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  • Spaker
    started a topic Passive low pass filter?

    Passive low pass filter?

    I've been trying to find a schematic or product that acts as a passive (half) crossover that cuts all higher frequencies and allows low frequencies to pass, presumably to an amp for a subwoofer. Anyone know how to wire one of these up? I can work off a schematic, crayon drawing or back of the napkin chicken scratch. Here is the link https://rover.ebay.com/rover/0/0/0?m...2F112403805811 Let me know if you can draw up a circuit. If you need a cut off, 400Hz will do if it can handle 200w Sent from my SAMSUNG-SM-G891A using Tapatalk

  • bard
    replied
    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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  • bard
    replied
    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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  • BennyD
    replied
    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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  • djg
    replied
    Beware.

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  • Chris Roemer
    replied
    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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  • BOBinGA
    replied
    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.

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  • Millstonemike
    replied
    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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  • BOBinGA
    replied
    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.

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  • bard
    replied
    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.

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  • Millstonemike
    replied
    Originally posted by Chris Roemer View Post
    Come ON you guys, you all KNOW this stuff! (what the hey's goin' on?)

    Mike calls it a high pass, but in fact it's a low pass (which IS what you want).
    I didn't do a good job structuring my post. The "this is a high pass filter" was referring to the quoted post (actually what I remembered before it was edited). The rest is textbook.

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  • BOBinGA
    replied
    Thanks Chris. Bard's first attempt was cap, coil to ground, cap which is of course a high pass filter. After my post, he changed the circuit to cap to ground, coil, cap to ground which is still not right. I didn't correct his second try but I probably should have. I was trying to not be too hard on a new member. I welcome new members and want to encourage them to keep posting. I was called out for mistakes more than once when I started posting, so, Bard, don't take it personally and keep posting. Mike'circuit is indeed correct for a low pass filter. I assume he knows better and just made a simple mistake. So, let's all be a little more careful before we post.

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  • Chris Roemer
    replied
    Come ON you guys, you all KNOW this stuff! (what the hey's goin' on?)

    Mike calls it a high pass, but in fact it's a low pass (which IS what you want).

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  • Millstonemike
    replied
    Originally posted by bard View Post
    From what you've requested, I assume you're looking for a good, fairly sharp, low-pass filter for a sub-woofer.

    If that is so, I would suggest a 3rd order, series, low-pass Butterworth filter with an F3 of roughly 240 Hertz.

    That would mean 3 parts: The input parallel cap to ground should be 35volt, 150uF non-polar electrolytic capacitor, the mid point series inductor should be 6 milli-Henries, and the output parallel cap to ground should be a 35 volt, 50 uF non-polar electrolytic capacitor. This will provide a fairly steep rolloff at 18 dB per octave with an F3 of approx. 240 Hertz. (Great sub-woofer specs) {edited}
    This is a high pass filter.A third order, 240 Hz the filter for a 4 ohm sub would be: Series inductor of 4 mH, a capacitor to ground of 220 uF and a series inductor of 1.33 mH.

    This would be used between a power amp and a passive sub. Per DanP's post, an entirely different set of components would be needed if you're passing a signal to a powered sub (or passing a signal to the amp that is powering the sub).

    Click image for larger version

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  • bard
    replied
    and hats off to Bob in GA for catching my topography error in my last post. It is now corrected.

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