OK. Before we get started, let me say this: I KNOW about impedences in different audio devices (guitars, XLR microphones, etc.), and by electronic standards, this DIY SEEMS as if it SHOULDN'T work very well. It does, however the exact opposite, and works very very well, especially considering the ease of implementation and the quality of sound recording! Perhaps one of you EE types can illuminate on this and my theory as to why it DOES work.
That being said, I present to you the USB Headphone Hack!
Step 1: What We'll Do
Basically, we will be attaching a 1/4" mono jack to the microphone input pads on the controller PCB to allow connection and recording of just about anything that uses that type of jack, including guitars, bases, microphones, etc.
Take a 1/4" female jack cord and cut the wires above the jack, leaving about 1-2". Strip about 1/2" of the covering, twist together the braided shield, then strip about 1/8" of the single wire inside. Tin these with solder if you desire to do so at this stage.
Step 2: Determining the PCB Layout
Use a small jeweler's flat head screwdriver to gently pry the top of the controller box off. It merely is snapped into place in this model.
Inside, we can see where the twisted shield and single wire from the headset mic are attached to the PCB inside the controller.
Step 3: Adapting the Cover for the 1/4" Jack
Step 4: Finished and Testing!
That's it! pretty simple... now you can plug in your guitar or other musical instrument and enjoy MUCH cleaner sound and wider frequency response than plugging directly into your PC's soundcard. I created a couple of MP3 Files that you can right click and download from my website (not very big) that demonstrate the difference in quality. RIght click and save as, and then give these a listen.
Audigy - Analog
USB Hack - Digital