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Homemade Audio Probe for debugging DIY pedals

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Homemade Audio Probe for debugging DIY pedals

Postby maz » January 9th, 2020, 2:28 pm

As most of us DIY pedal builders, sometimes we have to improve circuits, make modifications, or try to find bugs, so we have to get something to be able to trace the audio signal. Unfortunately an oscilloscope is not an accessible tool for everyone. Here's a simple homemade audio probe for debugging DIY pedals, which will let you trace the audio signal but will also let you hear it from different parts of the circuit.

All it takes is :

- 1/4 jack
- 100nF capacitor (must be rated over the max voltage you plan to probe, to have peace of mind I used 600V)
- Something to make the probe (I got a useless and cheap DMM probe)
- Aligator clip
- Wire
- A guitar amplifier (any small nor cheap will do)

So here's the simple wiring diagram :


Here's mine :

Get something that will make it easier to probe components in the circuit :

The same kind of jack used in DIY pedals, I got some stereo but mono will do

The capacitor is protected under some tape layers :

That's quite easy :
- Power the pedal (usually 9V)
- Connect your guitar (or a signal generator) into the input of the pedal/circuit as usually
- Connect the aligator clip (from the audio probe tool) to the ground of the circuit
- Connect a normal guitar cable from the output of audio probe tool to an amplifier
- Use the probe to trace/hear any part of the circuit

Nothing should be connected to the output of the pedal/circuit, since we are already using the output from the audio probe tool :)

Also, if you are using your guitar as the input audio signal, it could be useful to have a friend to play the guitar while you probe the circuit.

*** Be careful with the amplifier volume, while you will be probing in the circuit you may notice that some sections are louder than others.

That's a very useful tool, for example : you've just finished a nice Big Muff clone but the sound comes out a little bit too much gated for you, with such a tool you can find which transistor is the culprit. After all, it's all about how it sounds, in the end !! :)
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