Stereo analog input mod for the Sansa Clip+
The Sansa Clip+ features a FM radio chip that is either a Si4702/Si4703 (data sheet) or a RDA5802 (data sheet) depending on device but as the pinouts are identical, the type of chip is irrelevant for the following modification.
The signal path from the FM chip to the main MCU is analog line level and with Rockbox installed it's possible to record up to 16-bit wav 48kHz. Recording is available in the original firmware as well but with slightly lower quality. To record from an external stereo source it's possible to utilize the same internal AD (as used by the FM chip) by cutting the traces carrying the audio signal from the FM chip and connect an input jack there instead. Please note that even though the chip is left intact fully functioning, it won't be possible to listen to radio after theese changes. It would however be possible to retain the functionality by adding a small switch between the radio and the jack signal. Though, this would require some extra work/skills, might introduce noise and is probably not worth the effort (IMO).
Above: The audio signal traces that where cut are marked T1,T2. The wires connected to the input jack where soldered to the points marked S1,S2.
(The original photo was taken from the Rockbox wiki for SansaClip as I couldn't get a decent shot with my cell camera.)
Above: One of two traces cut using a pen knife. As you can see it's quite hard to visually tell if it's a successful cut. One way to verify this is to set the Clip in radio mode and measure that there's no signal reaching S1, S2. I used an oscilloscope and a sharp probe but it should work using a multimeter instead.
Above: The cables attached. Again, hard to tell visually if there's connection/short-circuit. Also the color coding is somewhat reversed as the red cable carries the left signal (should be the right).
Above: The ground/earth cable is connected to the ground of the headphone output. I realized later that this might me a mistake as the headphone ground is not directly shared with the ground plane of the FM chip. Even though it works fine this way, connecting to a point directly connected ground plane of the FM chip might give a better input signal.
Above: The cables are drawn next to the circuit board. To assembly the Clip some minor cuts in bottom case was made.
Above: Battery back together and check that the unit still works.
Above: Maybe not a glamorous solution for mounting the 3.5 mm stereo jack, but at least there's now a working audio input where there previous was none!