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Help with spherical sub for spherical speakers

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  • Help with spherical sub for spherical speakers

    Back in 2015, I made a pair of Bluetooth, battery-powered spherical speakers with a built-in 15WPC amplifier (Dayton DTA-2) - see the link below. These were made from Ikea "Blanda Matt" 8" salad bowls (so a little less than 0.15 cubic feet each). I've been interested in adding a subwoofer, swapping out the amp for a 2.1 channel board, and running it with a proper power supply instead of batteries.

    The speakers currently use Tang Band W3-1364SA Bamboo Cone drivers, which sound pretty good and have a fairly flat FR down to 200 Hz, but not much below that.

    The amp I was thinking of using is this one: 2.1 Hi-Fi Class D Audio Amplifier Board 2 x 15W + 30W(Part # 320-608). It has a fixed low pass at 150 Hz, and the left/right channels run full-range.

    Coincidentally, Ikea also makes those bamboo bowls in an 11" size - two of them would make a nice sphere with an internal volume of 0.4 cubic ft / 11.4 L - actual internal volume will be smaller once I saw off a section of the sphere to mount the driver.

    So here's my questions for the forum:

    1) It feels like there's a frequency gap between the Tang Band drivers and the LPF: the TB drivers drop off below 200Hz, but the sub doesn't pick up until 150Hz. Would this be noticeable and should I use different drivers for the L/R channels that dig a little deeper, even by 1/3rd of an octave (like the Fountek FE85). Or should I pick a different amp with a higher/variable crossover frequency?

    2) My original thought for the sub was to make a spherical version of Paul Carmody's "Voxel", but the enclosure volume seems off - my sphere is significantly bigger than 7L. Anyone have an alternate, not-too-expensive recommendation for a ported sub in a ~11L enclosure?

    3) The driver in the Voxel sub seems like it would need more than the "30W" supplied by the amp I'm considering, given the sensitivity is only 82dB @ 1W/1M. My other option is to use one of those Ebay TPA3116D2 2.1 channel amps that include a bluetooth chip and run on a simple laptop AC adapter for more WPC (those are often advertised as 50+50+100 W), or even the Lepai 2.1 channel amp that Paul originally used in his Voxel build. Thoughts on choice of amp?

    I apologize for what may seem like novice questions - I'm trying to move beyond just putting random drivers into an enclosure and hoping they sound good and am eager to learn from all of you. Thanks in advance!
    Show off your speaker projects by posting the details and images of your creations. We encourage you to discuss with the speaker project designer his methods and results.

  • #2
    The TPA3116 2.1 amps ... at 24 V you'll get ~70 W rms into 4 ohms, 35 W rms into 8 ohms (including the sub channel). The 100W rating is for a 2 ohm sub and is the thermal dissipation limit of the chip. But that's for a sine wave and music is not as dense. So a 8 ohm L-R and a 4 ohm sub at 24 V will put out 2 x 35W + 70W.

    The thing with the ebay amps is most of the offerings are based on the same design. But the specific mfg.'s (and retailers) have varying quality (I have had three and one design is a no go. Othera have run into issues as well.).

    If the board was populated with the correct parts, the sub channel has a 2nd LP filter circa 100 hz. That's easily changed to what you need by swapping out two small caps. For my build, I modeled the sub and L-R in WinPSD including the a passive LP filter (large cap and coil). Then I converted the resulting corner freq and Q into components for the active LP on the board. Worked very. I can help you through that if you like.

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    • #3
      you can reduce the volume by putting something inside, like a block of wood or a brick, to take up space.

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      • #4
        1) you're living without the bottom 3-4 octave today, so anything that plays below 200Hz is adding to the sound. From there, it depends on your sensitivity to linear distortion; can you hear a 1/2 octave missing band? I'm not asking if it's audible (it is), rather if you notice it. Only way to find out may be to build it and find you need L/R speakers that dig deeper.

        2) reading Paul's write-up, he was going for small. Based on my Unibox model for the W5-1138, building it in an 11L box just gives you another 5Hz in F3, with associated increase in LF extension. The downside is you stress the performance of an already too-small port. Chuffing will be a bigger problem at high output levels. Conversely, a big-enough port is twice the length, and 18" becomes a problem.

        3) lots of options, the best of which may be to use a subwoofer plate amp, like this:
        https://www.parts-express.com/dayton...ifier--300-784
        https://www.parts-express.com/dayton...ifier--300-802

        These two are designed for this type application. They accept speaker-level inputs, and have adjustable XO that reaches up to 180Hz where it should meet your current speakers.

        Hard to fit on a sphere, though...

        HAve fun,
        Frank

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