Introduction: Little Mobile Boombox DIY

About: Electronic Design Freelancer. Technical Author. Hardware βeta Tester. Product Reviewer» International

While I was sitting in my backyard, and listening to my favorite music through my Samsung phone’s built-in speaker, I got an idea spark: why don't build a little Mobile Boombox myself? When I started the build , I just wanted to get a compact Boombox, but found out at once that the construction itself is actually quite fun. To be honest, I already had most of the requisite parts in hand, so the overall experience was definitely pleasurable. How easy can it be? If you're interested in constructing an economical mobile Boombox yourself, then read on!

Step 1: Stereo Audio Electronics

At first, allow me explain what's going to go inside of the device. Since it’s meant to be a self-contained stereo loudspeaker system, finding and using some sort of stereo audio amplifier is a must. Even though there’re numerous cheerful options out there nowadays, my quick pick is a minuscule Class D stereo audio amplifier module. There may be much simpler/serious choices for the core part – the stereo audio power amplifier – but I'd this little Class D stereo audio amplifier module lying on a drawer for months. If I remember correctly, I bought it for a little Bluetooth speaker project but saved for another build, and left inside an old drawer.

The tiny stereo audio module is a PAM8403 based one available for cheap at twice the price. The pretty great 3W Class D stereo audio power amplifier module is designed to drive a small set of loudspeakers with zero external components. The module also has an onboard dual (stereo) potentiometer (volume controller) with a power on/off switch, so all that needed other than the module is just a pair of full range loudspeakers only. Its operating supply voltage is in 2.5 to 5.5VDC scale (5V typical), and the module can deliver guaranteed 3W audio output at 10% THD with 4Ω loudspeaker and 5VDC power supply source.

Step 2: Wireless Stereo Audio Adapter

Rest of the electronics is mostly just about wiring the loudspeakers to the amplifier module and the amplifier to the power source. For the audio, a stereo audio cable with a 3.5mm plug at one end is enough to link it to the audio source – i.e. the headphone socket of a mobile phone or portable music player. However, a wire-free connection will be a welcome option, so it’d be better to interface a cheap stereo bluetooth audio module with the amplifier module. Following is the image of such a 3.7-5VDC (5V typical) stereo bluetooth module marked as MH-M28.

The MH-M28 is a low-power bluetooth module solution that supports the latest bluetooth 4.2 transmission, automatic bluetooth connection mode, and two-channel stereo lossless playback. In an open environment, the bluetooth connection distance can be up to 20 meters. We can run the module from any USB standard 5V power supply or by a 1S (3.7V-4.2V) lithium-ion battery. A stereo headphone can be directly connected to the 3.5mm audio connector of the module, or it can be wired for the audio input of the power amplifier module, the latter is what necessitated here, though. Needless to say, the particular MH-M28 module is not very critical, so you can try any other similar 5V bluetooth wireless stereo audio adapter module for this project. Likewise, when it comes to the power amplifier module there are many options (but be attentive to what you’re going to do)!

Step 3: Battery/Power Supply

For the Boombox power supply, there’re at least two realistic choices: an external USB (5V) power source, or an internal 1S Li-Ion (3.7V-4.2V) battery. The former is more easy and cheap, whereas the latter is much more comfortable and convenient. Following is the proposed circuit diagram of the power supply section that combines both resources – the Li-Ion battery output as the main power supply source and the USB power supply input for ‘tricky’ charging of the Li-Ion battery. Remember to pick a suitable Li-Ion battery (with internal protection circuitry) that has ample capacity, dimensions, price, etc. and is able to deliver what the setup really calls for. The 3.7V-4.2VDC supply output from the Li-Ion battery is actually just above the midway of the potential input supply voltage rail that the audio power amplifier module can run on. However, there’s (hopefully) no chance for a devalued audio performance even at reasonably loud volumes.

The given configuration allows you to charge the internal battery with the help of an external USB power supply (a mobile phone power bank, for example) through the socket X1. The slide switch S1 is the master power on/off switch which extends the battery output voltage (+) to both the bluetooth and audio power amplifier module. The audio amplifier module (thus the loudspeakers) can be powered on or off by means of the on power on/off and volume controller switch (50K stereo potentiometer with switch) soldered onboard.

Even so, you can simplify your build by omitting the external components X1 and D1. And then you can recharge the Li-Ion battery (BAT) through the Micro USB connector soldered on the module. There’s already one diode on board (see the mark in the next image) and it’ll take care of the rest. Remember, then your enclosure must have a suitable window/slot to access the Micro USB port handily. As might be expected, this downward shift has a few limitations!

Step 4: Bluetooth Pairing

Note that after the Boombox is powered up, if the mobile phone searches for bluetooth devices, the Boombox is visible instantly there as MH-M28 (see below). Thereafter it can play the audio files wirelessly if the bluetooth connection is established (paired) successfully.

When the Bluetooth is not connected, the onboard indicator light (blue LED) mounted on the MH-M28 bluetooth module flashes speedily, but remains alighted incase of a valid bluetooth connectivity (it flashes very slowly during playback).

Step 5: Quick Run & Test Report

As usual, below you can see the basic prototype put down on my workbench. It’s intentional because I needed to assemble and disassemble the electronics many times before the final build. At first I did a quick test run to ensure everything is working as expected, and it’s an extremely cheering moment for sure (keep note, the loudspeakers won't perform well in open space).

Step 6: Enclosure Selection Views

A Boombox enclosure (preferably a waterproof one) is hardest to pick/design as there’re a lot of things that affects the overall performance of the loudspeakers. I’ve very limited experience in building wooden enclosures fro audio systems, so in the end I decided to suspend the enclosure design process (see the sample image) and planned to use the box of a pretty cheap (and nifty) PC multimedia speaker system. Since it’s well sealed, water and dust shouldn't be that big of an issue.

A side note on the efficiency of loud speakers. Overall performance of the Boombox depends heavily on the efficiency of its loudspeakers, as well. As far as I know, typical efficiency of a loudspeaker is measured as the volume the loudspeaker can produce to 1 Meter distance using 1W of power (for example 93dB efficiency – bigger is better). One suggestion for further read

Step 7: ​Conclusion

The build might take half a day to prepare the enclosure and half an hour to complete the electronics assembly. The whole thing cost less than $10-15 in parts and equates moderately well against pricey models available online. Admittedly, this little Boombox may have very limited application in comparison to luxurious models but it has a place in my collection and I’m sure in time I’ll find plenty of interesting ideas for expansion of my canonical concept. Good luck creating your own systems.See you again soon. Well, Go forth and Boombox!

Acknowledgment: Special Thanks to