Audio Probe - What do I listen for?

[jcwillow777]

OK - I just made an audio probe as shown by R.G. Keen. If I touch a spot in the circuit and it is working properly will I know because there is no hum? What is the tell tale sign that I have located the problem?

Thanks in advance

[Auke Haarsma]

with a probe you can follow what the circuit does to your signal.
-if you get no sound at a certain point you know there's a problem between that point and the last point you do get sound
-make sure you ground the probe correctly to prevent hum
-you can listen to what certain parts do to the signal. If you have three trannies you can listen to the sound at Q1, Q2 and Q3 and hear the differences.

hth

[jcwillow777]

This is going to sound stupid - but here goes. I am thinking about getting a small practice amp to use just for this. Does it really matter if I use a small solid state amp or one of my larger tube amps? Also, how can I generate sound/music/whatever? from my amp while I am using the input jack for my probe? I only have one input for my amp. I am really new at this, but very interested and excited about learning.
Thanks

[Auke Haarsma]

afaik you can just use your normal amp/ruby w/a.
you use the probe as the output of the pedal. at the input you can connect your guitar (that's what I do) and strum some. There are also schematics for signal generators around here, which you can use. In that cause you don't need to strum the guitar.

I often use my Boss DD-20 delay as a looper and just play a couple of seconds into the dd20, then set feedback to max and connect the output of the dd20 the effect I'm testing.

[petemoore]

  Take a typical circuit with wrapper.
  grounded probe w/cap, you should hear a buzz when the cap is connected to your thumb, clicks and other hash as you connect it to ground or anything else...should be 'live'...you AP is verified as working.
  Connected ground to box [test that box is ground w/DMM], you should hear guitar at input jack tip [if guitar is source and turned up and plugged in etc.] with probe. If not check that input jack tip isn't grounded.
  Next try switch where IJtip connects, then put switch in non bypass mode and listen through switch to the guitar input.
  Test input cap <, then input cap> Sides.
  Well say that cap connects to a transistor base, a buffer stage, the base should sound about the same as the emitter [Jfet would be a Gate/Emitter assigned as input output]. Emitter followers, Jfet source followers, opamp buffers should be about unity gain, 1/1.
  ok the buffer is passing signal, to through another cap, test where this cap connects to a transistor base, a Bipolar gain stage. [If the cap is small you may hear bass rolloff, otherwise you should hear the same signal as the buffer input]...if this gain stage is ok.
  Between the base [input of transistor] and collector [assigned as output on a gain stage], you should hear...an increase of gain, the collector should be an amplified version of the base and sound louder. If not there is a biasing problem with this stage.
  Work on through the circuit testing actives for unity [buffers] and gain [on gain stages], and passives..that they pass what they're intended to pass [a 10uf cap should pass all frequencies related to guitar/bass].
  Another tact is use a metal stick, and perform a 'thumbuzz from output' test. Connect your thumb to the output jack tip, hear a buzz, try getting same buzz through the BP Switch, then thumb to the VC and output cap<, next, the last gain stage should buzz on collector, buzz louder on base, buffer stage should sound same/same...work your way to the input. This does about the same thing, but the source is less of a 'frequency specific' source [your thumb's buzz sound], but I find it's a quick way [I just grab a skinny metal screwdriver] to work through a circuit to find a fault...Do Not Try This On High Voltage Circuits.
 

[jcwillow777]

WOW - THANK YOU!!!! The light has been turned on and it now makes sense. I never thought about plugging my guitar into the input ot the pedal and using the probe from amp to output of the pedal. DUH, like I said, I am new at this. Thanks so much again for shedding some light on my overloaded brain! I can already tell this is going to be an addictive hobby.

[cab42]

I can recommend building an oscillator as input. Only a few parts and you have your hands free for the audio probe.

Try R.G.s Quick and Dirty oscillator. You can find the schematic on his website and there is a layout in the gallery.

