Sennheiser Set SET 820 Instruction Manual page 8

Only use one of the two audio inputs on the transmitter (MIC or LINE).
Connect the power cable to the POWER socket on the transmitter. Plug the
power supply into a household electrical outlet.
Pull the power supply unit out of the electrical outlet when the unit will not
be used for extended periods of time.
Complete freedom of movement
It is not necessary that the transmitter and
stethoset receiver are in a direct line of
sight, so you can move around freely in the
home or outside in the garden. One
transmitter can be used to transmit to
several receivers on the same frequency.
The transmitter's range may decrease due
to the surface/composition of walls.
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