Introduction: Bluetooth Soundbar, 50W+50W Stereo & 100W Bass, Transmission Line Speaker Design

By Hoekstra (UK)
About: At heart an engineer, musician, polyglot, cook, computer programmer, wood worker, brewer and hacker.

Use scraps of wood and old speakers, plus a cheap class-D amplifier board and a laptop brick to build a quality-sounding, powerful yet portable soundbar in the style of top-range, transmission-line speakers.

No calculations were used when designing the speaker enclosures, the transmission line or the bass vent, so your build does not need to be an exact copy and will ultimately depend on what components you were able to salvage. I chose an amplifier board that has parametric bass equalization to achieve a balanced frequency response to compensate for all the vagaries and variances of the design. What I can say is that the result sounded very good!

Supplies

This is mostly a woodworking project, with a bit of electrical wiring and soldering thrown in. Therefore you will need the following:

The usual collection of woodworking tools:

  • Tenon and rip saw, or a chop saw
  • Electric drill
  • Hole saw or jigsaw
  • Mallet & chisel
  • Hot glue gun (will you ever not need one?)
  • Optional lathe if you want to make a fancy-shaped bass vent
  • Some marking and measuring tools

You may also need:

  • Wire strippers
  • Soldering iron

Consumables:

  • 95mm x 18mm / 4" x ⅝" wood of your choice
  • PVA wood glue
  • Screws
  • Solder
  • Hot glue sticks
  • Wood finish of your choice

From the salvage shed:

  • 2 x midrange drivers, try to get the 4 Ohm variety
  • 2 x tweeters, try to get the 4 Ohm variety
  • 2 x 2-way crossover networks, compatible with 4 Ohm drivers
  • 1 x low-frequency, bass speaker.
  • 1 oversize laptop power brick - the bigger, the better. 26VDC maximum.
  • Some high-value capacitors, rated at over 35V, usually salvaged from old amplifiers

From t'Internet (Amazon, ebay, Banggood, et al):

  • TPA3116 Bluetooth 2.1 50+50+100W class D amplifier board.

Step 1: Notes on the Amplifier Board

  • I chose one that has parametric bass equalization to achieve a balanced frequency response since only general acoustic design principles were used when designing the speaker enclosures, the transmission line and the bass vent.
  • The board cost about £15 a while back. Current prices are higher.
  • Make sure your amplifier board supports at least Bluetooth 4.0, and that your laptop or mobile phone does too.
  • I tried to power the board with 30VDC and it tripped out, so stick to a maximum of 26VDC. I only had 19VDC power supplies, which worked fine.
  • The power outputs of the amplifier board are maximum nominal power if you provide it with a 24VDC power supply that can deliver the current when needed. A 19VDC power supply will give you 40Watts maximum output power into 4 Ohm speakers.
  • You can "fatten" the capability of the amplifier by adding a huge capacitive source to the power into the amplifier. There will be less chance of drop-outs that result in nasty, loud snaps when you are driving the beastie at full tilt, and it seems to generally improve the sound quality.

Step 2: Resize Design and Cut Wood

This is where the first bit of creativity is already required: Size the holes in the front panel and bottom panel according to the size of your sound drivers. In fact, you may need to resize the entire design if your drivers are too large.

Here is a SketchUp plan of the design. I deviated from the design and made the entire front from a single piece of wood, and also made a widening funnel effect for each speaker's transmission line. The corners could have been more rounded and the exits should have been flared, since we will know from Acoustics 101 that the shape of the funnel needs to be an exponential curve, right? But compromises have to be made to get real-world production to meet beautiful theory.

I used mainly spruce wood from the local home improvement store and added a few spare pieces of oak.

Attachments

Step 3: The Power Supply

I used the largest laptop power supply that I could find, which is one for an old Dell laptop and weighs in at 135W power output. It kicks out 6.7 Amperes at 19Volts and should not have a problem delivering the peak outputs that the amplifier board advertises. I had also tried using smaller power supplies, but they simply could not deliver the required power, which often resulted in loud and unpleasant snapping sounds.

It is useful to align the brick so that the mains socket-end is flush with the outside back of the enclosure (see photo).

Note about Dell Power Supplies:

  • The lead is a double-shrouded co-axial cable, with the inner core being a digital signal line for connecting to a laptop. Ignore this line.
  • Use only the outer (Ground) shroud and inner (19VDC) shroud

Step 4: Adding Extra Power Capacitors

The power capacitors are added in parallel to the Ground and 19VDC supply lines and there are points on the board to solder this on. I was lucky enough to have been able to rescue a few large capacitors from an old amplifier If you do this, you can check that they work by performing a charge test on them: charge them to 12 VDC and if you short the leads out, the capacitor should make a nice little spark. See the photo for where the best soldering points on this particular amplifier board are: the orange cable is 19VDC, the blue is Ground

I used a cable tie to secure the capacitors to the box.

Oh, and do observe the polarity of these capacitors! Getting it wrong can result in much very stinky magic smoke.

Step 5: The Cross-over Network and Speakers

Screw the cross-over network boards and the speakers in place before glueing the pieces together. This will save you some frustrations later on. Try to form a funnel of increasing width with the baffle boards. (The baffle boards could have had their edges rounded a little better, in hindsight.)

Have all the components ready for wiring in the final assembly step.

Step 6: The Bass Driver Enclosure

The bass driver is downward facing and forces its sound from underneath the whole box. It has a vent on top, which can be embellished with a piece of flared plastic or, as I have done, a lathe-sculpted wood. A hole with rounded edges will also do.

Step 7: The Control Bezel

The controls of the amplifier board are forward-facing. Use the dimensions in the board layout diagram to mark the bezel material out and fabricate the bezel. I used a piece of English oak and polished it with Danish Oil for contrast with the rest of the speaker.

Step 8: General Assembly

You should now be ready to wire all the components together, using the markings on the PCB screw terminals and the board layout diagram as reference. Perform a functional test using your mobile phone's Bluetooth feature, before securing the power supply and the amplifier board into the chassis with hot glue and screws.

Only attach the top cover with hot glue once everything is in place. You should now be ready to wire all the components together, using the markings on the PCB screw terminals and the board layout diagram as reference. Perform a functional test using your mobile phone's Bluetooth feature, before securing the power supply and the amplifier board into the chassis with hot glue and screws.

Only attach the top cover with hot glue once everything is in place.

Step 9: Finished!

Finally, hot glue the bezel to the front and add the control knobs. I made some wooden control knobs, since the plastic knobs that came with the amplifier board looked cheap.

Enjoy!