i'm not smart enough to build an audio probe!

[JohnForeman]

ok, so i made the stupid simple audio probe and it creates horrible hum when i plug it into the amp.  I've tried two different plugs.  took the .1uf cap off, plugged it into the amp with nothing connected to the tip, all was good.  put the cap on the tip connector, got hum.  put the cap on the sleeve, no hum.  wired it up with the cap on the sleeve (ground side) and got hum.  what on earth am i doing wrong.  i'm obviously not smart enough to be building pedals

[JohnForeman]

update - i'm still stupid.  I cut one end off a patch chord i have, plugged it into the amp, no hum.  as soon as i touch the cap to the tip lead, it hums.
any idea what gives?

[armdnrdy]

If you don't have anything connected to the probe end, and the other end is connected to your amp, that it's like pulling a guitar cord out of your guitar with the amplifier on.

[JohnForeman]

If you don't have anything connected to the probe end, and the other end is connected to your amp, that it's like pulling a guitar cord out of your guitar with the amplifier on.

yup.  and when i do that, my amp doesn't hum.

[karbomusic]

If you don't have anything connected to the probe end, and the other end is connected to your amp, that it's like pulling a guitar cord out of your guitar with the amplifier on.

yup.  and when i do that, my amp doesn't hum.

Regardless, you can't assume it doesn't hum unless you have an actual signal attached and the circuit and closed because they hum without it except in somewhat rare conditions. If it hums when properly connected then maybe the hum is coming from the circuit you are testing LOL (Or you are using speaker or other completely unshielded wire). However, the takeaway here is assuming that it shouldn't hum with a raw unconnected wire is the wrong way to go about this, I promise. Properly hook it up and "probe" with the ground grounded and the positive to the signal source then see what happens.

This is where a small USB AWG (signal generator) + software scope saves the day every time: http://www.digilentinc.com/Products/Detail.cfm?NavPath=2,842,1018&Prod=ANALOG-DISCOVERY&CFID=5585804&CFTOKEN=bf2d43b9a7e2459b-C6C137F0-5056-0201-02D80FCBC9D6DDC3

[armdnrdy]

Build the probe exactly as shown in this drawing:

http://www.diystompboxes.com/pedals/debug.html

Connect the ground clip of the probe to any ground of the stompbox. Input a signal (strummed guitar works fine) into the input of the effect. Make sure that the effect switch is engaged. Touch the open end of the cap to any point of the audio path.