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Thread: New driver and horn for my 'speakers: JBL 2450J + Truextent BeX4016 + Arai horn

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  1. #1
    Member marco_gea's Avatar
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    New driver and horn for my 'speakers: JBL 2450J + Truextent BeX4016 + Arai horn

    I'm starting this thread to document the journey I've embarked on, namely replacing the mid-range section of my current 3-way loudspeakers:
    http://www.homebuilthifi.com/project/9409

    Intrigued by a post that I read here a while back:

    An interesting JBL driver candidate for diaphragm replacement with Be would be the 2450. This driver was originally developed to accommodate a Be diaphragm as JBL was considering going head to head with TAD with that design. It was the reason for the development of the "Coherent Wave" phase plug. It allows for output from the diaphragm to be recombined in-phase at the driver throat in comparison to the out-of-phase output from a traditional phasing plug with variable length paths. According to JBL, it really only works with a Be diaphragm. The Al and Ti diaphragms are in breakup mode over most of their bandwidth, and as one designer said, are so "phasey" that there is limited benefit from the "Coherent Wave". However, a Be diaphragm, being pistonic for most of its bandwidth, would theoretically benefit the most.

    This theoretically in-phase output would be a unique driver attribute compared to the TAD drivers and even JBL's current Be drivers. It would be interesting to have feedback if this two decade old technology actually results in a superior driver with a Be diaphragm.

    I decided to purchase a pair of second-hand 2450J drivers and send them to Guido Behringer to have Truextent BeX4016 diaphragms installed in them.

    I intend to pair these with Yuichi Arai's A320FL radial horns, which were originally documented in the May 1992 issue of the Japanese magazine "MJ":

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    This is an interesting design that adopts a Hypex expansion with a cut-off frequency Fc = 320Hz and T = 0.6, resulting in a very linear on-axis frequency response down to Fc when paired with JBL 2441 drivers, as well as good horizontal directivity control to approx. 10 kHz (I intend to use a supertweeter for the last octave or so):

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    I had the horns made out of baltic birch plywood by a French company specializing in bespoke audio designs:

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    Another interesting feature of this design is the fact that the throat adaptor operates a very smooth transition from the circular throat of the driver to the rectangular throat of the radial horn, while respecting the same throat exit angle of the "classic" JBL 2" drivers (244x and 2450) in the horizontal plane:

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    I will post again in the next few days/weeks as I proceed to make the first measurements using my set-up (calibrated Beyerdynamic MM1 microphone + HolmImpulse software)...

    Marco

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    Very interesting Marco. Thanks for posting. I will certainly be following up to see what you do and find. Great craftmanship on those horns!

  3. #3
    Senior Member gibber's Avatar
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    Quote Originally Posted by marco_gea View Post
    Another interesting feature of this design is the fact that the throat adaptor operates a very smooth transition from the circular throat of the driver to the rectangular throat of the radial horn, while respecting the same throat exit angle of the "classic" JBL 2" drivers (244x and 2450) in the horizontal plane:

    I will post again in the next few days/weeks as I proceed to make the first measurements using my set-up (calibrated Beyerdynamic MM1 microphone + HolmImpulse software)...

    Marco

    Hi Marco,

    i tried this 'plug & Be fram with the single piece of JBL476 that i own (offers for a second one welcome...)
    Used on a white whale, i found the "throatless" design of the 2450SL 1.5" core (476Be is very close to it) doesn't harmonise so well with short horns. Guess the length of throat-in-driver the EV HR series was designed for plays a role. I got better results using 2447 instead of 2450SL/476Be. It offers some 12mm or so length of throat and the same phase plug.

    Why do i write this? Your horn is also short even if it has a bit more of its own throat section than the EVs. But if you bump into the same issue as me and can't realize the 300Hz that Yuichi-san did, maybe try the 2446 core -- the "Coherent Wave" design was used as well, and maybe the 2446 also is longer than it's Neodymium sibling.

