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7 March 2015

How to build an audio probe

Yeah, I know it's only a matter of hours since my last post, but it seemed like a good idea to split this mini project out into its own post. The topic is debugging...


You already know that I'm building a Piezo Buffer circuit. It's a simple pre-amplifier and my circuit has a single 2n5457 n-JFET at its core. The journey has been long and painful and I haven't reached the end yet.

In my last post I talked about some challenges I'm having with audio "noise". I managed to eliminate most of it by tuning the DC supply voltage (you can see the extra resistors I've added top left to do this), but I'm still left with a sort of hum that is driving me mad.

My challenge is to come up with ideas for how to figure out what's causing it with a view to fixing it. I'll capture my thoughts as I go...


Having gone through rounds and rounds of checking as much of the circuit as I can using methods described previously, I've convinced myself that everything is where it should be and that all connections are good.

In the process of doing this I've realised that having things like the heavy jacks hanging free is causing me issues. They get caught and tug at the terminal blocks, pulling wires free or breaking soldered joints. My temporary solution is to have it all fitted inside a cardboard box which you can see above.

Back to debugging...

My research has revealed that a common source of unwanted noise can come from the power source, especially if that power source is connected to the Mains.

Here you can see me testing whether I have this problem by temporarily switching power over to a battery. It made no difference - the hum was unaffected. What I think this tells me is that my power supply is not the problem, so I'm crossing this off the list.


Next up is an idea for a project I spotted only yesterday. You'll find various posts around the internet for what is being termed an "Audio Probe".

Put simply it's a means of plugging any part of your circuit into an amplifier so that you can hear the audio signal at any point on the circuit.

The idea is that you trace the signal, checking how components are affecting it. The probe might help you to uncover a bad join; It might help you to uncover a faulty component or one that's not doing what it should, Who knows what else it might help you to uncover!

This sounds like an important debugging tool to add to my kitbag!


The design of the Audio Probe is as simple as the concept itself. I give you two implementations in the picture above.

The top one is the one I've made, though I prefer the idea of the bottom one better.

I cut the end off a guitar lead I'd bought specifically for cutting up into smaller lengths. Having pared back the plastic jacket, I soldered a long breadboard jumper lead to a capacitor and the capacitor to the centre tip wire. I then soldered a crocodile-clip connector wire to the ground. Finally, once I'd checked that it worked, I taped the joins. Job done!

The finesse to this design is to wire the probe to a 1/4" mono jack so that you can reuse existing guitar cables. I will probably find myself doing this in the future when I need to "reclaim" this length of cable.

I used a 0.1uF "104" capacitor for my probe and everything seems fine so far.


Here it is all hung up and waiting for action. I've only tested the input and output terminals on my circuit so far to prove that it works. I'll come back to this soon. That's your lot for now.

Update 8-MAR-2015: I've only just realised that I never explained how you use the audio probe. Do this: plug the probe jack into your amplifier; Connect the crocodile jack to your circuit's ground; and then touch the points on your circuit where audio signal is meant to be with the probe, and you should hear it through your amp.


See this poster from the up-coming Melbourne Ukulele Festival. I love it and I'm asking anyone who will listen to try and get hold of one for me. If you can help, then please do!

Pay special attention to who's sitting proud at the top of the artists' list. Oh yes... the bear-hunter himself Ukulele Russ! Wish I was there!

Hold on I just found out that this poster was designed by Jeanie Hague-Smith and is actually available as one of the pledges in a fundraiser aimed at generating funding for artists at the festival. Damn! I might be out of luck. Still... if there are any knocking about, please grab me one and I will love you forever! ;-)


Finally, I want to bring your attention to "mountain banjos". I saw a pretty special picture earlier this week (below) that has sparked my interest. I'm going to call out a kit that I just bought from Brian Carver at Carver Banjos. Postage was a real killer, but I figured I wanted to offer my support to Brian by buying one of his kits regardless. Expect me to have a go at making a mountain (aka "Appalachian") banjo once I get past my Highwayman banjo build.

I also bought myself a copy of Foxfire 3, a book from the seventies that features a section on mountain banjos. I'll maybe review that for you once I get that far. I can't stop myself at the moment. Soooo many things I want to do... ;-)


Here's that picture I was talking about. It's just too good not to share with you. I'm struggling to track down who made it, but for now I'll give the credit to The Masterpiece Fair in London who put on an exhibition in 2013 called "Folk Art in Harmony" that seems to be where this photo came from. I wish I'd been to that.

Brilliant! I want one! I want to make one! I want to play one! I WANT ONE!


Okay, one last mention before I shut up shop...

Check out the Moran Tinycaster pictured above. This wonderful little electric uke is being built by Jack Moran. He posted this update earlier this week and I must say, this little baby looks like it could do some damage! Ha ha,

Jack says that he's going to paint it and stick those missing frets back in too. Have I ever told you how much I hate fretting?

Looking great Jack. Can't wait to see it finished along with a screeching sound check!


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