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Thread: Truextent Diaphragms in JBL four-inch compression drivers

  1. #1
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    Truextent Diaphragms in JBL four-inch compression drivers

    These are frequency response and impedance plots. They don't convey how the various diaphragms actually sound on the various
    horns. They are used to aid in the development of passive filters.

    Yuichi A-290, a 290hz with a hyperbolic flare - it's a clone of the TAD TH-4001

    These are some of the last measurements I took. The impedance curve of the 2445 core just looked weird. I thought it might be the felt instead of the foam in the back cap so I swapped the felt 2445 back cap with the foam 2446 back cap and ended up with pretty much the same deal above ~ 500 Hz.

    The 2445 and 2446 have different phase plugs but it doesn't appear that the 2446 has any better top end response.

    The plots are 1/2 octave.
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    The 2446H on the three different horns.
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    2446H and Truextent on 2311
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    2446H and Truextent on the 350hz Edgarhorn (tractrix flare).
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    2446H and Truextent on the Yuichi A-290, a 290hz with a hyperbolic flare - it's a clone of the TAD TH-4001
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    2445 core
    2311 /2308 / baffled
    D16R2441
    D16R2445
    Truextent
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    2445 core
    350hz Edgarhorn (tractrix flare)
    D16R2441
    D16R2445
    Truextent
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    2445 core
    Yuichi A-290, a 290hz with a hyperbolic flare - it's a clone of the TAD TH-4001
    D16R2441
    D16R2445
    Truextent
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    These were taken a few nights earlier using a 2441 core with a D16R2441 diaphragm. The 2446H plot shown here was taken at that time too. That was when I ran into trouble. The D16R2445 diaphragm wouldn't fit right in the 2441 core and when I went to take a Truextent diaphragm out of a 2445 to put in the 2441 there were mold issues (#51) in the 2445 due to the Arlene's glue that was used to adhere the felt pad to the back cap. I called it a night.
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    Member originaltubino's Avatar
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    Great work! Some surprises

    Compared to the others, the Truextent diaphragms do not appear to have any special FR extension in the 14K to 20K range.

    The Yuichi and Edgar do not differ as much as I expected. This is all on-axis, right? Both are pretty well-behaved.

    THANKS for doing this and sharing it!
    deHavilland Aries SE 845; UltraFi Monaco SE 845
    Tannoy GRF-R, DMT15; '52 Jensen Imperials+JBL 2441, LE15B; TAD 2402 clones

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    Senior Member timc's Avatar
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    I'm surprised that there are acutually less HF extension with the Be diaphragm than with the original Ti's.

    Any idea why?
    2213 + 2435HPL w/aquaplas + H9800 (Matsj edition)

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    Senior Member Ian Mackenzie's Avatar
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    Here is a link to Yuichi's own measurement.

    http://www.geocities.jp/arai401204/H.../A290-S90.html

    I dont have a 2445 / 2446 core to compare but my own measurements of the 4001 confirm the HF extension.

    With a simple two stage passive EQ / impedance circuit per Tads design its quite easy to get a flat response and impedance from 600 hertz response to 18 khertz.

    Controlling or should I say damping the impedance has a marked effect on sound quality.

    I expect the Tad diaphgram surround and the 4001 phase plug play a big part on how it performs on paper. As to subjective impressions its a matter of taste.

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    RIP 2011 Zilch's Avatar
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    Quote Originally Posted by timc View Post
    I'm surprised that there are acutually less HF extension with the Be diaphragm than with the original Ti's.

    Any idea why?
    Absence of the diamond surround, perhaps....

  14. #14
    Senior Member timc's Avatar
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    Could be. Softer surround (higher compliance) should reduce HF extension. Good call Zilch.
    2213 + 2435HPL w/aquaplas + H9800 (Matsj edition)

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    Quote Originally Posted by originaltubino View Post
    Compared to the others, the Truextent diaphragms do not appear to have any special FR extension in the 14K to 20K range.

    The Yuichi and Edgar do not differ as much as I expected. This is all on-axis, right? Both are pretty well-behaved.

    THANKS for doing this and sharing it!
    They are nice horns. Yes, this is all on-axis. I did a few off-axis shots just to see where the horns started dropping off. I was especially curious as to how tight a sweet spot the Edgar might have.
    Quote Originally Posted by Ian Mackenzie View Post
    Here is a link to Yuichi's own measurement.

    http://www.geocities.jp/arai401204/H.../A290-S90.html

    I dont have a 2445 / 2446 core to compare but my own measurements of the 4001 confirm the HF extension.

    With a simple two stage passive EQ / impedance circuit per Tads design its quite easy to get a flat response and impedance from 600 hertz response to 18 khertz.

    Controlling or should I say damping the impedance has a marked effect on sound quality.

    I expect the Tad diaphgram surround and the 4001 phase plug play a big part on how it performs on paper. As to subjective impressions its a matter of taste.
    Thanks Ian. That link was helpful. I know nothing about these Yuichi horns. They look and measure nice. I'll have to hear a pair someday.

    It looks like he got a bit more extension from his 2441 than I get from either of mine. I tried both on the Yuichi and determined that they were probably a wee bit low on flux density. That's why I pulled their diaphragms and put them in the 2445 core.

    I'd like to see a schematic of the Tad circuit.
    Quote Originally Posted by timc View Post
    Softer surround (higher compliance) should reduce HF extension.
    It does. The Truextent uses less beryllium as well as the soft surround (as compared to the 476Be diaphragm) with the result being reduced extension. The Truextent also has a bit higher distortion according to JBL's measurements.



    The titanium and aluminum diaphragms can't really compete with the sound quality of the berylliums. The Tad and JBL four-inch beryllium compression drivers are very expensive (and in the case of the JBL, nearly impossible to get). These aftermarket beryllium diaphragms offer excellent cost/performance.
    Quote Originally Posted by originaltubino View Post
    THANKS for doing this and sharing it!
    You're welcome. I really don't enjoy handling other people's beryllium diaphragms and swapping them around in various cores. Everytime one is exposed I get real nervous.

    When I ran the impedance curve on the second 2445 with the Truextent installed I didn't like the way it sounded. There were unwanted resonances. I opened it up, cleaned out the little bit of mold on the felt in the backcap and then cleaned the gap out (there were a few metal particles in the gap) and then reassembled it. That fixed the problem. I'll check the first 2445 (the one with the mold) today so Dan can get two fully functional drivers back. Don't use Arlene's glue in these back caps guys. I used the Moyen recone glue in my 2445J's and there weren't any issues.

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