Take a look at http://www.cathodecorner.com/bikeboombox/ showing the system in detail.
The amplifier circuit board is of my own design. It uses a Texas Instruments Class D stereo amplifier chip to provide 15 watts per channel of clean sound with very low battery drain. I designed a circuit board with everything needed for a nice bike system, including a charging port for your MP3 player.
The amplifier board details are http://www.cathodecorner.com/bikeboombox/cdamp/
Step 1: The components
The amplifier circuit board is available from my one-man company Cathode Corner for $100 assembled. you may also download all the design info from my website and build your own if you are good at surface-mount assembly.
The 6" sewer pipe is available from any big plumbing supply house (not Home Depot or Lowe's, unfortunately) but needs to be cut carefully, a task that I describe how to do.
The bike rear rack is available at any decent bike shop. You may already have one on your bike.
A fully assembled CDAMP amplifier board with angle brackets
A battery pack, Li-ion 18650x4 series cells with protection circuit (Batteryspace)
A 14.4V Li-ion charger (Batteryspace)
A 5.5mm/2.1mm DC power plug to fit the amplifier board (Digikey or Mouser)
A pair of Kenwood KFC-1652MRB marine speakers (eBay)
Two each .250 and .187 quick-disconnect crimp terminals to fit the speakers
Three feet of two-conductor speaker wire, 18 to 22 gauge
A rear bicycle rack, standard aluminum type (local bike shop)
Two feet of 3/4" Sch. 40 PVC electrical conduit
18" length of 6" Sch. 40 ABS sewer pipe (big plumbing supply house)
One foot of 3/4" to 1"x1/8" 6061-T6 aluminum bar stock (hardware store)
A dozen #8x3/4" sheet metal screws
Two 8-32x1/2" machine screws, flathead preferred
Two 6-32x3/8" machine screws, nuts and lockwashers or nylock nuts
Electric drill and/or drill press
Fractional drill bit set, up to 3/8"
Countersink, #8 screw head
#1 and #2 Phillips screwdrivers
Sharpie marking pen
Small file, square or flat
(optional) Large miter box or the parts to make one and a crosscut saw
Step 2: Cut the tube
The boom box body is a 16 inch long piece of pipe. You also need to cut two pieces one inch long to be used as reinforcements for the speaker mounting screws.
Step 3: Cut the arcs
Step 4: Glue the arcs in the tube
Place the pipe end-up on a piece of newspaper. Apply ABS cement to the inside of the pipe where one arc will be placed. Use the arc itself to see how long of a cement spot to make. Then apply cement to the outside of an arc. Press the arc into the cement spot on the inside of the pipe, align the edges flush with each other, and apply the clothespin clamp. Align everything again, making sure the clamp is not skewed in our out of the pipe, which will make the arc wander up or down. Use a paper towel or rag to wipe off the excess cement from the joint.
Repeat for the other two arcs on that end. The gap between any two arcs will be about 3/4" to 1" wide.
Let the assembly dry for an hour (longer if it's cold outside). Then remove the clamps, flip the pipe over and cement the other three arcs into the other end of the pipe.
Let the whole thing dry overnight before mounting the speakers.
Step 5: Drill the amplifier holes
Center punch the hole centers onto the tube.
Drill all the holes out to 11/64" first, which is the size of the 8-32 mounting holes at the ends of the row of holes. Then enlarge them all except the two end holes to 1/4", then drill the bigger holes out to 23/64" or 3/8", then the three biggest holes to 13/32".
Countersink the two end holes to fit an 8-32 flathead screw.
File out the double hole with a small file to permit a USB plug to fit through it.
Check that the amplifier board fits in the holes.
Step 6: Drill the speaker holes
Set the tube up on end and place a speaker in one end. Rotate the speaker so that two mounting holes are centered on each reinforcement arc. Center the speaker on the tube and mark the holes with a Sharpie.
Remove the speaker from the tube and start each hole with the drill. Place the speaker back on the tube to double-check the hole positions and correct if necessary. Remove the speaker again and drill the six holes about one inch deep.
