Besides the hum (see noise filter mod), the GBA often suffers from a little high pitched whine. This annoying tone can be almost eliminated with a grounded shield near the audio amp.
This can be done internally and externally.
The easy way:
(Foil sticked flat on the case with some adhesive tape.)
If this works for you, you may consider the internal version:
Cut the foil into a square of about 3x3 cm with an appendix of 8 cm as shown here.
Insulate the square with a slightly bigger adhesive label.
Open the GBA and fold the PCB up.
Put the square on the PCB under the speaker and hold it in place with some tape.
Carefully flip back the PCB and fold the appendix up.
Put some insulation on the PCB, fold the appendix towards the screw hole marked with the arrow. Fix it with some tape so that the foil is right at the hole.
Fix the PCB by tightening the screws again. The screw in the hole marked red goes through the foil.
Before closing the case again, you may apply the noise filter mod to minimize hum, too: http://chipmusic.org/forums/topic/19565 olderless/
Last edited by oliver (April 12, 2017 2:42 pm)