards
    //Rob
    The solution to the problem changes the problem.
    -And always remember that all of your equipment was made by the lowest bidder

  5. #35
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    Quote Originally Posted by sebackman View Post
    Hi there,

    Sorry to be a party pooper but how do you measure and with what gear. Unless you can make certain your gear is calibrated and you are measuring both drivers with exactely the same position there is no knowing if the difference is real or just the result of poor placement. And pretty sweeps pointless if you are not using a echo free chamber or a MLS pulse. Sweeps are usefull in controlled environments but less so outside.

    Are your filters identical?

    Are you measuring on horn or just the naked drivers?

    kind regards
    //Rob
    I use a MiniDsp UMIK and REW. The mic comes with a calibration file specific to it. I think the consistency of my sweeps is an indicator they are somewhat useful.

    Filters are identical.

    Always measured on the 2311 horn

  6. #36
    Administrator Mr. Widget's Avatar
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    Quote Originally Posted by kelossus View Post
    I use a MiniDsp UMIK and REW. The mic comes with a calibration file specific to it. I think the consistency of my sweeps is an indicator they are somewhat useful.

    Filters are identical.

    Always measured on the 2311 horn
    Have you seen this document? It covers measuring diaphragms and has a lot of interesting info.


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  7. #37
    Senior Member ivica's Avatar
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    Quote Originally Posted by kelossus View Post
    Thank you very much for the responses guys. I attacked it this morning after with a fresh mind and had a slight improvement.

    I cleaned the gap again, shuffled the shims and I found only one quadrant needs to be shimmed. Once shimmed I installed 3 of the mounting screws and ran a 500hz-1200hz sweep. Edgewound on an earlier thread specified 3.5v for the 16ohm phragms but I didn't take it over 2.5v, 3.5v would have been insanely loud. Couldn't hear any fouling so I assembled it all back together. I am now an expert at removing/installing the 2440 driver from the 4350 cabinet by myself though I hope to never do it again.


    These sweeps aren't as good as my initial ones but the 6% THD from yesterdays sweep is now gone. If I can get another brand new 2441 Diaphragm I will send these interstate to an expert and get them fitted and recharged. Sadly in Australia there a few who can service these JBL drivers and even fewer that would back their installation up with measurements.

    BLUE TRACE = LEFT 2441, the one I had had issues with. RED TRACE = RIGHT 2441

    WITH 2308 ATTACHED, 1 foot away from speaker

    WITHOUT 2308, 1 foot away from speaker
    Hi kelossus,

    I would suggest to do a measurements at least 1m (3+ feet) from the horn mouth, may be 2m would be better. A baffle about 50cm x 50cm has to be applied too, if more realistic response is of interest.

    regards
    ivica

  8. #38
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    Quote Originally Posted by ivica View Post
    Hi kelossus,

    I would suggest to do a measurements at least 1m (3+ feet) from the horn mouth, may be 2m would be better. A baffle about 50cm x 50cm has to be applied too, if more realistic response is of interest.

    regards
    ivica
    My main goal was to sweep the horns in identical settings to confirm that they were both measuring close to one another. I understand this may not be the best way to get a realistic FR sweep but being in identical settings the sweeps should be close if all else is equal.

  9. #39
    Senior Member ivica's Avatar
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    Quote Originally Posted by kelossus View Post
    My main goal was to sweep the horns in identical settings to confirm that they were both measuring close to one another. I understand this may not be the best way to get a realistic FR sweep but being in identical settings the sweeps should be close if all else is equal.
    Hi kelossus,

    I think putting mic to near the horn a small differences in the distance, or mic angle can introduce larger error, so I would suggest You to put mic 1m apart from the horn (or lenses) mouth and the to do the measurements.....
    regards
    ivica

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