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Passive Lo Pass with Active Crossover?

Discussion in 'Live Sound [BG]' started by MrLeadFoot, Nov 15, 2012.

  1. MrLeadFoot

    MrLeadFoot

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    I have an 18" sub with a passive lo-pass filter rated at 125hz. I have a Mackie m1400i amp with a Subwoofer option and two settings: 63hz and 125hz. Interestingly, when the amp is set at 125hz, it cuts out noticably more of the upper end than the sub's built-in passive lo-pass filter does. For example, when I play an .MP3 file through the system, and set the amp output to be full-range, I can hear some faint vocals coming through. When I engage the amp's 125hz setting, those vocals are gone completely. This leads me to believe that the sub's lo-pass filter is "leaking" some higher frequencies, would you agree?

    While I can simply run the amp in Subwoofer mode, and set the frequency selector to 125hz, I can't help but wonder if removing the sub's lo-pass filter will result in noticably better sound and/or power to the speaker. I spoke to Mackie yesterday and I was surprised to hear from the rep that I should leave the lo-pass filter in the sub, and don't worry about it, as I would likely not notice any difference, and that the sub's lo-pass filter is probably doing something else to help the sound in the speaker design. It sounds a bit hard to believe, so I am posting here to get opinions from people in the "real world". :)

    It would be nice if Mackie was 100% correct in their answer, because it would offer more flexibility. For example, if in a pinch at a gig we needed to power a full range speaker (maybe a monitor amp dies, or whatever), having the lo-pass filter remain in the sub would enable us to set the amp in full-range mode and let the sub's lo-pass filter do the work, even if it is not an optimal lo-pass filter. ON the other hand, if removing it will yield better results in terms of sound and power to the speaker, I'm thinking I can always put the lo-pass filter in a project box, and run it inline if I ever need to, correct? Or, will it get too hot in a box? FWIW, I made a box with an L-pad that I run that way when I need to (albeit for a different purpose), but it does NOT get hot.

    Given that I am currently running banana plugs from the amp to Speakon at the sub on 12 gauge wire, if I do put the lo-pass filter in a box, and end up having to run it that way, would running Speakon to 1/4" adapters in and out of the box degrade the signal?

    Thoughts on ANY of this would be appreciated.
  2. WayneP

    WayneP

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    Hey LeadFoot,

    A crossover isn’t a brick-wall filter. It drops frequencies below the frequency setting at so-many dB per octave. For instance, if you have a 200 Hz filter with a 6 dB/octave slope, and play a 100 Hz sine wave signal (i.e. one octave below 200 Hz), the level of the sine wave would be 6 dB lower than a 200 Hz sine wave would be. A 50 Hz sine wave (two octaves below 200 Hz) would be 12 dB lower. Make sense? Six dB/octave is actually a fairly shallow slope; 24 dB/octave is common with electronic filters these days.

    So yes, it’s typical to be able to hear some vocals coming through the sub when it’s set as high as 125 Hz, as there are artifacts down that low, especially if the voice in the recording hasn’t been properly high-passed. It’s a function of not only the crossover frequency, but how steep the crossover’s slope is as well: The steeper the filter’s slope, the less of the signal that’s above the crossover frequency “leaks” through.

    It’s very possible that the sub’s passive filter has a 12 dB/octave slope. When you add the Mackie’s electronic filter, which is probably 24 dB/octave, you now functionally have a 36 dB/octave slope. This explains why you hear less of the voices when the electronic filter is engaged.

    As far as bypassing the sub’s passive crossover, that might be a good idea, since those things suck off a lot of power. Bypassing the passive crossover and going active you’ll notice that the sub is playing louder than it was with the same gain setting on the amplifier. IOW, you’ll get the same sub level with less power from the amp.

    On the other hand, as Mackie says, there might be something the passive filter is doing to enhance the sub’s sound quality. I’d expect it’d be negligible, though, and if not it could probably be corrected by equalization.

    There’s no problem putting the crossover in an outboard box with Speakon connections, so that you could still use it if needed.