Regards

Carsten

[Apehouse]

WOW - THANK YOU!!!! The light has been turned on and it now makes sense. I never thought about plugging my guitar into the input ot the pedal and using the probe from amp to output of the pedal. DUH, like I said, I am new at this. Thanks so much again for shedding some light on my overloaded brain! I can already tell this is going to be an addictive hobby.

i myself usually record a simple blues rhythm or something and loop it, as the input. Fascinating to hear how it changes thru the circuit too. good stuff!

[Cardboard Tube Samurai]

I got sick of strumming whilst probing so these days I use a program like Audacity (freeware) and generate a looping sine wave

[petemoore]

  Blearrooo...bleearroow...strumming notes to try getting enough sustain to have time to...rediculous..or not too bad?
  Some kind of source certainly helps, taping down a key on a toy keyboard set to organ [or some other sustaining setting] is another way to get a constant source.

[John Lyons]

I have my probe set up like this:

Old multimeter probe and wire connected to .1uf cap 600v rating... (for amps as well) and then to a 1meg pot that I can turn down or up depending on the circuit I'm working on.  The wiper goes to a guitar cable that I can plug into my listening amp. I ran a shielded foil wrap connected to the listening amps ground all the way up to about half an inch from the probes tip.
Seems like I always get some buzz with the probe touching any point in the pedal. Even just touching the guitar cable plugged into a guitar will get the guitars signal with a buzz.

Am I missing something? Should the ground be connected to the circuit under test ground?

John

[petemoore]

  The circuit in test needs to be grounded to the amp.
  The probe goes in the hot/tip connection to the amp.
  The probe wire will 'work' without shielding, but to reduce noise for tests I use a shielded cable, one end of the cable to 1/4'' plug to amp [ground/sleeve, signal/tip], the other side de-insulated, the shielding connected to a clip [so I remember to ground the circuit side], the hot shield-ed wire going through a cap for DC blocking/probing signals at circuit points.

[wampcat1]

If you have a delay pedal, simply set it up to loop (if it does) and connect it BEFORE the pedal you are probing. Play a passage, record, loop, and VIOLA! This is also my favorite way to breadboard new circuits.

bw

[jcwillow777]

OK, now for a probably really stupid question. I probed the first resistor after the input jack. The end of the resistor that goes to ground gets no sound, but I do get sound from the other end. Should I get sound from leads of componets that go to ground? I am assuming the answer is yes. Thanks again for all of your input. This has been extremely helpful.

[slacker]

you shouldn't get any sound from the side of a component that's connected to ground. If the other end of the component is in the signal chain then you should get sound at that end.

[GibsonGM]

I use Visual Analyzer 8 for oscilloscope work (ok, on the PC, so it's not super mega awesome, but I like it and it's free!).  There's a function on there that generates sine waves.  I recorded this into Cakewalk at 220, 440 and 1kHz for 5 minutes each.  Burned to a CD.  Now, I can choose whether I want lo or hi A, or 1kHz...I take the output from my CD player, measure the ac voltage and set to about 100mV.  Use this at the input to an effect, it's just like a guitar signal (altho more pure than a real guitar note).  Not great for audio tweaking the ckt for tone or the like, but great for troubleshooting!

When audio probing, ground should = :click: on contact and not too much more.   V supply should = buzzzz.  Anywhere in the signal path, you should hear.....signal.  The cap on the probe is very important, or you'll send DC voltage into your amp      I too use an old Marshall SS practice amp that I don't care about for this work...save the tube amp for playing if you can! 

[petemoore]

 Ground is a reference point. All the circuit voltages are referenced to ground, if you want to kill all signal, connect it to ground.

[GibsonGM]

Sure, unless there's a problem with the ground and it has voltage riding on it, which a nasty noise can tell you...


Of course, we all check our common reference point and power supplies before we audio probe, right?

[boyersdad]

I use Visual Analyzer 8 for oscilloscope work (ok, on the PC, so it's not super mega awesome, but I like it and it's free!)....

How to you hook up to the PC to take measurements? Is there a breakout with protection and probes or something? Or does it just do analysis through the sound card?