There is different hardware on the market available to convert a passive speaker to an active one. Here I present a variable solution with Bluetooth and the possibility to control it with an MCU.

FYI: If you have an old amp with 30-pin iPod Dock, see ipod-simu

• passive speaker
• amplifier (here: TPA3118)
• power supply unit
• optional: DC-DC decoupler / filter (to eliminate noise of PSU) / LC-filter
• receiver/player (here: BT201 KT1025A)
• optional: MCU

TPA3118 class D mono amp
BT201 module with KT1025A
MeanWell SMU01L-05 DC/DC Wandler, 5V zu 5V, 200mA, 1W

What is different with this module from other common ones: It has RX/TX lines for AT-commands, so it's easy to use an MCU for setup and controls.
It also has iR for a remote control. It can play audio files from Bluetooth, USB, SD-Card, AUX.

Latest(?) (V2.4 / 2020-04-17) Firmware is available here: http://www.szqyvhome.com/DownLoad/18685.html

Put “updata.bfu” in USB/SD card (FAT/FAT32, 32GB max), connect the power, wait ~10s, it will beep a few times on speaker output and stops when update process is done.
“updata.bfu” file location: 20240524154050.zip/99_BT201方案_KT1025A_B_资料_V2.4_20200426/02_用户手册/

https://web.archive.org/web/20240615231859/https://nettips.ru/book/BT201%20Module%20_KT1025A_B_User%20Manual%20_V2.3.pdf
https://elektrotanya.com/kt1025b_bt201_bluetooth_audio_chip_user_manual_v2.4.pdf/download.html
https://www.electronics123.net/amazon/datasheet/FN-BT91%20(BT201)%20Bluetooth%20Audio%20Player%20Module.pdf
https://www.scribd.com/document/857956764/BT201-Module-KT1025A-B-User-Manual-V2-4

example code links:

https://github.com/MooreSound/code
https://github.com/mehdi-adham/BT201
https://github.com/christian-kramer/JieLi-AC690X-Familiarization/issues/4

https://www.diyaudio.com/community/threads/qcc5125-and-qcc3034-qcc3031-programming.374322/
https://github.com/lorf/csr-spi-ftdi/issues/59

The amp needs >=12V, the player 5V. There are different choices as PSU:

For >=12V:

• Use a random AC-DC power supply unit
• Use USB-C PD trigger (module): see USB-C-ify everything

To get 5V:

• 5V module (either switching regulator or LDO): switching introduces noise, an LDO heat an inefficiency
  • use a 12V car cigarette 5V USB adapter
    

If there is noise, smoothen 5V on the player. Options are:

• use 5V DC-DC decoupling module (+ ~5€)
• try if an LC filter is sufficient: #TODO: add schematics
  
• use an LDO to get your 5V