Making a Bluetooth Stereo System from Broken Headphones, Old Speakers, and a Cheap Amplifier
Projects are most fun if you can build them from broken or unused parts you have already laying around. In my case, I had some old Bluetooth headphones from 2014 where the plastic headband had broken through. The electronics still worked, but due to the broken headband, the headphones were not usable as such anymore. The headphone model is “August EP650” and the headphones look like this (I forgot to take a picture myself before disassembly):
I decided to disassemble the headphones and take out the electronics, which look like this:
As I also had two old 4-Ohm speakers from a broken stereo system laying around, I decided to pair these with the Bluetooth module from the headphones to create a Bluetooth stereo system. In this post, I’m going to explain the structure of this system.
The device I constructed requires the following components:
- Bluetooth module from broken Bluetooth headphones (including the 220 mAh battery)
- 2x 4 Ohm speaker
- PAM8403 class-D dual channel amplifier board
- TP4056 Lithium-Ion battery charger/protection module
- 18650 Lithium-Ion battery (capacity unknown)
- Toggle switch (to turn the amplifier on and off)
- Button (for On/Off/Play/Pause of Bluetooth module)
- 3x 1k Ohm resistor
- 2x 500 Ohm resistor
- Blue LED (indicates whether the amplifier is on)
- Plastic chassis (a case from an old flashlight)
In the following, I will briefly introduce the most important components:
The heart of the circuit is of course the Bluetooth controller PCB from the broken headphones. The circuit seems to be built around the Qualcomm CSR8645 Bluetooth audio IC and supports Bluetooth 4.0.
I had two old 5 Watt 4-Ohm speakers from an old AEG stereo system (circa 2010) laying around. The stereo system itself broke years ago but the speakers still work fine. Note that this is not a real device from German quality brand AEG but just a cheap Chinese device. AEG went defunct in 1996 and they rent out their brand name to numerous other manufacturers.
Also, note that these speakers look more high-end than they are. It looks like each speaker box contains a woofer and a tweeter. However, the tweeter is completely fake and there is only one speaker driver in each box.
PAM8403 Amplifier Module
The PAM8403 (datasheet) is a 3W class-D audio amplifier IC. Amplifier boards based on this IC can be bought for cheap (1-2 euros) from websites like eBay or AliExpress. The amplifier supports 2 audio channels (left and right) which means that we only need one amplifier board for both speakers. The IC operates at supply voltages between 2.5 and 5.5V, which is good as we can directly power the board from a 3.7V lithium-ion battery.
We have to note that the claim of 3W output power only holds for a supply voltage of 5V. According to the datasheet, the output power is 1.6 Watts at 3.6V, so we can probably expect at most 2W of output power on a lithium-ion battery. This seems way too low when we consider that the speakers are rated 5 Watts each. However, in my tests, the speakers’ volume range was perfectly acceptable.
TP4056 Charger Module
The TP4056 (datasheet) is a standalone lithium-ion battery charger IC which regulates the charging voltage and current and terminates the charging process when the battery is fully charged. There are dirt cheap PCBs on eBay and AliExpress (so-called “TP4056 modules”) that bundle a TP4056 with a DW01A discharge protection IC (datasheet). These modules feature a micro USB port and can handle the safe charging and discharging of a lithium-ion battery.
18650 Lithium-Ion Battery
To power the amplifier, I use a 18650 lithium-ion 3.7V battery which I salvaged from an old laptop. Unfortunately, the capacity is not written on the battery and I also never got to measure it. However, I estimate that it has a capacity of around 1000 mAh which is totally sufficient for this project.
With the components I introduced in the previous section, I designed the following schematics for the Bluetooth stereo system:
The Bluetooth module is connected to the amplifier through its audio outputs. The amplifier gets powered by the TP4056 lithium-ion charging module while the Bluetooth module is powered by its own battery (which was already part of the headphones).
As we can see, there are two voltage dividers. These reduce the voltage level of the Bluetooth module’s left and right audio output by 67%. I first tried directly connecting the Bluetooth module’s audio outputs to the amplifier. However, the output volume was so high that clipping occurred at medium playback volumes (controlled by the sending device). The values of the voltage divider resistors were chosen experimentally and depend on the output volume of the Bluetooth module, the amplifier’s input range, and your personal loudness preferences.
I decided to add a toggle switch in series with the amplifier’s VCC line since the amp draws multiple milliamps, even when no music is played back. This would empty the battery way too fast to be acceptable.
The Bluetooth board from the headphones actually has 5 buttons: The main button (On /Off / Play / Pause) but also two volume buttons and a skip-forward and skip-backward button. However, as it is difficult to solder wires to the buttons on the PCB, I decided to only make the main button available from outside the housing. For me, this isn’t a big restriction since I prefer to control the volume and track from my phone anyways.
Why Two Batteries?
As you can see in the schematics, the Bluetooth board and the amplifier both have their own lithium-ion battery. You surely wonder, why we don’t just use the same battery for both. Initially, this was also my plan. However, I noticed that when the same battery is used for both parts, there is severe high-frequency noise that is clearly audible in the speakers. I identified the Bluetooth module as the cause of this noise: Apparently, the module injects noise into the VCC and ground lines, which is then picked up by the amplifier. This is a common problem with this type of application, and electronics YouTuber “GreatScott!” made an entire video about this topic.
In my case, the easiest solution to this problem is just to use two different batteries: One for the Bluetooth module and one for the amplifier. This way, the power lines for both components are isolated, and thus, no noise can be transferred. Another, probably more professional option would be to use an isolated DC-to-DC converter, but I didn’t have one laying around. Using two batteries is simplified by the fact that the Bluetooth module already comes with its own battery and has a micro USB port for charging. Of course, this brings the inconvenience, that you have to charge two batteries through two different micro USB ports.
I soldered the components together according to my schematics above. To build the voltage divider for the audio channels, I used a small piece of perfboard. After the circuit was constructed, I started to prepare the housing which is the case of an old Maglite flashlight. I created holes for the two micro USB charging ports, the toggle button, the play/pause button, and the power indicator LED. Additionally, I drilled two holes at the back of the housing for the speaker wires. Then I put the components inside the chassis and fixed them with hot glue. Finally, I connected the speakers to the amplifier outputs using screw terminals.
In the end, the result looks like this:
Now the homemade Bluetooth stereo system is complete. Time for a test!
Personally, I’m satisfied with the sound quality. I’m surprised that an amplifier for under 2 euros can provide such good sound. The only thing I have to criticize is that the trebles could be a bit louder. However, this might also be an issue with the speakers, as the tweeters in the speaker boxes are fake.
In this post, I showed you how I built a Bluetooth stereo system from old, broken, and cheap parts. You can use this system with any Bluetooth-capable audio device, most importantly smartphones and computers.