Item Type: Module Function / Model M18 M28 M38 Power supply MICRO No Yes Yes Headphone interface No Yes No Built-in amplifier No No Yes (5W+5W) Lithium battery power supply Yes Yes Yes Support USB sound card, free drive No No Yes Memory / volume adjustment Yes No No MUTE mute interface Yes No No Button expansion Yes No No for Bluetooth version For Bluetooth V4.2 Bluetooth protocol support HFPV1.7, A2DPV1.2, AVRCPV1.5, AVCTPV1.2, AVDTPV1.2 Format support Support undecoded WAV / WMA / FLAC / APE / MP3, two-channel stereo output Working voltage 5V / 3.7V-4.2V Not connected Broadcast status Connection operating status 5.5 mA 20 mA Deep standby 3 UA Transmission distance 20 meters (empty or low interference environment) Operating temperature -40 °C to +85 °C Sensitivity -87 dBm MH-M18: Pin Number  1 KEY Button control terminal (4 button functions, additional resistor required) 2 MUTE Mute control terminal (3.3 V high level output when muted, low level output during playback) 3 VCC 5 V positive power supply (3.7 V lithium battery power supply requires a short-circuit diode) 4 GND Negative power supply 5 L Left channel output 6 R Right channel output MH-M28: Pin Number  1 5V 5V positive power supply 2 GND Negative power supply 3 VBAT 3.7-4.2V lithium battery positive 4 L Left channel output 5 R Right channel output 6 AGND Audio ground MH-M38: Pin Number  1 5V 5V positive power supply 2 GND Negative power supply 3 VBAT 3.7-4.2V lithium battery positive 4 L+/L- Left channel output (direct speaker) 5 R+/R- Right channel output (direct speaker) Blue module