    Good luck with the project, eager to follow your progress here !

    Ralph

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    Quote Originally Posted by gibber View Post
    Hi Marco,

    i tried this 'plug & Be fram with the single piece of JBL476 that i own (offers for a second one welcome...)
    Used on a white whale, i found the "throatless" design of the 2450SL 1.5" core (476Be is very close to it) doesn't harmonise so well with short horns. Guess the length of throat-in-driver the EV HR series was designed for plays a role. I got better results using 2447 instead of 2450SL/476Be. It offers some 12mm or so length of throat and the same phase plug.

    Why do i write this? Your horn is also short even if it has a bit more of its own throat section than the EVs. But if you bump into the same issue as me and can't realize the 300Hz that Yuichi-san did, maybe try the 2446 core -- the "Coherent Wave" design was used as well, and maybe the 2446 also is longer than it's Neodymium sibling.

    Good luck with the project, eager to follow your progress here !

    Ralph


    JBLs model numbers are a bit confusing. Marco has the 2450, it is a 2" exit driver with a throat. The 2450SL is the 1.5" exit throatless driver. The 2450 would be closer in geometry to the 2446. Apologies if I misunderstood your post.

  5. #5
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    What is the current price of these Truextent-diaphragms?

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    Senior Member Odd's Avatar
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    43XX (2235-2123-2450-2405-CC 3155)5235-4412-4406-4401-L250-18Ti-L40-S109 Aquarius lV-C38 (030) 305P MkII

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    Member marco_gea's Avatar
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    Crossover design (II)

    For the second crossover between Mid and (super)-Tweeter, I opted for a simple symmetrical 2nd-order "Constant Power" design at 7kHz, with both drivers down -3dB at Fx.

    Inevitably, given the short wavelengths at play and the physical separation between the acoutst centres, the super-Tweeter will not be time-aligned.

    However,

    (i) this was not deemed important at such high frequencies, where the human auditory system's sensitivity to phase is reduced;

    (ii) the resulting comb filtering between the Mid and Tweeter outputs falls within the Equivalent Rectangular Bandwith (ERB), and is thus essentially inaudible.

    Greg Timbers also seems to agree on this, as he said:
    "The comb filter effect is normal and cannot be eliminated, even with the use of electronic delay. [...] In spite of the curve, we have had no negative comments about the 045Be and the choppy response through the crossover range"

    Additionally, having an intentionally misaligned constant power crossover at such high frequencies results in a 1/6th octave-smoothed summed response that is essentially invariant over a +/- 30 degree listening angle, since any additional misalignment introduced by moving the head laterally is swamped by the >360 degree original misalignment.

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    Finally, the resulting modelled impedance curve of the complete system is very flat and with only moderate phase angles throughout:

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    ...MORE TO COME! :-)

  8. #8
    Member marco_gea's Avatar
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    Crossover design (I)

    For the Woofer-Mid crossover, I got inspiration from the design used by Pioneer in their TAD/Exclusive monitors

    (e.g., http://audio-database.com/PIONEER-EX...del2402-e.html),

    and later reprised by Shozo Kinoshita in his Rey Audio monitors

    (http://www.reyaudio.com/large-e.html),

    which is based on an asymmetrical 6th-order electrical low pass / 2nd order electrical high pass, with both drivers down -6dB at Fx.

    In my case, the chosen crossover frequency is 600 Hz, so as to stay approximately one octave above the horn's cut-off.

    When both the Woofer and Mid are connected with positive polarity and the Mid is physically set back so as to create a suitable offset between the two acoustic centres, this crossover results in the emissions of both drivers to be in phase over a relatively wide frequency range around the crossover frequency, as can be seen in the figure provided. As further proof of the excellend phase match, when reversing the polarity of the Mid, a deep notch at Fx is observed.

