The circuit is quite simple. Once you open the earbuds, solder wites to the audio output that goes to the samll speakers and also remove that samll speaker. Also solder wires to the supply of the earbuds and to the push bhutton for turnning on/off the earbuds. Connect the BMS to the battery pack as below and the output from the BMS will go to the charging plug and to the ON/OFF main switch. From the switch we supply 16.8V to the amplifier and to the 5V regulator. Before you connect the regulator output to the earbuds, check with the multimeter if the output is below 5V, otherwise you will burn the earbuds BT receiver. Connect the speakers at the output of the amplifier and add the filters at the input. To these filters, connect the audio signal from the Bluetooth receiver. That's it.