Simple Audio Probe / Signal Injector (AF)

Introduction: Simple Audio Probe / Signal Injector (AF)

In building and troubleshooting your DIY audio circuits there is often need to probe the leads or components for the signal path or / and inject signal to the audio path. The simplest way is to use aligator clips and a capacitor and hook it to an amp or a signal generator, but this contrapment tends to fall apart or even electrocute you, so it is nice to have a simple device, which is already made and doesn't include looking for the appropriate parts every time.

There is a HV warning sign here not because building this device would be dangerous in any way (it is a 5 minute craft) but the application in HV circuits demands extreme caution!

This contrapment is really a no brainer, it is just a capacitor hooked up conventionaly. But I wrote down all the steps to help avoid any headaches.

This is made for the audio frequency (AF) range. Radio frequency range (RF) requires more delicate approach (additional shielding, active circuit) and is covered elsewhere. Also this is a contact probe, not a wireless one.

This is not a scope probe and don't use it as such.

The finished product on the picture was made before this instructional and so differs in some minor points from the process pictures.


  1. capacitor of choice - F value doesn't matter much, it is possible to have some bass cut on the signal, but for the basic troubleshooting this probably is neglectable. If there is need for a better sound quality, calculate the value for coupling. But it is crucial to choose the high enough voltage - this mostly goes for the tube circuit builders, for almost all others a cap, which is so big that it is practical to use is also of a high enough voltage. Cap voltage tells you a safe DC (!) voltage, which the capacitor wil block from entering your sensing /signal generating circuit. As you can see I used an axial capacitor, this is an easy way to use it as a sort of a "sensing pen". You can always use a radial cap and fiddle a little with the shape of the probe. You can even use an old marker (US: sharpie) case to put it in. I did it that way to be able to see the value and also the voltage of the cap all the time. (Those knowing a thing or two about caps probably cringe over my waste of a nice Mallory - I feel it still too; but I didn't have a other one at the time building it). Use non polar cap here.
  2. shielded wire - It can also be an ordinary two wire chord I suppose, but this way you are more safe from "airborne" noise entering your lead. Make it a small diameter - to make this contrapment smaller and more suitable to use. As for the length, think about your use; from the socket on there would be another cable (to the amp or signal generator), so cca. 30 cm of it will probably be enough.
  3. ground clip - there are really two main variants to this; I like to use "scope style" with a very small protected "hook" on the end, it tends to slip less form the element leads, if needed, use same clip also on the signal side; the prevailing approach is the aligator clip - both are on the picture
  4. piece of ordinary wire for grounding - so it is appropriate it is black - on picture there is quite a short piece, feel free to use longer one - approx. 20cm is probably optimal
  5. 1/4' (6.3mm) jack - I use the plastic enclosed option, it is more practical; if it is switched, take care to hook up wire to the correct terminals
  6. heatshrink - is not on the pictures, but is quite essential for this build > to protect from high voltage, to isolate signal from the ground and to prevent connection breakage
  7. (optional) duct tape and cable ties - to make even stronger build


  • soldering iron
  • diagonal cutters
  • isolation stripper or a knife
  • heat gun / lighter

Step 1: Soldering to the Socket

6,3mm sockets (1/4') can be switchable or not. If you use switchable socket take care not to solder wires to the wrong side. Take look at the contacts on the socket. If in doubt, insert a plug and look (or check with the continuity meter (that is voltmeter set on continuity mode)). If you have a stereo socket (tip, ring, sleeve), use tip and ring.

  • strip insulation from the shield mesh (~2 cm), collect it ALL and roll them together (one practically invisible hair of the shield in contact with the inner wire can complicate all the project and has given grey hair during the making and troubleshooting of much more intricate devices ...
  • leave approx. 1cm of inner wire isolated then remove inner insulation
  • solder the shield of the shielded cable to the sleeve of the socket - with the switchable socket it is always a good practice to conect ground also to "off-switchable" contact of the tip (to connect it to ground when nothing inserted)
  • solder the inner wire of the shielded cable to the tip contact
  • check for the mechanical integrity of the solder joints
  • to make the connection even stronger, use the empty jack lugs and cable ties to secure the wire to the socket

Step 2: Soldering to the Ground Clip

There is no special need for the grounding wire or the clip to be black, it is just kind of "common language" in the field ...

