Sennheiser Set 820 Instruction Manual page 41

Adjusting the max. volume:
Potentiometer turned fully counterclockwise
Potentiometer turned fully clockwise
Adjusting the bass response:
Potentiometer turned fully counterclockwise
Potentiometer turned fully clockwise
The adjustments can also be made when the receiver is
being used.
When replacing the operating control, make sure the
"projections" on the inside of the operating control point in
the direction of the earbows.
After replacing the operating control, synchronize the
operating control with the volume control by once turning it
fully counterclockwise and then fully clockwise.
18
0 dB
125 dB
attenuation of -25 dB at 100 Hz
attenuation 0 dB