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Thread: 2123H question please

  1. #1
    Senior Member SteveJewels's Avatar
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    2123H question please

    Hello All,


    I hope that you are happy and well.


    After a bit of a delay (I spent a year working in China.) I can resume my speaker project.


    I am planning a tri-amped system with a 2235H for the low freqs and maybe 2446/Truextent Be/2380a’s for the high freqs.


    I’m looking a 2123H’s for the mids and pulled the JBL spec sheet. There is a chart that shows the freq response as pretty much flat from 350 Hz or so to 5kHz. If I am reading this right it specifies that this is the performance in a 7 liter/0.25 cubic foot closed (sealed?) box.
    Am I missing something? A 7 liter sealed box?


    This is great if it is correct. I am looking for confirmation/experience with putting a 2123H in a sealed enclosure. Especially one that small.


    Here is a link to the spec sheet containing the chart I am referring to. The link is hot. If you click it the spec sheet will download.


    http://www.jblpro.com/pub/obsolete/2123.pdf


    Thanks!

    Steve

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    Regarding enclosure, the 2122 was used in the 4345 series in an enclosed 'dog house' style box to isolate it from the woofer. The inside dimensions were 14.5"x10'x6', which is 1/2 cubic foot. I believe the crossover point was 290hz.

    My hybrid 4345 project here is utilizing 2123's, as other builders have, and I expect great mid bass range performance.

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    Have a look at the JBL 4344 mkII to get an idea of how it´s used correctly.

    An enclosed subchamber is definetly the way to go!
    The 2123 specsheet says 7 litres, but I can´t find anything in the 4344 mkII specs... The 4343 and 4345 used a 14litre subchamber, but the 10" was a 2121 or 2122 back then...

    If you´re using a large-format driver you should target a crossover much below that stated 5khz, rather something between 1-1,5khz...

    Using the Truextent diaphragm I suspect you will need a tweeter, as this diaphragms use a soft surround and won´t go as high as TAD beryllium etc..

  4. #4
    Senior Member SteveJewels's Avatar
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    Thanks!

    I have some 2405's.

  5. #5
    Senior Member Ian Mackenzie's Avatar
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    I had time to look at this today

    There are a few considerations

    The box volume effects the resonance frequency of the driver / enclosure

    The smaller 1/4 cu ft3 enclosure resonance is close to 170 hertz
    The larger 1/2 cu ft3 enclosure resonance is close to 130 hertz

    Ideally you don’t want the enclosure resonance any where near the crossover point and preferably at least 1 octave below the crossover point.

    Modelling the 2123 including the dcr of the inductors in the 4344mk11 crossover the 1/4 cu ft3 enclosure is a QTC 0.7 Butterworth sealed alignment while the 1/2 cuft3 enclosure is closer to Bessel sealed alignment.

    In practice a 1/4 cu ft3 enclosure is tiny and may not provide breathing space around the driver after putting fibreglass on all surfaces (lightly fill).

    My suggestion is try the 1/2 cu ft3 enclosure and see how it goes.

    There are proportions in the 4345 plans or use a sono tube.

    If available l recommend you use an active crossover initially for woofer to midrange and midrange to horn transition initially. It’s much easier and quick to assess ball park options in a diy pursuit than a passive crossover which without a calibrated mic, analyser and crossover software and considerable experience is very difficult .

    You will also need to consider CD compensation on the horn if it’s a constant directivity type

    Good luck with your project

  6. #6
    Senior Member SteveJewels's Avatar
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    Excellent Ian! Thank you.

    I have a DBX DriveRack PA that I can use as a crossover initially, as you suggested. This also allows me to have an amplifier connected directly to the 2123H so no crossover network if that makes a difference for your calculations. I am planning to start at 300 Hz for the crossover.

    Once the crossover freqs are determined I have a Marchand crossover. There will be 3 amps, one for the 2235H's, another for the 2123H's and another for the 2446's and 2405's with a cap to perform the frequency discrimination duties.

  7. #7
    Senior Member Ian Mackenzie's Avatar
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    Direct amp to the 2123 is a good option. Your 1/2 cu ft enclosure will be okay.

    By all means try 1/4 cu ft3 and evaluate its subjectively if you wish.a

    You can try different crossover frequencies but I think you will find 250-300 hertz satisfactory.

    The 2245H is quite capable of operating up to a 300 hertz crossover point.

    I suggest 10 cu ft3 net tuned to 27-30 hertz with one inch fibre glass wadding on all surfaces except the front panel.
    There is some very good guidelines under plans in the JBL Library.


    Design example for the 2245H

    Net volume = Gross volume -Less all solid objects that occupy space inside the enclosure like the woofer, ports, bracing then +Add 10% volume for effect of the fibre glass virtual volume added to the enclosure.


    Gross enclosure volume 289.75 litres per dimensions below (10.23 cu ft3)

    Less volume of solid objects =31.75 litres (1.2 cu ft3)

    Woofer 6 litres
    3 x ports 6.75 litres. (10.3 cm ID x 26 cm long) (4 " ID x 10 " long) (27 hertz tuning)
    3 x ports (10.3 cm ID x 19 cm long) (4 " ID x 7.5" long) (30 hertz tuning)
    Bracing 19 litres (2"x4'" bracing)

    =258 litres (9.1 cu ft3)

    Add
    fibreglass virtual volume 25.7 litres (normal fill as simulated by Bassbox5.1)
    =Net volume 284 litres (10 cu ft3)

    Box internal dimensions 107 (H) x 66 (W) x 41 (D) cm (42 (H) x 26 (W) x 16 D) inches)
    Use one inch mdf and brace it like the 4345 plans.
    Three 4 inch round PVC drainage tubes for the ports on front panel)

    FB 27.2 hertz
    F3 32.2 hertz
    QL = 7

    Bassbox 5.1 is a Professional Box Design program amp and is very accurate.

    if you plan to installed the 1/2 cu ft3 mod enclosure I inside the main enclosure you will need to add the volume of this enclosure based (exterior dimensions) and re calculate the internal dimensions.

  8. #8
    Senior Member SteveJewels's Avatar
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    Hello Ian,

    Thanks for the info.

    Actually I have 2235H's rather than 2245's.

  9. #9
    Senior Member Ian Mackenzie's Avatar
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    Regards crossover aim for 300 hertz on the 2123 and 1000-1200 hertz.

    The 2380a requires CD EQ to raise the upper mid response and flatten it then a 2nd order low pass @8000
    You can do that with the active crossover or passively. I posted details in another thread recently.

    In the end you might experiment with other horns such as the 2311/2308, the 2397 or diy wood horns on the www.

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