There were two situations where you could get this type of problem as I recall and they were:- Using a Beosound like Ouverture or BS9000 as a master, the amplifier will be turned on a few seconds before the sound is plugged in through the relay; causing a blop in the speakers if they haven't been turned on for while, say an hour. When Beolab Penta is connected to Beovision AV 9000 or Avant, a plop could be heard in the speakers when the speakers were muted. The solution to the first problem was apparently to:- Keep a continious small load on the big 10000µF electrolytic capacitors 6C1 and 6C2 by means of two diodes and a resistor: On PCB1 at the bottom of the amplifier/speaker. You need to put two IN4002 diodes in series with a 2.7 K ohm resister. Place the end of the string with the anodes of the diodes at the plus (+) of 1C18 which is 22 Volts  Place the end of the string with the cathodes (and resistor) of the diodes at the minus (-) of 1C20 which is -22 Volts. The second issue was resolved by:- The control voltages connecting the speaker signal in Beolab Penta in/out on 22IC2 pins 5/12 are determined by the voltage dividers 22R13 to 22R16. As the control voltages are higher than the supply voltage of 22IC2, a positive DC voltage will occur on the Power Link input. When the Beovision is muted, the speaker signal is pulled to chassis. Thereby a DC loop occurs which results in a plop in the Beolab Penta speakers. The solution is to change 22R13 and 22R15 from 390 Kohms to 470 Kohms.