My kids love to play music from their phones so i wanted to build them a portable amplifier, a low cost version of the Blueshift, without Bluetooth, and with recycled laptop batteries (2x 18650). As i also had a pair of decent speakers lying around, the biggest expense turned out to be the wood, at around US$7, and the 3 PCBs costing less than US$1 together! (on Taobao..)

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Most of the work was making the wooden box, more on that later, let’s start with the electronics. The heart of the system is an ultra cheap (US$0.12) 2x 3W class D stereo amplifier board based on the PAM8403. It only needs 5V to reach the 3W output, but needs 4Ω speakers; my speakers are 8Ω so i only get max 1.8W per channel according to the datasheet. I was going to power this with a pair of Lithium 18650 batteries that i salvaged from an old laptop, with nominal voltage 3.7V in parallel, so i needed to boost this to at least 5V to use the full power of the amp. Actually the datasheet mentions it is rated to 5.5V so i used a cheap (US$0.26) DC/DC booster and put the output at 5.5V, with a large decoupling capacitor on the output. Below the schematics as i built it.

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For the Lithium charger i used a cheap (US$0.4) model that has an output that can continue to deliver power while charging. Below the prototype playing music from an iPod.

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The Li charger module comes with 2 LEDs on board: a red one to indicate charging, and blue for completed charge. I wanted to see this status so un-soldered the 2 SMD LEDs and wired them to 5mm LEDs. This was a bit messy and i lost one pad on the board; luckily the share the positive so i only needed 3 connections on the board, as below.

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Now it was time to build the box. I got 5mm plywood from my local wood shop, cut to the right dimensions. Unfortunately they couldn’t do the speaker holes. I used 1 inch triangular corner supports with while glue, and drilled the holes for the LEDs, switch, and cables.

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I added some screws on the top and bottom panel for extra strength, and painted the box with a water based varnish to protect it against the tropical humidity of Hong Kong.

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Next i used hot glue to fix the 3 PCBs on the back panel, the batteries on the bottom, and the LEDs in the side panel. Previous attempts to solder 3.5mm jack connectors had gone wrong, so i decide to cut a good quality music cable in half to use it with the 3.5mm jack connector and solder the other end to the 3 inputs of the amp: R, L, GND. I also did not want to try to expose the mini USB connector on the Lithium charger and cut a good quality USB cable in half, so i could plug in the USB connector to any phone charger. I also made a knot in the 2 black cables so they would not pull on the PCB connections.

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For the front panel, i used a laser cutter at DSL to drill the 2x 10cm speaker holes. The first try went completely wrong so i ended up with a slightly different type of wood for my front panel. I also engraved a label, and cut an oval piece for the wire holder on the side panel. Below the unit is charging from a standard USB charger.

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We’re very happy with the result, the volume is quite loud, but the sound is a bit bass heavy, with relatively less treble. Probably due to the speakers, i’m not sure they are full range. I looked into sealed vs ported speaker designs but a ported design (box with opening) would only increase the bass sound so that’s not what i need. The box is not really sealed either, and i may need to glue the front and top panels to get rid of the vibrations at higher volume. Of course it would be better to use more than 5mm thick panels, i may try that for a next project with a more powerful amp.

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