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Thread: 4331 Crossover help please

  1. #1
    Member Mazza's Avatar
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    4331 Crossover help please

    Dear all,
    I have managed to find the 3131a crossover schematic for the 4331 that I'd appreciate some help interpreting please.

    My objective is to recreate this crossover (without what seems like the multi-tapped transformer L2?).

    Here's the original schematic that EarlK very helpfully posted here many moons ago...

    Name:  N3131a.jpg
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    Here's my trace of the primary circuit path - Red = woofer positive, Green = tweeter positive and black is ground.....


    Name:  InkedJBL 3131a_LI.jpg
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Size:  49.6 KB

    A question: is R6 5.1 ohms?


    Here's my interpretation of the 3131a in XSim (the blue highlighted resistor pair are an L-Pad block to recreate R7 in the original 3131a circuit)....


    Name:  JBL 3131a XSim.jpg
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    Have I worked around the tapped inductor appropriately and got the XSim version correct?



    Many thanks!

  2. #2
    Senior Member Doctor_Electron's Avatar
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    You've got a fun project going there.
    I would like to mention a sometimes overlooked electrical value pertaining to the inductors used in "clones" of JBL professional crossover networks.
    The DC resistance values of the original inductors may or may not have been a significant parameter in the design of the original network.
    The DCR value of an inductor will affect the steepness of the attenuation slope of the filter pole. So when choosing or winding an inductor for your clone, its DCR should be included in your calculations.
    I mention this because I did a lot of experimentation with 3107 (used in the 4350) networks and 2440 drivers. In the process I found that the 1.8 mH inductors in the section of the 3107 that "crossed" between the 4350's 2202 midbass driver and its 2440 midrange compression driver each had a DCR of 3.5 Ω. This was obviously an intentional and and important consideration in the desired response of the network.
    I have tested the crossover inductors in the 4410 and 4412 Monitors as well as those used in L150’s. Their DCR values were much lower.

    A drawing of the original networks (only) should specify the inductance and DCR values of the inductors, and the tapped autotransformer of the HF section.
    Regards, D_E
    Last edited by Doctor_Electron; 11-18-2021 at 04:09 PM. Reason: Refinement
    "Why don't you Mine your own Bismuth, so you won't be mining mine?"

  3. #3
    Member Mazza's Avatar
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    G'day DE,
    Thanks very much for your help.

    Yes I understand the importance of the DC resistance of the inductor and you rightly note that this information is indeed missing from the original JBL schematic.

    The benefit of these modern crossover modelling tools such as XSim is its very easy to adapt the design to accommodate variations in parts like this.

    Appreciate your insights.

    Cheers,
    Maz.
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