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Thread: Need help reverse engineering crossovers

  1. #1
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    Need help reverse engineering crossovers

    I had a pair of three-way crossovers made a number of years ago. Unfortunately, I don't remember what the crossover points were on them. Can someone who is much more crossover-savvy offer some advice on how to determine the xover points? A posted picture? An attempt at a description? A layman's circuit drawing?

    I would like to use the crossovers for a DIY project but, of course, I can't know which components to use unless I know the crossover points.

    Help!

  2. #2
    Senior Member duaneage's Avatar
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    It depends on what you have for test equipment.

    If you have two 8 ohm resistors, a signal generator, and a voltmeter with frequency display you can connect the resistors to the crossover outputs to simulate speakers and by sweeping from 100 hz to 10000 hz observe when the output from the woofer connection is 50% less, and the output from the tweeter connection begins and reaches 50% of full output. THis is generally where the crossover points are for 8 ohms drivers. If you have a three way the same rules apply for the LP to MF to HP points.

    You could also measure the capacitors and inductors with testers specific for that task but testing with a dummy load will show how it works with a plain 8 ohm load.

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    Duanage,


    I have zero test equipment. I was hoping it could be done by listing the components and/or showing a picture and/or trying to draw a circuit diagram.

    Is that possible?

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    RIP 2011 Zilch's Avatar
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    Quote Originally Posted by barkingdog
    Is that possible?
    Yes. You figure out the schematic and then, according to the topology, use the appropriate formulas to figure out the frequencies of each of the filters.

    Alternatively, the schematic can be entered into a circuit simulation program to see the theoretical drive characteristics.

    An RTA works with duaneage's approach, above, as well....

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    Senior Member duaneage's Avatar
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    Listing the components will give an idea as to the intentions. When you put a real driver on it that can be another matter. Crossovers interact with the drivers so that the same network with different drivers can yield very diferent results.

    The topology matters most.

    A simple circuit with a inductor for the woofer and a cap for the tweeter is real easy to determine. If you have an inductor, caps, resistors, maybe another cap or two for the low pass, and 3 inductors, two cps, 4 resistors, and a L-pad on the tweeter be prepared to do some calculating.
    Most high quality networks have 12 db/oct slopes and that means a inductor in series with the woofer and a cap in parallel after the inductor but before the driver

    The tweeter circuit is the exact opposite. Resistors are added to the tweeter to lower it's output and these usually don't affect the crossover point much as long as they are placed correctly.


    Describing all the combinations is beyond this thread, draw up wht you ahve and scan it to a file, maybe someone here can help figure out what you have.

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    Quote Originally Posted by barkingdog
    ....(snip).....(snip)..... draw a circuit diagram.
    - Yes, draw out the circuit diagram with clearly written values for all the passive components .

    - Forget pictures .

    - Someone can approximate what's going on from a circuit diagram as long as all the elements have values .

    - FWIW ; don't get your hopes up too high that these networks are useful ( stock, without modification ) when using JBL drivers . I'm not aware of any JBL network that actually follows the "Classic LC values" / giving standard slopes . In other words ; all JBL networks, are designed to also implement some frequency tailoring ( to some degree ) for the components that the networks are designed to work with .

  7. #7
    Senior Member duaneage's Avatar
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    Earl makes an excellent point.

    Some examples of this are:

    The L100 had NO filter on the woofer and this is normally not the case.

    The L100T had 19 ohms of resistance across the woofer directly, normally this is done in conjunction with a capacitor as a Zobel.

    Lpads were placed in the middle of midrange crossovers to depress the middle range of the driver more than the overall level.

    The same tweeter and woofer could have 3 different crossover networks

    L20T, L20T3, and 4406 all used the same exactly drivers but with different networks.

    The L112 and 4411 use the same drivers but VERY different networks because of the arrangement of the drivers on the baffleboard and a different sound.

    So use them as a starting point but you'll probably end up changing them. The most important thing that xovers do is protect the drivers from the wrong frequencies, and even if they are not perfect they should be able to do that properly

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