What is an audio probe and why do I need one?

Ok, you got your soldering iron and nothing is going to hold you back, but you have no clue where to start or what to build. There were others before you with the same questions... read them first.

What is an audio probe and why do I need one?

Postby modman » 28 Jul 2009, 03:23

JNewby wrote:I built a circuit and although it's powered and all cables tested ok, it is still not sounding the way it should. There is no sound, noise or noisy sounds. I have checked the board by eye but didn't find anything wrong. What now? Help, help!

Take an alligator clip, connect that to the output of the circuit. On the other end you attach a 0.1uF/50V (or about that value) capacitor to protect your amp's speakers.

STEP 1: Bypass mode
Now connect the capacitor lead to the input of the circuit. This way you should be able to hear your guitar coming out of the speaker, because the circuit bypasses the circuit through the alligator clip. If you still hear nothing, there is a problem in the guitar, amp or alligator clips, but you can still make no conclusions about the circuit.
Fix this first. When you hear a clean signal, you can proceed to step two.

STEP 2: Start probing
Take the correct schematic, look for the input and now connect the capacitor lead to circuit after the first component. The sound level may be a bit lower (with resistor) or different (with capacitor) but you should be able to hear your guitar sound coming through. If you do, the circuit is ok up until that point.

STEP 3: Tracing the signal path
From here on follow the signal path through the circuit all the way to the output. Always use the shortest path through the active devices (transistors, ICs). There is no use in probing the 9V rail, but you will quickly learn how 'ground' sounds.

STEP 4: Find the problem
As probe across the circuit, you will encounter a location where the sounds dissapears or does not sound good anymore. If you do, check the surrounding components and connections carefully for mistakes, backwards connected caps or diodes.

References on how to build a permanent audio probe
http://www.geofex.com/FX_images/audioprb.gif
http://www.diyguitarist.com/PDF_Files/D ... Tester.pdf
http://www.diystompboxes.com/pedals/debug.html

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