You may want to build this input probe from the first diagram. http://nte01.nteinc.com/nte/NTExRefSemiProd.nsf/$$Search is a good place to go to get equivalent part numbers. Also included are other probes that are not as system safe.
Quote from linuxquestions
I made a little progress....
I started first with using the line in jack. I made a probe with a 1 meg resistor inline. The tip of the stereo plug is the left channel or channel 1 on Xoscope display. So the ring is the right channel or ch. 2 on the display. I only used one channel to start with. I used the Sleve as ground. I found the line jack seems work a lot better "cleaner" than the mic input.
With xoscope running, trigger off, I opened Kmix and selected the red button under "line" and went over and selected the green button above "IGAIN" and set the gain mid way and it started to work. This was after much knob dicking and fiddling around without a clue.
I seemed to have bleed over from the other channel, but I think its just the setting of the scale I used 1/5 and the noise dropped way down. I think being familure with how it works will make it easier to use.
I think its possible to make a 555 timer oscillator maybe with some clipping diodes to create a voltage reference?