By Dan Goldwater
A good mobile sound system really adds to any bike event – both for riders and spectators. Today’s project uses high efficiency components to make a convenient 25 pound (10 kilogram) setup that mounts on any rear rack and gives those 150 pound (60 kilograms) sound trailers a run for their money.
Rear bike rack, mp3 player with belt clip/holder, small wood screws, mount from an old bike light, mp3 player cable and speaker cable, amplifier, Tupperware container, four 3" x 5/16" (80 x 8mm) bolts with eight washers and four nuts, length of wood about 1" x 5/8" (25 x 15mm) size.
The key to this project is efficiency. Home and car sound gear is not designed to be efficient or light – it just isn’t a concern in those applications. Low efficiency means a big heavy amp and a car battery. An efficient speaker can produce five times the volume using the same amount of power. An efficient amplifier is twice as good as a standard one, plus it’s far smaller and lighter.
PA and DJ speakers are the best place to look for efficiency, measured as “SPL 1w/1m” in the specs. Every three points means DOUBLE the efficiency: 90 is bad, 95+ is good. Large speakers are usually more efficient. At parts-express.com I got a Behringer S1020 for $100 US, it weighs about 20 pounds (9.1 kilograms). Just one speaker is fine – stereo doesn’t make a lot of sense here.
For amplifiers there is only one choice: the Tripath amp, sometimes called a T-amp or Class-T amp. These ultra-efficient wonders are inexpensive but obscure. parts-express.com sells the DTA-1 for $45 USD and it includes a built-in battery holder. It runs at full power for three to five hours on eight rechargeable AA’s – wow! On eBay there are several vendors with higher power T-amps, search for “TA2020.”
Putting it Together:
We need to get four bolts into the bottom of the speaker to hold it on the rack. Unscrew the grill and woofer from the speaker to get access to the inside. Match up your bike rack to the bottom of the speaker and mark where the four holes will go. Drill the holes and put the bolts in so they stick out the bottom of the speaker. Put some glue into the holes to keep the bolts in place and to waterproof it.
Cut two lengths of wood about 9" (230mm) long. Mark and drill two holes in each to match the bolts in the speaker. These wood blocks clamp the speaker firmly onto the rack. If you have trouble with vibration, wrap your rack with an inner tube.
The fabric box of most PA speakers is waterproof. If you use a home speaker, then spray paint it. The woofer cone on many speakers is made from thick paper; waterproof it by applying a thin coating of water-based urethane wood finish – such as Varathane, available at home improvement stores.
waterproof amp holder: Screw the Tupperware bottom onto the top of the speaker. Drill two or three holes in one side of it for wires. Cut a piece of foam to fit in it and put in the amplifier.
Separate the mounting from your old bike light. Glue or screw your standard mp3 player belt clip onto the bike light mount. Now you have an mp3 player bike mount! Put it on your handlebar or top tube. Route the stereo cable from the holder to the amp.
Connect one channel of the amp to the speaker, add batteries and prepare to rock out!
Lots more build info at instructables.com/group/momentum