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Forum Home > Tips, Tricks and Tutorials! - Builder's Guides for GuitarPCB Boards! > Sticky: Using an Audio Probe to Debug Builds

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There's a really cool way to debug your circuit besides the look and guess/measure with voltmeter technique.

You take a plain old amp cord, cut off the 1/4" connector. Put a .1uF non polarized capacitor on the hot lead, put the shield/ground to the ground of your circuit. (If you use alligator clips, then the shield clip goes to ground) Now plug the other end into an amp and turn the volume low.

Now you can use the tip of the capacitor to probe around the circuit. You can hear how the circuit works by touching different places in the circuit. Start from the input of your stompbox and work your way through the circuit. Follow the signal path to see where your problem lies.

You can plug your guitar into the circuit and have someone else strum it while you probe OR you can carefully use one hand to thrash a few strings yourself while you probe or you can use a signal generator or a keyboard or a CD player etc.... to send signal into the circuit. You can hear the effect change the tone of the input signal.

This simple technique can alert you to cold solder joints, if you hear audio before a joint, then none right after, it's probably a cold solder joint. You can hear how a transistor is amplifying the audio. There is a lot you can learn by probing around the circuit!


The Above Audio Probe can be Built for about $10 in Radio Shack parts or you can buy one.

A photo essay is now available on this forum that describes how to make an audio probe yourself. CLICK HERE

Here is a thread with info on how to build one from a Guitar Cable:

There are certain points in a circuit where you should be getting signal and by signal, I mean signal that you would test with an Audio Probe. Basically think about it like this. The sound of your guitar goes through your cord tip into the Input Jack and eventually makes it out to the Output Jack into the next cable and at some point to the amp. Basically it follows an Audio Path. Along these points, you should get some sort of signal. Some points the volume may be low (maybe after a coupling resistor or cap) or uneffected, but some points loud (right after a transistor) and effected.

So how does it pass to there? What components could be bad that are "hindering" the signal from getting through?


Generally the audio path will travel between the IN and OUT. It will not travel through +9V and GROUND. So any Cap, Resistor, Diode etc. leading to GROUND or +9V off the IN>OUT path will not be part of the audio path. Now you may get signal at that point that the component hits GROUND or +9V, but it is not part of the signal that will eventually get to OUTPUT of the circuit.

For Example:

Here is the Circuit Audio Path For the GBOM Fire Red Muff.

Download Pics to your Computer and view with you favorite view for an enlarged view!

Here is the Audio Path w/ voltages in a Schematic for the DSOTM fuzz.

The Yellow and Blue lines show the basic audio path. The other components, do other stuff like supply/limit voltage, feed to ground, filter off frequencies, etc, but are not part of audio path. Some of them will let signal pass through, but that is not what you should be testing.


So in the OP's findings, he finds signal is lost after C10, but C10 is not in the audio path, so that is to be expected. So find out where you are losing signal in the audio path and you will at least find the spot that is causing the problem. Now from there it could be a bad component in the path. Or bad voltages. Or wrong orientation, cold joint, solder bridge etc... But at least we know where to focus on.


This Topic also goes Hand in Hand with this one Located here:

May 18, 2011 at 9:42 AM Flag Quote & Reply

Posts: 550

Great post, Barry. I honestly can say an Audio Probe is the single most valuable tool for building pedals outside of a soldering gun. I can the find the problem area on any of my builds within a couple of minutes (The cause of the problem and fixing it can be another story:(). However, just being able to find where you are losing the sound/signal is invaluable.


Build one immediately.

May 18, 2011 at 10:11 AM Flag Quote & Reply

Posts: 655

Very Nice....Ive had to use mine a couple of times....and it was time well spent.  Thanks for this post.  Would you say that this would be a good reason to start to learn how to read or atleast look at the schematic when building a circuit?  I never used to look at the schematics, but when you begin to build more and more pedals, and you run into problems, ideas for modifications, or you just want to expand your knowledge base, schematics actually show the flow of the circuit.  Its like a language with the components interconnected, each conveying some part of the message that will come out at the end.

May 18, 2011 at 1:49 PM Flag Quote & Reply

Posts: 22

I second that, T.  I had to debug an original FZ-1B Maestro  not long ago and had to read the schem to get it working.  I took my time, but I will tell you that my schem reading literally went up 10 fold because of fixing the pedal for a friend.  And he paid me to do it :)


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May 25, 2011 at 12:20 AM Flag Quote & Reply

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This Guide Topic goes Hand in Hand with this one:

June 2, 2011 at 9:50 AM Flag Quote & Reply

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