Hey Marko,
I will take a guess at your answer even though I don't completely know the circuit... Yes, battery negative is always connected and plugging a cable into the 1/4" jack switches on the positive side of the power. Purple wire is traditionally the audio output signal. Red/white is most likely how the battery power gets to the circuit board. So, red and black from the battery, black and red/white power to the active board, purple audio output to the tip of the 1/4" plug. Make sense?
Jimmy J