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    This type of crossover is handy in several ways:

    (i) the front-to-back offset allows the convenient positioning of the horn-loaded mid atop the Woofer box without requiring any form of delay (be it analogue or digital);

    (ii) the same offset also results in the two impulse responses to be almost perfectly 'time aligned', i.e. both traces leave the horizontal line simultaneously;

    (iii) the 6th-order low pass effectively does away with any unwanted resonances at the top of the woofer's operating range.

    Two woofer section will adopt a balanced topology to reduce crosstalk with the mid and tweeter sections, and the compression driver will be attenuated with the Fostex R100T autoformer (http://www.fostex.jp/products/r80br82br100t/), whose 8 Ohm shunt resistor also handily leads to an almost perfectly level 8 Ohm impedence.

  9. #9
    Senior Member gibber's Avatar
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    Quote Originally Posted by JeffW View Post
    JBLs model numbers are a bit confusing. Marco has the 2450, it is a 2" exit driver with a throat. The 2450SL is the 1.5" exit throatless driver. The 2450 would be closer in geometry to the 2446. Apologies if I misunderstood your post.
    Hi Jeff, actually i am not sure if you misunderstood my post or not.

    I was careful to point out that my trials were based on 1.5" drivers. There, the 2447 fitted with 476Be fram did better than the 476/2450Sl core on a short-throat horn (i believe it was HR6040A, but maybe it was HR9040A). These EV horns offer only 13mm of throat length before rapidly expanding.

    In the 2nd paragraph of my post i tried to recommend 2446 (the 2" equivalent of 2447) in case the experiment with 2450 does show odd results (the horn Marco uses has 50mm of throat length before serious expansion occurs, so maybe this is a non-issue).
    I was also careful in only hinting at a possibly longer throat section inside 2446 (compared to 2450). I actually never measured throat length of these myself, but if 2446 is mechanically done the same way like 2447, then it might offer a longer throat than 2450.

    The issue of combined driver&horn throat length being too short was brought to my attention first when the JBL presenter on an AES convention here in Munich hinted at differences he got using 2447 and 2251 drivers on 2352 horns.
    Ralph

  10. #10
    Member marco_gea's Avatar
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    Quote Originally Posted by gibber View Post
    I was also careful in only hinting at a possibly longer throat section inside 2446 (compared to 2450). I actually never measured throat length of these myself, but if 2446 is mechanically done the same way like 2447, then it might offer a longer throat than 2450.
    This is interesting.

    Based on what I could find and read, I thought the internal structure of the 2446 and 2450 drivers was identical (apart of course the former's Ferrite vs. the latter's Neodymium magnet).
    In fact, I was under the impression that the internal conical throat section had remained the same all the way from the original 375 (=2440) to the 2441, 2445, 2446 and 2450.

    If anyone has reliable information to the contrary, I would be most interested in reading it.

    Marco

  11. #11
    Member marco_gea's Avatar
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    (Super)-Tweeter choice

    In my current speakers, I am using the Fostex T925A ring radiators (http://www.fostex.jp/products/t925a/).

    For this new endeavour, however, I decided that I wanted to try and stay with Beryllium diaphragms all the way to the very top of the frequency range.

    I therefore located and purchased a pair of Pioneer PT-R7 III
    (http://audio-database.com/PIONEER-EX...t-r7iii-e.html)

    This is an extraordinary ribbon tweeter which is unfortunately no longer manufactured.

    It features a Beryllium diaphragm loaded by a short front horn machined from solid aluminium, and a massive AlNiCo magnet, resulting a sensitivity of 97dB/W(m) and an outstanding frequency response that reaches out to 120 kHz (!).

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    The associated Impulse Response, calculated by way of reverse Fourier transform using the published frequency response (unfortunately, I do not have access to a microphone that is linear beyond 20 kHz) is very clean and "fast", as expected:

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    Senior Member jerv's Avatar
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    1.3 inch throat?