    Regards,
    Wayne A. Pflughaupt



    Administrator, Pedulla Club #45
    Administrator, Tobias Club
    Big Cabs Club #23
    My Rig: Stage and FOH Friendly


  3. MrLeadFoot

    MrLeadFoot

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    Yes, the passive filter has a 12dB slope.
    So, if, as a test, I put jumpers at the sub's connector plate to bypass the filter (albeit doing this means the filter would still be in parallel with the jumped circuitl) and use only the Mackie's active filter, it is possible I might still hear SOME of the vocals on that same .MP3, given that instead of a 36dB slope (with the two filters combined as you describe above), I would have 24dB slope, correct?

    This is exactly what I was asking, thank you.:) Would the same test above enable me to hear the difference, or would the fact that the filter still being connected in parallel "rob" some of the signal just by virtue of it still being connected?

    I can't imagine what that "something" might be, can you? After all, unlike the position of the speaker in the box itself, it's only a filter, and I can't see how a filter would EHNANCE sound in any way other than limiting unwanted frequencies, can you?

    Right now I am connecting the amp to the sub via a cable that is banana male==>speakon. So, I was thinking I could mount the filter in a box with 1/4" female inputs and outputs, then make a couple of short pigtails that were:

    1) banana male to==>1/4" to connect from amp to filter box
    2) 1/4"==>banana female
    then connect by banana male==>speakon cable for the long run to the sub. Would that work or would that many connections compromise signal strength, and rob the chain from even more power? The reason for using 1/4" females on the box is because speakon and banana connectors built into the box would make the box less universal for other applications (should I ever come across one), and 1/4" females might be much easier to work with on a compact box, you know what I mean?
  4. uhdinator

    uhdinator

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    I would consider wiring the sub driver directly to the Speakon input jack.
    Use an active crossover rack unit and set the crossover where you want it.

    Most will recommend 90-100 hz if using top cabs with 15's and horns.
    115-120hz for top cabs with 12" driver/horn.
  5. MrLeadFoot

    MrLeadFoot

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    Right now I am indeed wired straight to the sub. The question of wiring pertains ONLY if I remove the sub's passive filter, put it in an outboard box, AND for some reason need to use it, and only likely in a pinch. Hope that makes sense.

    I'm currently running 1 powered mixer and two submixers and the Mackie amp with that 63hz/125hz internal active filter to drive the sub. I'm trying to get by with the Mackie's internal crossover for now until the next inevitable upgrade, which will likely be a new amp with built-in crossover, as I don't want to carry yet ANOTHER unit even if it is a crossover that only needs 1U. My racks are full so that one additional rack space means a lot right now, you know what I mean? If only I had made my last rack with just 1 extra rack space; in fact, I had enough rack rails for one more space, and in the end decided I didn't want more box than I needed, especially since I modularized to keep weight down during transport. If I only knew then what I know now!:meh:
  6. will33

    will33

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    +1 to the above.

    The subs passive filter is likely at most a big coil and cap = 2nd order filter.

    Yes the better way is wiring the 18 straight to the jack and using an active crossover, and yes, you could take that passive filter and mount it in an external box with an "in", "low out", and "high out".

    The passive filter would likely be put to better use in a bass rig add-on woofer. Say something like a combo amp that sounds alright on it's own for small stuff. Adding the box and good woofer to it could really up it's performance for bigger stuff. Even something like a little 110 can get damn loud when you take the <100hz excursion pressure off of it and hand that off to a woofer that can actually do it.
  7. will33

    will33

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    Well, you'd also need a passive HPF for the "high out", but you're half way there.
  8. lowfreq33

    lowfreq33

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    You should leave it alone. The filter section is probably on a circuit board with the jacks and a fuse of some sort. Even if you do manage to properly remove the right parts, you still won't be improving the performance of the sub. Not even a little bit.
  9. Jazzdogg

    Jazzdogg Less barking, more wagging! Supporting Member

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    I find the operation of high-pass filters to be far more critical, especially with regard to intelligibility of sound, management of room modes, and power amp utilization.
  10. MrLeadFoot

    MrLeadFoot

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    While the filter is indeed on a board of it's own, that's about it. This is a passive filter, so there is no fuse or other active circuitry. The input jacks are wired to the filter, which is then wired to the 18" woofer. FWIW, there are two coils, each of which is quite large, which makes me think that they do suck a bit of power as the signal makes its way through the windings.

