Audio Probe

The audio probe is a great troubleshooting tool.

After a basic multimeter, you must have either an oscilloscope, or an audio probe, or both.

A missed connection or faulty audio stage may be hard to track down with dc voltage checks, or ac voltage checks for that matter. Sometimes, just listening to the circuit is the easiest way to find a problem. This is known as "tracing a signal."

While an oscilloscope is a luxury item for the beginner, the audio probe can be as simple as a 1/4" jack with some wire leads soldered on. See the illustrations below.

Straight wire connections will work in a pinch, but you should be careful poking dc voltages with straight wire. The 1/4" jack gets connection via a 1/4" patch cable to a small amp's input. Large amp works too; it's just overkill for tracing. Either way, keep the volume down in case you probe something LOUD. If using just wire, be aware that probing voltage will result in *pops* in the amps speaker, and may interact with your amps input circuitry... or your amps input circuitry might interact with the stage you are probing!

To free yourself from worry, simply solder a capacitor to your probe wire, and use the stiff lead of the cap (the end you didn't solder) as your new probe. Choose a higher voltage cap for 1) allows you to probe higher voltages, and 2) it will be bigger, making it easier to hold/probe with.

Capacitance value needs to be just large enough to couple the signal audio. For midrange tones (1k, etc), anything in the tens or hundreds of nano Farads range should work.

Example values: 0.01µF (10nF), 0.047µF (47nF), 0.1µF (100nF), 0.47µF (470nF). The input circuit of your amp will determine exact frequency response, but any of these values, or anything close to these values should function for all we require.

My favorite audio probe, if you want a deluxe version, is to use an oscilloscope probe. These can be bought separately from an oscilloscope purchase. The only catch is that they use BNC connectors, which aren't typical items if you mainly use 1/4" phone plugs. A panel mount BNC jack can be bought from almost any of the big electronic retailers. Mount the jack in a box or panel... somewhere convenient for you... and run the "hot" signal to the couple cap (now inside the box), and then to a 1/4" cable, or 1/4" jack mounted inside the box. Below is a photo of an actual box with probe and 1/4" cable attached.

Photo of an actual probe to 1/4" audio probe. The box contains a coupling cap and a 100Ω resistor. The idea with the resistor was just to add a little extra protection, but it is not a necessary component.

The oscilloscope probe will have a little alligator clip attached to it initially. This is optional - if your signal source (the sound you are tracing) has a common ground with your scope adapter box, you won't need the little alligator clip. Simply pull it off the probe, and save it for when you do need it. If you need it, but it isn't long enough, clip another alligator clip to it. (When do you need it short? When observing frequencies well above the audio range. So don't worry about ditching it for audio work.)

I like to have 2 BNC to 1/4" adapter setups, so I can trace and inject signal.

This can be very handy. For instance, often you'll want to poke around a circuit board outside it's enclosure, whether it's an amp, pedal, keyboard, etc. It will often be more convenient to be able to flip the board, or access spots that may be impossible to reach while the board is mounted normally. Sometimes this means the board will need to disconnected from the regular input and output jacks, or perhaps just an input, or just an output. This can be cumbersome, and may hinder your troubleshooting session if you have to first figure out how to inject or trace signal without your trusty 1/4" connections. Sometimes a alligator clip can save the day, but all too often the clip is too big, the connector too small, and you wish there was another way to put signal into your circuit. The scope probe saves the day by providing an easy solution, and it's shielded (low noise) as well, unlike alligator clips or the primitive audio probe.