Making an audio probe

[robotboy]

OK, I'm convinced I need an audio probe, so I cut the 1/4" adapter off of a crappy radioshack guitar cable I had in my closet. After stripping the external plastic away, I'm left with a stranded copper wire wrapped around a very thin wire that's wrapped with white plastic. Could someone clue me into which one of these goes to ground and which one gets the cap? The debugging page says put the cap on the hot lead, but I'm too unitiated to know which that is   :? .

[Peter Snowberg]

The 'hot' lead is the one in the center while the ground is the shield wire wrapped around it.

You're doing the right thing by adding a probe to your tools. What an excellent tool!

Welcome to the forum too.

[robotboy]

Thanks very much Peter. I'm having waaaay too much fun doing this stuff. I just finished my first "completed" project, and I'm just itching for my next order from small bear to arrive!

[robbiemcm]

Does someone mind telling me... do we earth our pedals through this ground wire or through some other method? To be honest, earthing things is the main thing that doesn't make sense to me in electronics. Not that I know much at all.

[petemoore]

Earth generally equals ground, and that is the bottom lug of your amps power supply cable that plugs into the wall AC supply, also the frame and output jack are grounded.
 When plugging a shielded cable to output jack, the sleeve of the cable plug, and the shielding [the 'wrap' surrounding the signal wire in the center of the cable] are now grounded, the ground SHould be continuous through all the cable/plugsleeves/shielding, so all the boxes [metal ones anyway] are grounded as are your strings and Gnd. points in your guitar.
 If working correctly, this holds everything connected to gnd. at 0V [a good place for it to be to prevent electrical shocks/electrocution]. This Gnd., 0V, is used as voltage reference point in these analog circuits.
 Using 0V as a reference point, most of these circuits...all of the other voltages are referenced from ground, like 0 inches on a ruler...if you're power supply is 9V, it's like a '9 inch' ruler, ground is 0 'inches' [the reference point] V+ would be '9', anything inbetween would be an operating voltage, say 4.5v, which is 1/2 of 9v.
 IF the oa output is 'sitting' at about 1/2V of supply it has room to swing to some higher and lower voltage before running out of room at the power supply rails ['V- or 0V or Gnd.'   /and/    'V+ or 9v or top rail' of neg gnd circuit with 9v supply].
 IF the opamp output is sitting at 8/9ths of supply, it has little or no 'room' to swing any more positive, and would be considered misbiased for general audio amplification applications.
 You should be able to find Gnd. in your amp, all Stompbox circuits, guitar..mixing boards etc.
 When you start measureing voltage between Gnd points that should all be at zero Voltage...it's time to call Houston.

[R.G.]

so I cut the 1/4" adapter off of a crappy radioshack guitar cable I had in my closet.

Too bad about that. The simpler way to do this is to leave the cord alone, and get yourself an enclosed plastic jack. Put the jack on the end of any normal cord, solder the cap on the lug that contacts the tip of the plug (you can see it inside the jack) and a ground wire with clip on to the sleeve connection of a jack. Now you have an audio probe that works with any guitar cord.

Like this
http://geofex.com/FX_images/audioprb.gif

[robotboy]

Yup... I read about the improved audio probe shortly after I snipped the cable. Oh well, at least I got to learn a little about the anatomy of an instrument cable.

[Peter Snowberg]

Knowing how cables are constructed is a very good thing.

That type of cable is called "coaxial cable" or more commonly "coax", because it has two conductors that share the same axis.

Better cables will have a shield made of conductive plastic, surrounded by copper wires while cheap cables have a braided shield with very poor coverage. If the braid isn't tight, it isn't working very well. Lots of good coax is made with an aluminumized plastic shield that has a loose braided shield wrapped around it. This is great cable for TVs, but it doesn't hold up to flexing as well as the conductive plastic type.

[petemoore]

FTR ,,,Unsulation is usually plastic or rubber of some sort always non-conductive
 SHielding...OTOH must be conductive and provides a shield from incoming electrons hitting the SP [signal path] wire and injecting noise, which otherwise would be an antenna.
 Correct me, because sometimes I get those two mixed up when typing fast.

[robbiemcm]

Aha... unfortunately I must say most of what you said about earthing things didn't make sense to me. Lucky it will be a good reference to other people on the forum. But just to clarify... Do I need to earth my stomboxes through use of the metal enclosure, or does it get sent through the cable to my amp and use the power supply?

[James P]

Or you could buy a Velleman signal generator kit for about $7 which has all the components needed and a ready drilled pcb. It would

a) function as your audio probe

b) help further your understanding of how circuits work

c) give even more experience of putting projects together.

Either way RG's probe will work fine (I used mine for quite a while) but with the kit i'm able to generate a 1kh constant sine wave which is great for debugging.

The choice is yours robbie!

James P

[Johnny Guitar]

How important is it to use shielded cable for the probe for you people?

I *finally* built a probe a couple of moths ago, and I felt very clever in that I was able to use the one reamining good clip from an aligator test lead set I have and an old orphaned test probe with a useless tip plug instead of banana. I attached both to a 1/4" plug with the cap inside. The only things new were the cap and the 1/4" plug.

In my inital tests to make sure it was working, I noticed (of course) that it was buzzing. Having never used one I didn't think that was important, but now I'm starting to wonder if I should have used sheilded cable.

John

[Johnny Guitar]

Let me try this again.

Is s/n ever important for an audio probe or is it good enough to just be able to trace a signal through a circuit?

Is my single unsheilded cable audio probe so inferior I should rip it apart and use shielded?

Thanks,
John

[Peter Snowberg]

Don't worry about shields... the probe gets it's major utility from just being able to hear a signal ruinning through the circuit. Less hum is better, but reducing hum is ususally more of a matter of grounding than a foot or two of unshielded wire.

Is your amp grounded with a three prong plug?

[Johnny Guitar]

Don't worry about shields... the probe gets it's major utility from just being able to hear a signal ruinning through the circuit. Less hum is better, but reducing hum is ususally more of a matter of grounding than a foot or two of unshielded wire.

Is your amp grounded with a three prong plug?

Thanks Peter.

For my bench work I usually use my Pignose (on batteries) but will probably start using a Smoky to save space.

Thanks again!
John
(BTW, since we are talking about three prongs and amps, and since I'm in the process of "tunning up" all my amps, I've decided to replace both of the AC cords on each of my Champs since neither has a three prong and I hate getting zapped from them -- I shouldn't have waited so long to do this)

[Peter Snowberg]

No problem.

You might want to try grounding your pignose by connecting it to the ground of an outlet via a paralleled .1µF 400V cap and 1 meg resistor. That should kill the hum.