Let's start with a reminder of what this project is all about. About 2 years ago I blogged about how it can be hard to get a good sound out of piezo pickups and that I had a hunch that this problem could be solved with some clever circuitry.
The nub of the issue stems from something called "Impedance". Electric guitars with your standard wound pickups are typically what you might call high impedance. Piezo transducers tend towards what I've been calling ultra-high impedance.
Most amps and recording equipment is tailored to handle high impedance signal, but not necessarily ultra-high. As such you can find that piezo pickups sound quiet and flat through them. Worse still, as you try to compensate for the poor signal by upping gain, you can end up adding noise. It's often a no-win situation.
Enter the "Piezo Buffer" circuit, which attempts to do some impedance matching and also to provide a little bit of gain before the signal hits your amp or recording studio. You can buy these circuits or you can make your own. I've chosen to make one. Bloody hell it's been a journey of discovery!
Let me introduce you to the King-Z Piezo Buffer...
An explanation for C1 and C1...
These capacitors are there to isolate the signal flowing from IN to OUT from any DC interference coming from the JFET. Again, they're predominantly filtering.
Both are tiny. I'm using ceramic capacitors here.
When it all goes wrong...
Let's be honest and admit that things are going to go wrong. Two years ago I rage-quit this project and shoved everything in a box. I had started out with high hopes, but quickly discovered that a winning smile just wasn't going to cut it.
I hit all the same sorts of issues the second time around... only this time I managed to get through them! In the interests of learning from my mistakes, I'll share some of them here for you.
The biggest issue I've had all along has been noise in the circuit. Every time that I've bread-boarded a circuit I've never got the clean sound I've been looking for. It's a common problem. I've tried all sorts of things. Let's discuss a couple:
Power Supply Noise: You'll hear about the potential for AC noise to make its way into your circuit from the power supply. It's easy to check for this by swapping the wall wart for a battery. I don't think I had this problem. Even so I still think the C3/C4 capacitor filters are a good idea.
Grounding: Grounding is important. I think I subscribe to the school of thought that all grounds within your circuit should converge at one point, but to be honest, I have this sneaking suspicion that a lot of people make too much of this a cause of circuit noise. In my experience it seems that you'll get one of 3 outcomes irrespective of how you do your grounding: 1. No sound at all... the circuit is broken; 2. A godawful buzzing like a laser... things are touching that shouldn't (i.e. a short-circuit); 3. It works. If it works, any noise you hear is most likely coming from somewhere else.
Breadboarding: Breadboards are pretty flaky; It's easy to have bad connections and wires crossing over each other. Housing a circuit properly isn't guaranteed to fix noise issues, but it might go a long way towards it. Give it a go.
Input Noise: There's not a great deal you can do here, but there are a couple of things to check. Long cables are bad. Try short ones while you're building. Try different piezos for comparison. Just like anything else, you could simply be dealing with a bad one, or one that is suffering from shielding problems. Rule this out before you throw in the towel.
Output Noise: Check that your amp isn't introducing the noise. Cut the gain. Make sure that no effects are being added. Cut all EQ. Switch to the clean channel. Use a short cable. Rule this out as a cause.
Shielding: In my opinion poor shielding is the biggest cause of noise in any circuit. You only need to pinch the positive line in from your audio signal with your thumb and finger and hear the noise intensify to know that this is a real problem. The good news is that boxing your circuit will go a long way to solving this problem for you. While you're bread-boarding, switch off plugs in the vicinity that you're not using. Hide your phone. Move your light further away. They'll all be contributing.
You've been very patient to get this far, so I'll reward you with a listen to the King-Z Buffer in action. It's set to about 680k and I've got my Sharkfin homemade uke plugged into it. The line from the pedal is fed into my Line6 Spider IV 15watt guitar amp. It's on the clean channel, a little gain, no EQ, with a little chorus and lots of echo.
The first thing you need to take note of is how quiet it all is while I show you around the set-up, before embarking on a trip of buffered ukulele psychedelia :-)
Noise = nada
Thanks where thanks are due...
I slip in a quick picture of Rod Naylor who wrote a wonderful book on woodcarving that I bought last weekend. I reckon Rod might just become a hero of mine in the not-too distant future ;-)
Back to circuits...
I want to make a few quick call outs before I forget. The first is to Scott Helmke who's Mint Box Buffer circuit has been truly inspirational. This is the design that made me think that I could actually do this. I've pondered it so many times trying to unravel the dark magic. I never did manage to get his circuit working the way he says it should, but that doesn't matter. Thank you Scott for sharing your work.
I also want to thank Petre Tzv Petrov whose circuit I stumbled across towards the end of my quest. It turns out that our two solutions share a lot of the same sort of ideas, and better still, Petre is able to explain his circuit far better than I am able to explain mine. Thank you Petre for sharing your work. It helped me to validate a lot of what I had done and taught me some of the reasons why I did it!
That's your lot. I never mentioned that I've published a new comic. Perhaps I'll do a post on this soon. In the meantime check this out.