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Passive low pass filter?

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  • #16
    Bard, my apologies. I did miss the word series. I stand corrected (again ). I guess I should take my own advice and be more careful.
    -Bob

    The PEDS 2.1 mini system
    My A7 Project - another small desktop speaker
    The B3 Hybrid Dipole - thread incomplete and outdated

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    • #17
      Originally posted by bard View Post
      Bob, I did edit (correct) my original post, and it now stands correct. It is a 3rd order SERIES filter. Apparently, you didn't see the "SERIES" in the description. This design only takes one inductor, rather than two. Again, the corrected post is a 3rd order, series, low-pass filter with an F3 of 240 Hertz. BTW it is spec'ed for 6.8 ohms Re, which should accommodate an 8 ohm speaker. I see later that the driver in question became a 4 ohm unit. Would have been good to know, up front. Sorry about the confusion.
      I'm not conversant in series XOs. But those components between an amp and a sub will cause problems. At 1 Khz, the filter's impedance is 2 ohms and near 0 ohms at 10 Khz. So while it does roll off the frequencies for the sub, it creates impedance problems for the amp.

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      • #18
        I've never done a series XO either, and I was confused by it, too. But come to think about it, with a normal series XO on a two way speaker, you would have the tweeter circuit also in series, so it would keep the impedance up, like in this example. Mike your observation looks right and it might not be wise to run just a sub on a series filter. It's also why you usually see at least a 1 ohm resistor in series with a grounded cap.
        -Bob

        The PEDS 2.1 mini system
        My A7 Project - another small desktop speaker
        The B3 Hybrid Dipole - thread incomplete and outdated

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        • #19
          Not sure he understands the diff. between parallel and series topologies.
          You can call that filter anything you want.
          To ME it looks like a fairly std. 3rd order "parallel" low pass filter (cuts highs).
          Values look appropriate for a 4ohm woofer that'd be down about -6dB around 240Hz or so.
          Last edited by Chris Roemer; 08-03-2018, 12:21 AM.

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          • #20
            Beware.

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            • #21
              Originally posted by djg View Post
              Beware.
              Omg, that is horrible! On a good note, when you do blow up that cap, at least it's contained! Sent from my SM-G920V using Tapatalk

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              • #22
                Mike: a 4 ohm load usually measures in at about 3.2 ohms RE roughly, and at 240 Hertz my software cranks out the following for values. Roughly 3mH input coil, 275 uF midpoint to ground, and 1mH output series cap. Third order Butterworth. At 4 ohms RE, your values are correct, however, as we know, 4 ohm impedance loads are rarely 4 ohms resistive. Right, bob?

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                • #23
                  To Bob, Mike, Chris, and others: I apologize for posting one half of a series filter circuit and causing all the confusion. Of course the series passover only works with the pseudo center point created between the two sections. In a senior moment, I posted only the bass section, which, by itself, is useless. Mea Culpa.

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