Vista M22: How to Replace the Batteries
The M12 amplifier can use either 2 "AA" batteries or an AC power supply (sold separately):
The battery compartment of the amplifier is located on the left side of the unit.
To replace the batteries:
Remove the battery cover by pushing it down towards the bottom of the amplifier.
Insert the two AA batteries into the amplifier. Be sure to match the + and - symbols inside the compartment.
Replace the battery cover by sliding it up into place.
When the batteries are low, you will hear three beeps through the headset.
You also have the option of powering your amplifier using an AC power supply, which is sold separately