This setup is easy to build and if you buy all new parts will cost about $150. The main cost is the speaker and you may be able to re-use one you have around.
If you need a more complete reference on off-grid party sound systems, check out my other article.
This article is sponsored by Momentum magazine and MonkeyLectric. The article appears in Momentum Issue 45. Here on Instructables i've posted an expanded build section that has tons more inspirational photos annotated with building tips.
Step 1: What you need
- rear bike rack
- mp3 player with belt clip/holder
- small wood screws
- mount from an old bike light
- mp3 player cable and speaker cable
- tupperware box
- 4 bolts around 3" x 5/16" (80 x 8mm) with 8 washers and 4 nuts
- about 2' (70cm) of wood around 1" x 5/8" section (25 x 15mm).
- if you want 2 speakers instead of 1: a piece of plywood to mount them on
- Speaker (1 or 2)
PA and DJ speakers are the best place to look for efficiency. Auto and home speakers are usually poor. Large speakers are nearly always more efficient than small ones - this is one of our real tradeoffs because large speakers also weigh more.
The smallest speaker I recommend is the Gemini RS-308, you can get that at Amazon and other places for $70. It fits into a milk crate and weighs 15 lbs.
For the current project I used a somewhat larger Behringer S1020, I got it from parts-express.com for $100, it weighs about 20 pounds. American Musical has a lot of speaker choices too.
Go MONO! Stereo doesn't make a lot of sense when you are riding around on a bike. Using one larger speaker you get will more sound and less weight than two small ones, and its less work to mount it.
How big a speaker can you fit on a bike rack? The speaker in my project photos has a 10" woofer, most models with a 10" woofer are about this size. The Gemini 8" speaker is quite a bit smaller, it fits into a milk crate that you might already have on your rack.
Hacker Tip: Occasionally I've seen hackers try to use a bare speaker cone with no box to save weight. Do not do this! The box of a speaker is critical to its ability to create sounds. When you get rid of the box you lose most of your sound output.
For amplifiers there is really only one choice: a Tripath-based amp, sometimes called a T-amp, Class-T amp or Digital amp. These ultra-efficient wonders are inexpensive but obscure. T-amps are available in a couple of different sizes. parts-express.com sells the 10 watt DTA-1 for $45 and it includes a built-in battery holder. It runs at full power for 4-6 hours on 8 rechargeable AA's - wow!
On Ebay.com there are several vendors with higher power T-amps. For a 20 watt amp I recommend the TA2020 amp from ebay seller indeed-hi-fi-lab. It's only $20 and is very good quality with durable construction. indeed-hi-fi-lab seems to have 2 variations called the "Tripath TA2020 Mini Cute Class T Amp Amplifier Ipod MP3" and the "NEW Class T Amp TA2020 Amplifier Tripath Chip TA 2020". Rest assured the one I bought is the "mini cute" type, but the other one looks equally well made in their photos.
The larger T-amps don't include a battery holder, you will need to supply an appropriate 12V battery. 12V "SLA" type batteries are very inexpensive, and also easy to find at recycle yards. For a new one try batterymart.com. A 7.0 Ah capacity should last you all day and costs $20. They also have a nice charger for $15. Amazon has these batteries too.
Much more details about speakers and amps I have tested if you need it.