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

General rules and hints to succesfuly bring back that old pedal to life
Post Reply
User avatar
a d m i n
Posts: 4807
Joined: 19 Jun 2007, 16:57
Has thanked: 4265 times
Been thanked: 1977 times

Post by modman »

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!
  1. Take an audio amplifier and turn down the volume (IMPORTANT!).
  2. Attach an aligator clip to the shield (ground) lug of a female jack, with a long enough wire.
  3. Attach a 0.1uF/50V (or about that value) capacitor to the tip (signal) lug of the female jack, with a long enough wire. The capacitor is there to block the direct current going through your amp input.
  4. Connect the alligator clip to the ground of the circuit to be probed.
  5. Connect the female jack to the amplifier's input with a normal guitar cable.
  6. Turn up the amplifier's volume as needed.
  7. You can now probe the circuit either by using the free leg of the capacitor as a test probe or by attaching a proper test probe to the said free leg.
Audio probe (source: http://diy-fever.com/misc/audio-probe/)
Audio probe (source: http://diy-fever.com/misc/audio-probe/)
audio_probe_socket.png (5.67 KiB) Viewed 3684 times
Practical example of audio probe build
Practical example of audio probe build
Notice two 1u capacitors connecting the probe to the T and R pins of the jack to make a stereo output
Notice two 1u capacitors connecting the probe to the T and R pins of the jack to make a stereo output

STEP 1: Bypass mode
Now connect the capacitor lead/test probe 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/test probe 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 you 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.diyguitarist.com/PDF_Files/D ... Tester.pdf
http://www.doitfuzz.com/forum/diy_tutor ... DIY_pedals
https://www.coda-effects.com/2015/07/tr ... uitar.html

User avatar
The G
Grease Monkey
Posts: 559
Joined: 21 Jun 2009, 11:43
Location: Bucharest, Romania
Has thanked: 405 times
Been thanked: 276 times

Post by The G »

A practical example of using an audio probe is this:

Practically one should follow the signal through the circuit, testing all available outputs, which usually are active components outputs - like op amps' or CMOS inverters' outputs, BJTs' emitters, JFETs' drains and so on.

Another example is this attempt at debugging the Mesa Flux Drive:


Post Reply