Critical Mass Boom Box

As a supporter of the local Critical Mass Movement I wanted to entertain my fellow cyclists with some music. In order to get some good volume and to support my DIY interest i build my own portable boom box.

What It takes

Everything can be bought second hand. The whole setup should not cost more than 50EUR (except mediaplayer).

  • car battery (12V 45Ah, lasts round 10h in my case)
  • 2 channel car amplifier with cinch input and stereo speaker output (Example Amp)
  • 2 bookshelf speakers (4 Ohm in my case)
  • a bluetooth audio receiver (12V powered) with 3,5 inch output
  • power and audio cables
  • power switch from car supply
  • carrying case (old wine box in my case)
  • a mediaplayer/smartphone with your favourite music 😉


The Boombox is quite heavy (around 20kg) so putting it on the rack of the bycicle is not the best idea. I prefer transporting it in a bike-trailer.


  • The smartphone connects wireless to the bluetooth receiver, which is hooked up to the cinch input of the amplifier via klinke-cinch-cable.
  • The receivers powercord is connected to the “battery” and “ground”-connector of the amplifier in which also the cables from the battery are mounted. Make sure the bluetooth receiver is capable of running 12V power supply from the battery.
  • As already said the battery (take some thicker cables) is connected to the amplifier using “+”-pole to “bat”-socket and “-“-pole to “ground”-socket. With the bluetooth receiver, two cables are then connected to “ground”.
  • I have installed a power switch with one leg connected to the “remote”-socket and the second leg to the “bat”-socket. Please note: to the “bat”-socket 3 wires are connected: “+” from battery, “+” from bluetooth-receiver and one cable from the switch. (There might be no remote control available to your amplifier, you then need to switch of the power cable from the battery itself)
  • Lastly connect the speakers to the speakers-socket of the amplifier.
  • Pro Tip: Make sure you have some fuses ready… you might burn one or two during the installation 😉


I am maintaining a playlist on spotify from which I choose tracks randomly…