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

  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":

    Name:  A320FL_01.jpg
Views: 3780
Size:  268.4 KBName:  A320FL_02.jpg
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Size:  232.2 KB

    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):

    Name:  A320FL_08.jpg
Views: 3316
Size:  287.7 KB

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

    Name:  My A320FL_small.jpg
Views: 3729
Size:  92.8 KB

    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:

    Name:  My A320FL throat_small.jpg
Views: 3122
Size:  44.2 KB

    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!

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    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.

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    What is the current price of these Truextent-diaphragms?

  6. #6
    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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    Very nice horns, thanks for sharing!

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    Senior Member ivica's Avatar
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    A320 horn show

    Quote Originally Posted by marco_gea View Post
    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": Name:  A320FL_01.jpg
Views: 3780
Size:  268.4 KBName:  A320FL_02.jpg
Views: 4709
Size:  232.2 KB 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): Name:  A320FL_08.jpg
Views: 3316
Size:  287.7 KB I had the horns made out of baltic birch plywood by a French company specializing in bespoke audio designs: Name:  My A320FL_small.jpg
Views: 3729
Size:  92.8 KB 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: Name:  My A320FL throat_small.jpg
Views: 3122
Size:  44.2 KB 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, Very, very nice design and realization. So You have adopted FL design, so , may be horizontal dispersion would be a problem. Regards Ivica

  9. #9
    Member marco_gea's Avatar
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    Quote Originally Posted by ivica View Post
    Hi Marco, Very, very nice design and realization. So You have adopted FL design, so , may be horizontal dispersion would be a problem. Regards Ivica
    Hello, I'm aware that the Yuichi's other designs that have "fins" (aka "vanes") manage to control horizontal directivity up to higher frequencies. This is to be expected.

    However, the fins are also problematic in terms of increased internal reflections and diffraction.

    In fact, TAD themselves also went from a horn with fins (the TH4001) to a "fin-less" (FL) one with the later TH4003 model.

    As always, there's no free lunch.

    But, according to the polar measurements published in MJ, the horizontal dispersion of the A320FL horn appears to be just fine up to approximately 10kHz. And I intend to use a supertweeter crossed over at around 7kHz anyway (yes, despite the Be diaphragms. I'll post more on the reason why soon).

    Marco

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    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

  11. #11
    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

  12. #12
    Member marco_gea's Avatar
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    First measurements

    OK, here are the first measurements (MLS gated at 200Hz) performed outdoors.

    First, the on-axis IR and frequency and phase response:

    Name:  2450J_Be+A320FL FR&IR.jpg
Views: 2850
Size:  165.6 KB

    +/- 2dB from 600Hz to 9kHz, not bad! Also, a very 'clean' IR with very little overshoot and ringing. I guess this is one of the main benefits of the Be diaphragm, but it also says something of the horn in terms of lack of reflections etc.

    Incidentally, to my eyes this response curve looks nicer than that of JBL's 476Mg + H9900 horn as used in the K2 S9900 system:
    http://www.audioheritage.org/vbulletin/attachment.php?attachmentid=48630&stc=1&d=12902702 92

    It will also allow for a much simpler electrical high-pass, without all the parallel notches that have become so common in JBL's recent systems.
    I'm not sure, but I tend to think that the fewer acoustic issues to 'solve' in the electrical domain, the better.

    Then, the 30º off-axis response:

    Name:  2450J_Be+A320FL MLS.jpg
Views: 2884
Size:  102.2 KB

    This is less smooth, with some minor peaking at ~2kHz and then some beaming after 7kHz or so.
    I was expecting this, hence my intention to use a supertweeter.

    More to come...
    Marco.


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    Senior Member ivica's Avatar
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    Quote Originally Posted by marco_gea View Post
    OK, here are the first measurements (MLS gated at 200Hz) performed outdoors.

    First, the on-axis IR and frequency and phase response:

    +/- 2dB from 600Hz to 9kHz, not bad! Also, a very 'clean' IR with very little overshoot and ringing. I guess this is one of the main benefits of the Be diaphragm, but it also says something of the horn in terms of lack of reflections etc.

    ......

    It will also allow for a much simpler electrical high-pass, without all the parallel notches that have become so common in JBL's recent systems.
    I'm not sure, but I tend to think that the fewer acoustic issues to 'solve' in the electrical domain, the better.

    Then, the 30º off-axis response:

    Name:  2450J_Be+A320FL MLS.jpg
Views: 2884
Size:  102.2 KB

    This is less smooth, with some minor peaking at ~2kHz and then some beaming after 7kHz or so.
    I was expecting this, hence my intention to use a supertweeter.

    More to come...
    Marco.

    Hi Marco,

    Very nice result, especially very good off axis response relative to large 2inch horn throat.
    It would be nice to explain us total sound picture when all drivers applied.

    Regards
    Ivica

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    Senior Member gibber's Avatar
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    I agree that the 0° amplitude response looks very good, indeed, in the range that interests first&foremost.
    A loss of -20dB @19 kHz is however, more than i observed with Be frams. Not that it is of much consequence.
    It could be the the fram is too far from the phase plug. Does the driver have something close to a "0", or does it have a larger number felt-pen written on the back plate just outside the fram mounting area?

    on-axis phase response doesn't look like it's going to allow use below 800 Hz. Actually group delay display would be nicer for judging this.

    How about K2/3/4 -- can you bring that into the plot ? -- would be interesting to see how the BW-fram fares mounted to this 2" horn.
    On his site, Guido published comparisons between 476Be and Radian 951PB mod'ed by himself to BW-4016 fram and it looked like same-same to me on this 9800(?) 1.5" horn clone.

    Ralph

  15. #15
    Member marco_gea's Avatar
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    Quote Originally Posted by gibber View Post
    on-axis phase response doesn't look like it's going to allow use below 800 Hz.
    One must be mindful that a gated MLS measurement has a resolution equal to the gating frequency (in this case, 200Hz). This makes it unsuitable to discuss the low end of the response.

    By way of comparison, here's the same measurement but with no gating, and with 1/9th octave smoothing:

    Name:  2450J_Be+A320FL FR ungated.jpg
Views: 2778
Size:  82.2 KB

    Marco

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