GTA02 bass fix
Resulting sound is good with 32Ohm headphones (AKG K-55). Possibility of GSM and WiFi interference not yet verified!
Here i'll try to describe my take on rework that should radically improve the ability to use FreeRunner as a portable music player.
This rework is fully based on information from Joerg Reisenweber (e.g. in this mailing list post: http://lists.openmoko.org/pipermail/hardware/2008-September/000552.html , and several others). If you decide to short the original caps, you should also remove R4116, R4117 and place them after your caps.
A known issue is that output decoupling capacitors are too small, only 1uF (4.7uF in v7). One of the possible fixes is to add some decent 100uF C in parallel.
I used two 100uF 10V tantalum caps. They should be connected in parallel to C4110 and C4111. Make sure you connect the plus terminal (marked with a thick line) to the side that goes to U4101.
The can should be lifted very carefully. Use a pin-pointed knife for that and try to lift a little in every point you can reach going in circle. After several rounds the can will be easily dismounted. It can take about 10 minutes, please be patient. You can use a plastic sim-holder for a lever, but be very carefull.
To avoid interference from GSM going into the can, use a piece of aluminium foil isolated from the caps with paper. Be sure to provide electrical connection between the foil and the can. Placing small ferrite bead on every twisted pair just before it enters the can is recommended.
After the rework Bass and Treble regulators in mixer should be placed at the middle as it corresponds to 0db attenuation.