50 hz mains hum in the speaker output of a Logitech LS21 LS 21 stereo sub-woofer combo.
The amplification circuit of this combo has the following problems:
-poor transformer placement (induces hum into the sub-woofer speaker)
-no voltage regulator in the circuit (mains 50 hz ripple goes amplified to the output of the amplifier circuit)
-case is glued together (had to use an oscillation tool to cut open the case).
the underside of the sub-woofer:
removing the sub speaker shows how close the transformer is placed to the speaker, inducing the 50hz mains hum.
the amplifier PCB. No voltage regulator included.
I un-soldered the rectifier diodes in order to attach my DIY voltage regulator + capacitor PCB.
This is what the audio output of the amplifier looks like when it is off.
When turned on. As you can see the 50hz mains hum is not smoothed out by the capacitors on the amplifier PCB.
So I decided to insert a (7809) 9V voltage regulator and a 25V 2200µF capacitor between the rectifying diodes and the rest of the amplifiers circuit.
The voltage regulator was getting a bit warm so I added an improvised heat sink.
The finished regulator/cap combo.
The backside of the sub-woofer was glued to the case, so I had to use force and an oscillation tool to open it.
The transformer was removed.
The regulator/cap combo attached to the rectifier diodes.
A DC power-in jack attached to the backside. I had to drill a hole in it and use hot glue to fixate the DC jack.
The transformer was mounted to a little piece of ply wood and latter fixated with 2 screws to the underside of my PC table.