***Warning! If you mess this up, you could seriously screw your POD! Use this at your own risk!***
Ok, I didn't create this mod so I take no responsibility in what happens to your POD. If you're not comfortable with doing this sort of thing, DON'T DO IT! I'd hate to see somebody mess up their POD cause of this! Good luck!
I got this from a guy named Rich who sent me the following...
I didn't document the CD jam jack mod but it is a pretty simple mod if you are confident enough to solder to the POD pcb and drill a hole in the case for the jack. There are two opamps that drive the headphone output and you just add two resistors to the board to sum in the external CD input. The easiest way for me to document this is to take my POD apart and snap a digital pic or two.
Here are the pics...
First one shows the pod on its back and the wiring added near the "output level" pot. As you can see I drilled a small hole in the bottom case where it wouldn't interfere with anything. The jack is a standard 1/8 stereo from Radio Shack. You have to add two resistors to the circuit - I used 20K but you could go larger if your input signals are higher levels. 20K is about right for CD line level inputs but you MUST be able to adjust the signal level externally since the CD input bypasses the output level pot. One 20k resistor goes between pin 2 of U13 and the tip (right channel) contact of the jack - thats the orange wire in the picture. The other resistor goes between pin 2 of U18 and the ring (left channel) of the jack, its the yellow wire in the photo. The ground contact of the jack is wired (green wire in photo) to the outermost leg of the output level pot (circuit ground). I secured the resistors to the top of the pot with hot glue, and I hot glued the jack in place too.
Circuit description - U13 and U18 are quad opamps. As I remember two opamps are used to drive each headphone output channel. The 20K resistors sum the external CD input with the POD's internal signals at the opamp inputs. Since the 20K resistors are fixed the gain of the CD input is fixed (larger resistors = less gain) and you need some means of adjusting the CD input level to get the internal/external levels matched. The output level control works normally to set the POD volume level.
It works great ! I plug the line out of my computer into it and play MP3's while I jam away with the POD. It sounds like you are right in the mix. The POD is a great box but it's missing some features, this being one.
Hope it works for you. If you are not familiar with electronics work I wouldn't attempt it because I'd hate to have you blow your POD up trying this. Soldering to the opamps requires a fine tipped soldering iron and a steady hand. I'm an electronics engineer so I have quite a bit of experience hacking devices like this ; )
Regards, Rich Heslip