The advantages of -MT21S compared to -MT21: 1. Added U disk/sound card interface 2. Added encoder for up and down song selection, for pause/play, for AUX/BT/USB audio source switching selection 3. The mid-range adjustment of the stereo part has been added, and combined with the subwoofer + treble to form a full coverage of the low, medium and high sound effects range, compared to the MT21, the increased translucent vocals are prominent. 4. Compared with MT21, the APP function is added. The APP supports changing the bluetooth name, password, prompt tone size, prompt tone switch, sleep aid song, mode switch, sound effect selection, U disk list, etc. 5. The frequency adjustment of the subwoofer is changed to the internal debugging of the board. After debugging, the customer will not change it at will, effectively protecting the speaker and power amplifier. 6. Remove the stereo bass adjustment (many customers feedback that the stereo bass is unnecessary, because it is repeated with the subwoofer), and change it to the midrange to make the sound more transparent. color:Black Material:plastic size:93 x 96 x 22mm Package Contents: 1 x -MT21S Power Amplifier Board Only the above package content, other products are not included. Note: Light and different displays may cause the color of the item in the picture a little different from the real thing. The measurement allowed error is +/- 1-3cm.