Turn the tube over and repeat the above procedure for the other end, being careful to align the holes at one end with those at the other end so that the speakers aren't rotated relative to each other (that would look bad).
Step 7: Cut and drill the conduit pieces
Drill three 11/64" holes in each piece at the marked locations using a vise and drill press if possible, but a handheld drill will work. Drill all the way through the pipe since the other side needs large holes to let the screws through into the pipe.
Turn the pieces over and enlarge the holes on this side to 3/8" diameter. I used a Unibit step drill to save time.
The last photo shows what the finished holes should look like.
Step 8: Mount the conduit pieces on the tube
First, temporarily install a speaker in the end closest to the amplifier holes using two screws. This is the front speaker. The rack's upturned front end will have to clear the speaker, so it has to be there to get the fit right.
Place the tube on the workbench upside-down, that is, with the bottom of the speaker pointing up. The amplifier holes will be about 30 degrees above center. The speaker has a center line molded into the grill to make it easy to align with the center of the rack.
Place the two conduit pieces on top of the tube and hold them in place with the upside-down rack. See the picture to understand what this means. Position the two conduit pieces about 1/4" away from the end of the tube with the speaker grill. This end has the upturned end of the rack also. Check the centering of the rack with the speaker grill to be sure the rack is properly positioned on the tube. While holding everything just so, get out your Sharpie (you had it handy, right?) and mark the tube at each end of the conduit. The important end is the one near the speaker, unlike the photograph.
Now take off the rack and conduit, and hold up one conduit piece at the mark. Using a handheld drill with a 7/64" bit, drill a hole through the conduit hole closest to the speaker. Install a #8x1/2" screw in the hole you just drilled.
Align the conduit carefully with the direction of the tube. Drill a hole in each of the other two conduit holes and install screws in them.
Repeat the procedure for the other piece of conduit.
Hold the rack on the tube-conduit assembly and see how it fits.
Step 9: Mount the rack to the conduit
Cut the bar with a hacksaw, then cut another piece of identical length. File down the saw burrs.
Center punch the hole marks, then drill a 11/64" hole in each position with the two bars stacked on each other to get the holes in the same place. Or drill one bar and transfer the marks to the second bar, then drill it.
Now that the bars are made, use them to hold the rack onto the conduit pieces. Hold the assembly together with one bar in place. Put the bar against the rack's center support rod as shown. Drill 7/64" holes through the bar holes into the conduit, then install screws into these holes. Repeat for the other bar, but it doesn't have to be in any particular position. See photos.
Step 10: Take it apart
Remove the screws that hold in the two rack clamping bars. Pull off the rack.
Remove the screws holding in the speaker and take it out.
Step 11: Install the amplifier
I used some wire from a standard audio speaker kit - 22 gauge zip cord with a black stripe on one wire for polarity identification. I made the black stripe be the minus wire and gave it the .187" terminal.
Strip about 3/16" of wire from the other end of the cables and attach to the speaker screw terminals as shown. Then connect the battery wires - red to positive and black to negative. Finally, mount the amplifier board in the tube using two 8-32 x 1/2" flat head screws.
Put two lengths of double-sided sticky foam tape on the battery pack along both edges of one side, then stick it into place on the bottom of the tube.
Step 12: Install the speakers
Now is a good time to test the system by plugging in a music player, throwing the power switch and seeing if music comes out.
Mount each speaker in its end using the six screws supplied. It's hard work. You can use an electric screwdriver (that's what we call a variable-speed drill with a #2 Phillips driver bit) to make it easier, but be careful as the screw reaches the end of its travel.
Step 13: Finishing up
It's time to mount the music player to the bike's top tube or wherever you want it. There are commercially available devices to do this such as the iConsole, but it's expensive. I use a 2x4" sheet of industrial-strength Velcro, wrapped around the top tube and stuck to the back of my iPod Nano.
A one meter long 3mm to 3mm audio cable is usually just right for connecting the player to the boom box. Use the USB cable supplied with the music player to keep it charged via the USB charging jack.