    Out of curiosity, why do you say that you don't think removing it will help the performance "even a little bit"?
  11. MrLeadFoot

    MrLeadFoot

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    It's actually comprised of 2 BIG coils. Imagine something like what I linked to below, except with BIGGER coils and BIGGER wire in the windings, and TWO of them. It's quite a beast, I magine due to the fact that the speaker is supposed to handle 1000 watts RMS and 2000 watts Peak. Because of it's size alone I wonder if it does suck down a significant amount of power WAYNEP stated. I will say this, though, bass guitar sounds awesome through this filter, so you're right about its application for a bass cabinet. :)

    http://www.parts-express.com/pe/showdetl.cfm?partnumber=266-446

    Do you agree with WAYNEP and also think I will see more power delivered to the speaker without the passive filter in the loop?
  12. lowfreq33

    lowfreq33

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    Because that's not how filters work. It rolls off frequencies above 125hz. It doesn't absorb raw power.
  13. will33

    will33

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    2 big coils could indicate a 3rd order filter, depending on how they're laid out, but I doubt it. 2nd order is usually enough, and I'd be more likely to think the 2 coils are both in the first half of the filter, to increase power handling in the crossover. That's important when you start putting together stuff the does anything that low in frequency.
  14. will33

    will33

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    Yes, using an active crossover and separate power amp, you will get more performance out of the speaker, especially relative to the rest of your system.

    I wouldn't worry about the insertion loss/power sucked up by the crossover parts. It takes at least a 50% in power difference for even discerning ears to hear a difference at high volume/live gig level volumes. You might hear some difference in a quiet, dedicated listening room, focusing on small differences, but who listens to music like that..........other than people who design speakers?:D

    But yes, music will be delivered cleaner and more "correct" by taking the passive filter out of it and using active multi-amping.


    When you say the bass guitar sounds good through it, that is of course, a subjective thing, but I bet it sounds pretty good. That is, after all, a musical instrument reproduction system and not a "hifi recorded music listening to system"........goals differ, and a bit of "character" is often a good thing.:)
  15. MrLeadFoot

    MrLeadFoot

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    I understand that, wouldn't the fact that there is so much more wire in these coils inherently causes a signal decrease?
  16. MrLeadFoot

    MrLeadFoot

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    Wait, just to be clear, I already AM running a dedicated amp that happens to include an active lo-pass filter to the sub. But, I was wondering if I was robbing the chain of signal/power by using the active lo-pass filter in the amp in conjunction with the lo-pass filter in the sub. And, if WayneP is correct about the 36dB slope resulting from two filters in series, then I am indeed robbing the chain of something.

    I'm more concerned with having just that much more headroom, should I need to push just a LITTLE bit more here and there, dependent upon venue, you know what I mean?

    What do you mean by "active multi-amping"?

    What I mean is we sometimes run bass guitar straight through the PA, and I gotta say the combination of the sub with the passive lo-pass filter in play, and our 15" mains sounds REALLY good. In fact, the best settings in this setup is to send full-range from the Mackie amp and let the passive filter do it's job. Conversely, when I set the Mackie's active filter to 125hz, we lose a significant amount of bass guitar in the sub, I presume because of what WayneP said about the doubling up of the filters. Thus, I'm trying to get as much info as possible, lest I do the tedious work of removing the passive filter and box-mounting it for backup, only to find that doing so really doesn't do much. :meh:
  17. modulusman

    modulusman

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    It might be easier to give you advise if you would tell us what brand/model the sub is.
  18. will33

    will33

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    Does it state what frequency the subs passive filter is set to? I may've missed it.

    I'm thinking when you engage the active on top of it, depending on settings, there may be some destructive overlap in the crossover region, or the 2 together might be rolling out too much mid/high bass. Meaning the 80-150hz thickness and "beef" in the bass tone, and not so much the 40-60hz really deep stuff. Depending on the settings and how the 2 filters interact, could be causing a hole in the response somewhere down there.
  19. 1958Bassman

    1958Bassman

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    Yes, it does. Passive filter components cause insertion loss and that does "waste" power- it's one of the main reasons active filters are superior.
  20. Steve Dallman

    Steve Dallman Supporting Member

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    That PE lowpass crossover has a coil and a cap...standard 12db/octave lowpass.

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