Blaupunkt GTw 190 A Manuel D'installation page 14

CONTROLS AN THEIR USE
1 Power indicator
Lights up when power is turned on.
2 Gain control
Use this control to set the desired gain level.
3 Frequency control
Turn this knob to change the frequency during sound reproduction
Set the knob to bass setting of your choice (continuously variable
from 50 Hz to 125 Hz)
4 Phase switch
Use switch to select the phase according to the system.