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
The original idea was much more complicated than this, but I decided to try this out first as it would be easier for the majority of people who wanted to try it. The addendum (last step) will have my full idea (which I'm DEFINITELY doing =) for those who wish to try a slighty more compicated hack.
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
From what I've seen of USB headphones, the basic control scheme and layout appears fairly identical. The particular USB set I used was an inexpensive Gigaware USB Headset sold at Radio Shack. Earlier models of this headset were complete garbage, so ask the dealer if these are the new revisions or not. The newer ones apeared to address all the customer complaints of the originals, and actually perform very well.
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
This is fairly straightforward. I used a circular file to notch the cover to accomodate the 1/4" jack cord. Take your time and repeatedly test fit the cover until it JUST seats snugly. This will apply pressure to the jack cord to hold it snugly to avoid slippage. For added protection against accidentaly pulling out the 1/4 inch jack, you could alternatively use some hot glue inside around the jack cord to create a makeshift slipage stop (somehting I do quite frequently =).
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
Step 5: My Theory on Why It Works Well
The ADC is designed simply to convert the signal from analogue to digital before passing the data to the USB port. Signal Hum is eliminated because the PC component interference is largely blocked by the PC case. As for the signal quality, I believe that impedence mismatching is not nearly as large an issue with the ADC chip in the headset controller, as it merely takes the signal it recieves and converts it to digital data and into the PC, where digital to analog conversion then takes place, and amplification is applied by the drivers and software in the PC.
Step 6: My Ultimate Plan for This
I want to create a breakout box with a 1/4" jack AND an XLR microphone jack with the +5 volt microphone power being provided by the USB port. I also want to add two stereo jacks into the box for connection of ANY analogue headset in lieu of the USB headset. I plan on incorporating a 1:1 isolation transformer between all teh breakout box connectors to make the signal absolutely the cleanest and interference free as it can possibly be. PLEASE feel free to build on this idea, or post questions or comments!