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An underdesk sub for a 2.1 system

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  • An underdesk sub for a 2.1 system

    I thought I'd share a sub I built recently which uses the venerable Tang Band W6 1139 SIF along with a Dayton SPA250 plate amp. The design goals here were;

    - tune down into the 20s so it could try to do a better job with movie soundtracks
    - fit under my desk
    - extend high enough to reach the desktop speakers, i.e. about 90-100Hz XO, as those speakers are a wall mounted MLTL tuned to about 80Hz using a pair of SB Acoustics sb65wbac25-4)

    The space available is the middle shelf as shown, this gives a max dimensions in the region of 40x30x30cm.

    After some time in the models, I arrived at a ~20-22L box and a ~90cm long slot port (about 15cm x produces a workable result, here it is with about 70W applied.

    Note that this includes the 2nd order 20Hz HPF in the SPA250. If you wanted to run it to the edge, and had external EQ, then you could switch to a 4th order hpf 1-2Hz higher and you could then apply a few dB more power. I don't think that really matters for the intended use but the option exists.

    Obviously this isn't the tiniest sub in the world as it needs to fit a 90cm long port in there but it is doable in the space available. The resulting enclosure wraps the port around 3 sides of the box like so

    This also the SPA250 to fit on one side as follows, you can see it slots between the top and bottom part of the port and fits between the amp and the driver.

    The external dimensions are as follows

    Next up, the build itself

  • #2
    I had purple 8mm valchromat to hand so used that to build it. I lined the sides with 4mm silentcoat and the interior with 12mm felt. The decision to use a plate amp came after I'd built it so I had to then route a hole out for the amp, good job it fit in the space


    • #3
      the finished result in room together with the computer/console desk I put together


      • #4
        some measurements to verify the result, looks like it came out on target to me

        final Z vs modelled

        in room with an FDW to try to see the sub response itself

        nf port and driver measurements (not scaled)


        • #5
          a quick look at the accompanying main speakers, got the idea for these from the diya thread on foamcore board speakers. It makes for quite a discreet computer system for when you're short of space.


          • #6
            Nice looking sub. What is the finish?


            • #7
              Thanks. I used a few coats of danish oil which brings out the colour quite nicely.


              • #8
                Very nice set up. So your wall speakers fire downwards?
                That Valchromat looks very interesting. I wish it was available around here, I see there is only 1 distributor in NY. Where did you get yours?
                I have made 1 small sub with that driver, but it chuffs so badly that it can only be used for music and at moderate levels, I would like to rebuild it with your design and see if that helps with the chuffing.
                Keep up the good work.


                • #9
                  the main speakers have a down firing port, perhaps these pics makes it more obvious

                  I have a local supplier of valchromat however I'm on the other side of the Atlantic in London so that probably doesn't help you

                  no noticeable chuffing here & I have given it a good run out at reasonably spirited levels on various movie test clips. You could reduce the port length a bit as well if you wanted to push the tuning up towards 30Hz, e.g. if you want to keep the port to 2 sides of the box.


                  • #10
                    My W6 box is really only about 2" longer (ODs w/0.75" stuff: 12h x 10.5w x 18.5d - or long) and uses a 2-1/2" straight port that's 14" long. Slightly larger volume (+10%) and slightly less port area (80%?), but almost twice the area of a 2" port. Easy to build. SA70 exceeds the 50w RMS rating of the voice coil.


                    • #11
                      note that I used a bigger amp because I am on 240V, US based builders could use something smaller and cheaper