  • strip the insulation from the shielded wire just as you did in the previous step
  • solder the black grounding wire to the shield wire
  • put heatshrink over the joint covering it all; shrink it with the gun (or a lighter, or a side (not the tip) of the soldering iron)
  • solder the black grounding wire to the grounding clip - !! (see the red clip cap in the middle right of the picture!) don't forget to put parts of the clip which need to go there on the wire or you will have to desolder the joint, put the part (cover, cap, sheath ...) on and then resolder it (you do it again and again and sometimes even remember in time), then assemble the clip if needed

Step 3: Soldering to the Capacitor & Probe Protection

Capacitors are encased in plastic and many made from kind of plastic foil. So you can damage them with excesive heat. Make your soldering and heat-gun use quick, and don't solder too close to the cap body.

Capacitors, although non-polarized, are better to use in one direction than in the other (also in this application). There are ways to detect that (look it up if interested), but I don't expect much difference, so I haven't done it for this project. If there will be trouble, I will turn it around.

  • put wide enough heatshrink tube on the shielded wire, approx. 2cm length
  • solder the inner wire of the shielded cable to the capacitor lead. Make a strong joint. Don't snip off the remaining lead before, it will help dissipate the soldering heat away from the capacitor body - snip it off after, but not too short, not directly after the solder joint, this way it will provide structural stability to this fragiule part of the tool
  • pull over the heatshrink and heat it, not blowing in the cap body (so my pic is wrong!)
  • I made few rolls of duct tape over all this to make the thing soundly strong and not fall apart or loose contact
  • the other lead of the capacitor will be the probe. In ideal conditions I would not only insulate it, but also shield it, but in AF world there is no need. But it certainly must be insulated! If working with high voltage not to get electrocuted by accident; in any case not to make short circuits or touch the wrong places. Use appropriate heatshrink and heat it in place. Leave only 1mm or so of lead exposed in the end. Shorten the lead to your taste - longer lead means better reach in the circuit, but can be hard to navigate sometimes.

Step 4: Use


This tool has no fuse, no current limiting and no safety switches so you can be electrocuted if not using it correctly and with extreme caution!

The capacitor in it blocks DC (direct current), but passes AC (alternating current), so if you stick it in wall socket there will still be all the mighty 230V AC there to kill you. Mind that. The same goes for other sources of high voltage, eg. power transformer secondaries, amp outputs ...

If you used a lower voltage cap, don't try your luck. Replace it if working on HV circuits.

Those warnings go for the use in HV circuits. If you are a humble pedal DIYer, you are probably laughing already.

Again, this is not a scope probe! Scope is infinitely superior tool to this, but requires some knowledge.



  • connect one end of the guitar cable to the tool socket and the other to the amp input
  • touch the probe end to the different parts of the circuit to determine signal paths, sinks and distortion
    WARNING! - too high of a signal (AC) can still get through and fry your amp, or give you a shock of your life. Don't probe randomly, use the schematic and common sense. Don't touch the wrong spots. Also, signal can be pretty strong on some places, set the amp volume accordingly.

On the picture there is me trying to find a source of strange noise in my hybrid amp.



  • connect one end of the guitar cable to the tool socket and the other to the signal generator or a guitar
  • touch the probe end to the different parts of the circuit to determine signal paths, sinks and distortion
    WARNING! - too high of a signal (AC) can still get through and fry your pickups, or give you a shock of your life. Don't probe randomly, use the schematic and common sense. Don't touch the wrong spots.
  • you sending the signal doesn't mean your project will just "listen", it could be sending AC your way; probing the spot previously is not a bad practice.
  • in the circuit areas where you use sockets / jacks switched to ground (input circuits, effect loops), don't forget to defeat this, otherwise there will be dead silence

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