Reuse an Old Phone and Old Speakers As a STEREO

Turn a pair of old speakers and an old smartphone into a stereo installation with radio, mp3 playback podcasts and internet radio, using a few common components that cost less than 5 euros in total!
So we have this collection of 5-10 year old smartphones that are partially functional but impractical: the memory is too small, the Android version is too old and typically the battery capacity is down to a tenth of its original. They are great for music playback though! Many have a built-in FM radio, a built-in mp3 player and if it connects to the home WiFi, it can also play internet radio, podcasts or Spotify. The built-in speaker is tiny, but it can be amplified. If you have a stereo with an input jack or that connects with bluetooth, plugging it in is all it takes. If not, read on!

So our stereo died. But I kept the speakers. I tried the tiny and cheap (<1EUR) amplifier module that are based on the PAM8403 chip. The sound quality is great, and there is more: the amplifier is so small that it doesn’t need its own box: it easily fits on the back of one of the speakers!

The touchscreen interface of the phone is far superior to the lousy user interface that most stereos com with. And local radio doesn’t match quality podcasts that are easily followed with the phone. However, there are a few pitfalls in the use of the PAM8403 module, so here I share my setup that worked well the first failed attempts.

• 2 old speakers, an old smartphone and a USB charger + micro USB cable
• 1 PAM8403 amplifier module
• 1 5x7cm prototype board
• 1 double pot, 10kOhm, with switch plus a knob
• 1 LED + 1 kOhm resistor
  
• 2 resistors of 22Ohm
• 1 electrolytic capacitor of 1000muF
• 2 2-pin screw terminals
• 1 3.5mm stereo jack socket
• 1 micro-usb to DIP adapto
  
• 1 double pot, 10kOhm, with switch plus a knob
• 1 LED + 1 kOhm resistor
• 2 resistors of 220Ohm
• 1 electrolytic capacitor of 1000muF
• 2 2-pin screw terminals
• 1 3.5mm stereo jack socket
• 1 micro-usb to DIP adaptor
• 1 3.5mm male-to-male audio cable

These are all pretty standard components that can be ordered for very little from ebay or Aliexpress. Note that the 3 connectors (screw terminals, stereo jack socket, micro-usb to DIP adaptor ) can be omitted by soldering the speaker cables, the audio cable and the USB cable directly to the board, but the result is a neater stand-alone board with the connectors.
The double pot can be omitted as well, but for volume control you depend on the phone, which is a pain.

The component values are not critical: the pot can be anywhere from 200Ohm to 50kOhm; the 220Ohm resistors can be 1kOhm as well or left out completely. The LED with resistor can be anything that lights up at 5V or be omitted. I used a flashing RGB LED, it gives a very visual indication that the amplifier is on.

Solder the components on the prototype board according to the schematics shown. The picture of my board is a bit messy, since it evolved a bit over time. There is nothing particularly critical. Concerning the double pot, make sure to put the ground on the side where the switch is ‘off’, and the signal on the side where the switch is ‘on’: this way you get minimum volume when the amplifier is switched on, and then it increases as you turn it further.
Once ready, the board doesn’t need a box, it can simply be screwed (or glued) to the back of one of the speakers, just make sure that the knob of the pot comes out on the side or the top.

Play some music on your phone and connect it (or your MP3 player or anything else with headphone output) to the amplifier and switch it on. Turn up the volume and the check the sound quality.
I use two USB power supplies to power the amplifier and the phone separately. When I tried to run them together from a single supply, the noise was horrendous! An alternative solution is to use a double-throw double-pole (6-pin) switch, so that the phone recharges when the amplifier is off.

You may want to set up the phone for optimal use as a music player. I did the following steps for 3 old phones:

• Create a gmail account: the phone will be left unlocked in the house. You don’t want kids or thieves to access your gmail or google drive through this phone!
• Do a factory reset. This erases everything, but keeps the latest Android version. The phone will run much smoother!
• Disable all the unnecessary apps that slow it down
• Disable auto-updates
• Disable screen-lock
• Connect to Wifi Install your favorite music apps (Tune-in radio, BBC iplayer, spotify etc)
• Stick in a micro-sd card with your music collection