There are times when you are modifying or troubleshooting sound equipment and you wish you could test parts of a circuit to help identify the problem. In these cases an audio probe can come in handy.
An audio probe is basically a musical signal that you can use to provide a signal to any part of some audio equipment, its usage will become apparent in a further How-To that is in the works for modifying an old car stereo for auxiliary-in capability.
The diagram below shows the layout of the audio probe and probably explains as much as a thousand words.
The most important feature is the capacitor as it ensures that a DC current from the audio source cannot occur (music being an AC signal), which will help save your equipment should you place the probe in the wrong place! I have found that the size of the capacitor doesn’t really matter too much (I used a 47uF axial capacitor).
Also, it doesn’t matter which side signal you use (either Right Hand or Left Hand, so long as you use a capacitor, just cover up the used signal so it can’t short circuit).
An improvement upon my design is to attach a crocodile clip to the ground cable to allow easier attachment to the audio equipment although I just wrapped my ground cable around a small bolt and tightened it.
To use the audio probe simply attach the ground cable to the ground of the audio equipment and poke around with the signal end while connecting the 3.5mm jack to an audio source!