    Quote Originally Posted by gibber View Post

    ....used on a white whale, i found the "throatless" design of the 2450SL 1.5" core....

    Ralph
    The EV HR9040 is a real interesting horn, and I have alwas wanted to try a pair - but the documentation

    (http://www.google.no/url?sa=t&rct=j&...PZDIVMP5yb2DGw)

    states that the horn throat is only 1.3".
    How does this match up with the JBL 1,5 inch throat?
    Have you done any modifications?

    Best,
    Espen

  13. #13
    Senior Member gibber's Avatar
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    Quote Originally Posted by jerv View Post
    The EV HR9040 is a real interesting horn, and I have alwas wanted to try a pair - but the documentation
    ...
    How does this match up with the JBL 1,5 inch throat?
    Have you done any modifications?
    Hi Espen,

    it almost doesn't matter if you insert an adaptor or not. The HR series even has ex-factory that three hole mounting arrangement for the Altec 1.4" drivers of the time. Invitation for users to create a 2.5mm mismatch.
    For fear of nasty visitor comment i wouldn't dare a permanent install of a 1.5" without a conical flange piece, but truth be told i never saw a significant effect, just a bit of ripple here or there, sometimes even smoothening a response taken with the "correct" transition fitted .

    EV published measurements of even more gross throat size mismatch in one of the "PA Bible" articles as a quick fix guide for roadies during perfomances. Check it out.

    Ralph

  14. #14
    Member marco_gea's Avatar
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    Woofer choice

    Hello,

    here's some more info on this project as it unfolds.

    The Woofer will be the same one that I was previously using, i.e., the Fostex FW405N (http://www.fostex.jp/products/fw405n/)

    This is a modern woofer of recent design, with a hybrid cellulose+carbon fibre cone and a newly developed aluminum die-cast frame based on FEA.

    Magnet is ferrite, and sensitivity is medium-high at ~93 dB/W(m).

    The features that attracted me were:

    (i) Medium Efficiency Bandwidth Product (EBP) = Fs/Qes = 56.

    This allows the use of an overdamped bass-reflex box characterized by an almost 12dB/oct initial roll-off and associated low Group Delay, while still obtaining an F-3 of approximately 40 Hz in anechoic conditions (which leads to a nice almost flat response in room, given the typical Room Gain over the last octave).

    Also note how the excursion is contained under Xmax down to 20Hz with 130Wpc in input.

    Vb = 2 * VAS * Qt^2 = 180 L
    Fb = 0.39 * Fs/Qt = 23 Hz

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    (ii) Very low mechanical damping = Fs/Qms = 4.0.

    This, in my experience, is a recipe for excellent low-level detail retrieval. In other words, the woofer sounds good and does not mask or smear micro-details also at very low SPLs.

    By way of comparison, this is roughly the same mechanical damping as that of the TAD TL-1601a (Fs/Qms = 4.1), while only slightly higher than that of the JBL 1501AL-2 used in the flagship Everest DD67000 system (Fs/Qms = 3.7).

    Instead, it is HALF of that of the more PA-oriented JBL 2226H (Fs/Qms = 8.0) and of that of the venerable JBL 2235H (Fs/Qms = 7.9).

    (iii) Extremely low distortion.

    The levels of 2nd and 3rd harmonic distortion are amongst the lowest ever measured by the German magazine "Hobby-Hi-Fi":
    < 0.1% at 90dB(1m) all the way down to 20Hz (measured on an infinite baffle).

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  15. #15
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    Quote Originally Posted by marco_gea View Post

    The Woofer will be the same one that I was previously using, i.e., the Fostex FW405N (http://www.fostex.jp/products/fw405n/)
    This Fostex has a Rated Input of 50w, that seems very weak to me for a bassdriver. Musical power is 150w, but again thatīs only half of that what the JBL 2235h is capable of.
    Does Fostex has different power-ratings or are these drivers just not intended for driving high levels?

    Besides that, all the other parameters look very temping.

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