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Need advice re: enclosure crossover

Discussion in 'Amps, Mics & Pickups [DB]' started by membluz, Sep 7, 2008.


  1. membluz

    membluz

    Sep 3, 2001
    Winnipeg Canada
    I'm building a cabinet (using the Eminence Disigner software) with an Eminence CB15 (8-Ohm 300W 48Hz-3KHz) and an Eminence Alpha 8MR (8 Ohm 125W 400Hz-4.8KHz).

    Each speaker will be mounted in its own 'compartment' in the enclosure.

    I'll be powering this enclosure with an Acoustic Image Focus 2R Series III amplifier and using it for my upright bass and my Fender Precision bass.

    I plan to connect them in parallel for a 4 Ohm enclosure. Do I need a crossover or some other electronic component to prevent the low frequencies below 400HZ from having a negative impact on the Alpha 8MR speaker?

    Thank you.
     
  2. robgrow

    robgrow Supporting Member

    May 1, 2004
    Cincinnati, Ohio
    Building your own speaker cabinet can be a lot of fun. Unfortunately, the results are often not predictable. You don't really know what the cabinet is going to sound like and if you are going to like that sound or not until your project is completed. Even having commercial design software is no guarantee that your project is going to sound great. There are so many variables and lots of ways to go wrong even for experienced designers. Having built many speaker cabinets in the past, I'd much rather go with a proven design these days. But that's just me.

    To try to answer your questions, you won't need a separate compartment for the Eminence Alpha 8MR, since it has a sealed back and is effectively isolated from the woofer. You will need a high-pass filter, probably set at about 500Hz with a 12 dB / octave slope to use with your Alpha 8MR. In musical instrument speaker cabinets, the woofer is usually connected directly to the amp, so you won't need a full 2-way crossover, just a high-pass filter for the Alpha 8MR. You will probably also need a good-quality, high power, 8-ohm L-pad to control the level of the midrange driver. For practical purposes, you can consider this as an 8-ohm cabinet, since system impedance is usually measured at somewhere around 250 Hz where the Alpha 8MR should be effectively out the picture with the high-pass filter. You can probably get the high-pass filter and L-Pad where you got your Eminence drivers.

    I won't get involved in the 15's versus smaller drivers for double bass issue, but I think that you are probably going to end up with a bigger cabinet than I'd want to have to tote around.

    Have fun and good luck.
     
  3. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    I had good results with the 800 Hz passive x-over made by Eminence. BLT Sound has them (www.bltsound.com). I think they may also have a 500 Hz model.

    Don't connect them in parallel, even for testing. The low end will kill the midrange. Like robgrow says, the end result of connecting these drivers with the crossover will be an 8-Ohm system.

    I think it's worth at least trying the system without an L-Pad, since if you can get the tone you want with the EQ controls on your amp, then you're home.
     
  4. robgrow

    robgrow Supporting Member

    May 1, 2004
    Cincinnati, Ohio
    The crossover frequency for the Alpha 8MR could be anywhere from 500 Hz to about 1000 Hz depending on how you want to voice your cabinet. A higher crossover frequency should give you a subjectively brighter sound. A lower crossover frequency will give you a little fuller sounding mids. One advantage with the 8-inch midrange driver is it has much wider dispersion at higher frequencies than the 15-inch woofer.

    I would highly recommend using an L-Pad to allow adjusting the relative level of the 8MR midrange driver. I think you will find that it has significantly higher sensitivity than the CB15 woofer, and the cabinet could easily be too bright unless you can pad the midrange driver down a bit. I'm not a fan of using the amp's tone controls to try to correct problems with the speaker cabinet, and the EQ on the AI amps is not particularly effective and works best for making small adjustments IMO. It's best to have a speaker cabinet that sounds about the way you want it to without having to use any EQ.

    The reason a standard two-way crossover is not used in many musical instrument speaker cabinets is because even a very high quality inductor in series with the woofer can have significant insertion loss. This can easily be 2 dB (or more) and is not very desirable. Often it's best in this application to connect the woofer directly to the amp and only filter the low frequencies from the mid / high frequency driver(s).
     
  5. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    Aha. Thanks for the info.

    I have to admit that I have reverted to a 1-way system. I added a midrange, and measured the response of the system to confirm that it was doing its thing, but for the life of me I couldn't tell the difference when playing bass through it.
     
  6. robgrow

    robgrow Supporting Member

    May 1, 2004
    Cincinnati, Ohio
    Another issue is low loss inductors are made with iron cores. They need to be rather large to handle high power, and are also fairly expensive.
     

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