Published 23.4.2013 (built mid-2012)
- Battery powered small and lightweight speaker with long playing time
- 3,5 mm plug and bluetooth for connecting music source
- Mostly existing components used – rather cheap and quick to build
- Audio Pro stage 22, 41Hz AMP32-PS class D amplifier, 12 V 3.2 Ah gel battery
The goal of this project was to build a portable battery powered speaker which could be easily taken for a summer picnic or festival camp to create some party atmosphere. I wanted to build it without using too much time or money since it would be a bit vulnerable to damages in its use. Thus, I ended up using an old speaker, D-class amplifier kit, lead gel battery and bluetooth audio receiver. Basically it was putting several individual components together and building the wiring and mechanics around them. This was not the first boomblaster I built, though. The first one was at least 5 times bigger, heavier and more time-consuming. It could create a party for the whole camp instead of one tent but it was simply too big and heavy to carry around. And to be honest, where this small fella loses in quantity, it performs better in quality.
Rear panel contains switches for power, mute and input selecting. 3,5 mm plug is one input and bluetooth another.
Frame for the project is an old Audio Pro 2-way speaker. I removed speaker cones and connector panel and sawed most of the back wall off. That was replaced with a metal plate with artificial leather around, acting as a panel for switches but also as a hatch to take out the battery or doing service work. Battery is not the most modern and lightest type but it is a robust 12 V 3.2 Ah gel battery which is not very demanding when it comes to operating conditions, charging or discharging. Amplifier is from 41Hz, model AMP32-PS, a tiny D-class stereo (only one channel used here) amplifier working with 12 volts and providing few watts of power with high efficiency. The combination of the battery and the amplifier provide music at full power for 15 hours or more. I have a spare battery to make sure music won’t stop during a weekend trip.
With the old big boomblaster I learned that having a cable plugged from your phone to the boombox is not so handy. Thus, I ordered an audio bluetooth receiver from eBay. It was a small black box with USB and audio out connectors (3,5mm) and also had a small battery inside. I reaped it into pieces, removed the battery and soldered wires to attach it inside the speaker enclosure. To power the receiver from 12 V battery I bought a small car USB-connector which I also reaped into pieces and soldered it into the circuit. I could have used simple 7805 regulator circuit but to achieve better efficiency I wanted to have a switching power supply. However, taking into account the idea of not using too much time for this project (they take lots of time anyway), I didn’t design my own supply circuit but just bought that 5 € car adapter. Unfortunately, it turned out that using the BT has lots of high frequency noise which is most probably coming from the switching power supply. So, I need to fix it later. I guess it still holds that if you want to do good – do it yourself.
However, the bluetooth works and I also included 3,5mm plug for better connectivity options. BT-receiver also draws significant power in the whole system since the amplifier is very efficient. Because of the noise issue the cable also provides better sound.
The meter on the side of the speaker cabin is a battery voltage meter. It is activated by pushing a button on the back panel – to save power. There is no monitoring circuit for low battery voltage so it has to be monitored manually. I also found a protective grill for the woofer cone to prevent it from damaging in more rough use. It already took a bit hit but it is only cosmetic drawback.
This was just a project to get something I needed for summer activities with little effort. Therefore I didn’t start designing everything myself. Maybe one day I will do so if I need a fancier boombox. Below are some photos of the old heavy backpack-model and parts of this small one.