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Use sealed enclosure as 2nd order high pass for 2.1 system?

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  • Use sealed enclosure as 2nd order high pass for 2.1 system?

    I'm looking at building some sort of small desktop system. Either satellites with a small sub like the Voxel, or a pair of speakers combining a micro 2 way design or full range driver with a 4" side firing woofer in each sharing power from the .1 channel.

    There are lots of 2.1 amplifier boards on the market. I've yet to find an inexpensive one with a proper 2 way crossover. On most the satellite channels are full range and the sub channel has a higher than normal low pass (150hz +)

    Would there be any reason not to use the satellites in a sealed enclosure to approximate a 2nd order high pass if I can hit the target frequency while still keeping the Qtc reasonable? I'm assuming the filters in most of the 2.1 boards would be a 2nd order LR, so I'd aim to be -6db at the x-over frequency of the board.

    Jason

  • #2
    What type of amp's have you looked at (pic.'s or links)? I've analyzed several of the class D 2.1 amps. On a few, the .1 LP Fc is easily modified. High pass on the L-R is more is little more complicated but can be done.

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    • #3
      Using only the acoustical roll off of a small sealed cab doesn't accomplish the main job of a high pass filter, which is to keep low frequency content from being sent to the satellites.
      www.billfitzmaurice.com
      www.billfitzmaurice.info/forum

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      • #4
        Originally posted by Millstonemike View Post
        What type of amp's have you looked at (pic.'s or links)? I'e analyzed several of the class D 2.1 amps. On a few, the .1 LP Fc is easily modified. High pass on the L-R is more is little more complicated but can be done.
        One I've got in mind is sold on a big auction site under the name Bluetooth 4.0 Digital 2.1 Class D HIFI Power Amplifier Board 3CH Super Bass Ampl

        Not sure I'm allowed to post a link here.

        It has a actually has a 180hz-250hz variable crossover. Might be possible to swap out a different value pot to change the range? I'm open to suggestions for other 2.1 boards with bluetooth as well especially if there is one out there with a proper high pass or one that can easily be modded to have a high pass.

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        • #5
          Originally posted by billfitzmaurice View Post
          Using only the acoustical roll off of a small sealed cab doesn't accomplish the main job of a high pass filter, which is to keep low frequency content from being sent to the satellites.
          True, but it may be an acceptable compromise. They should blend well if the slopes and frequencies are a reasonable match. And the small sealed enclosure should keep excursion under control.

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          • #6
            It will help, yes. But keeping excursion under control (if you are pushing them hard) will be helped by the use of a cap. This will result in a 2nd order acoustic filter.
            https://www.facebook.com/Mosaic-Audi...7373763888294/

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            • #7
              Keeping excursion under control doesn't protect the voice coil from overheating. That would be a problem even if power density was a constant with frequency, but it isn't, it doubles with each octave lowering of frequency content. The power the amp is producing below the speaker pass band still heats the voice coil.
              www.billfitzmaurice.com
              www.billfitzmaurice.info/forum

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              • #8
                Originally posted by jasonb View Post

                One I've got in mind is sold on a big auction site under the name Bluetooth 4.0 Digital 2.1 Class D HIFI Power Amplifier Board 3CH Super Bass Ampl

                Not sure I'm allowed to post a link here.

                It has a actually has a 180hz-250hz variable crossover. Might be possible to swap out a different value pot to change the range? I'm open to suggestions for other 2.1 boards with bluetooth as well especially if there is one out there with a proper high pass or one that can easily be modded to have a high pass.
                That amp is expensive compared to others and doesn't have a good review. Check out this post on DIYAudio. The first link is in use (you can read the post). On that amp, the blue poly box caps near the middle pot may be the two caps in the LP filter (infinitely easier to replace them than SMD caps unless you have a SMT rework station). I'e been looking at the similar amp in the second link to analyze and post here but I hae another 2.0 amp coming so that;s a long way off.

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                • #9
                  Originally posted by billfitzmaurice View Post
                  Keeping excursion under control doesn't protect the voice coil from overheating. That would be a problem even if power density was a constant with frequency, but it isn't, it doubles with each octave lowering of frequency content. The power the amp is producing below the speaker pass band still heats the voice coil.
                  Given the amp category the OP is considering and if he's using 8 ohm L-R speakers, he won't get more than 30 - 35 W rms into the drivers before clipping / distortion sets in. So if the box can control xmax and keep the low end down and the drivers are rated for 30 W thermal, that might be a fairly good compromise.

                  A cap will never hurt and would cost peanuts if an NPE is used. That would be advised sans $100+ drivers and a critical ear.

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                  • #10
                    Originally posted by Millstonemike View Post
                    A cap will never hurt and would cost peanuts if an NPE is used. That would be advised sans $100+ drivers and a critical ear.
                    ​Note that you can't just add a cap to a sealed speaker and expect a THIRD ORDER (not second order - a sealed speaker's rolloff is already second order) rollof. The speaker's impedance at Fb will affect the rolloff a bit. In fact, in some cases it might even extend the speaker's frequency response.

                    The system I made for my daughter uses sealed boxes that naturally rolloff below 100 Hz and a bass unit that handles the bass frequencies. It works fine.
                    Brian Steele
                    www.diysubwoofers.org

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                    • #11
                      Originally posted by Brian Steele View Post

                      ​Note that you can't just add a cap to a sealed speaker and expect a THIRD ORDER (not second order - a sealed speaker's rolloff is already second order) rollof. The speaker's impedance at Fb will affect the rolloff a bit. In fact, in some cases it might even extend the speaker's frequency response.

                      The system I made for my daughter uses sealed boxes that naturally rolloff below 100 Hz and a bass unit that handles the bass frequencies. It works fine.
                      Yes, (I've just never see that effect in any sims I've done in WinPCD).

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                      • #12
                        Originally posted by Millstonemike View Post

                        That amp is expensive compared to others and doesn't have a good review. Check out this post on DIYAudio. The first link is in use (you can read the post). On that amp, the blue poly box caps near the middle pot may be the two caps in the LP filter (infinitely easier to replace them than SMD caps unless you have a SMT rework station). I'e been looking at the similar amp in the second link to analyze and post here but I hae another 2.0 amp coming so that;s a long way off.
                        The seller in the second link has a very similar board with bluetooth. I may look at that one as I'd like to have bluetooth as on option. The one thing I couldn't find is any indication what the frequency range is for the variable low pass crossover. I'm going to try contacting the seller.

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