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7 March 2015

Perfboarding the Piezo Buffer

Time for another update in my Piezo Buffer project. If you've only just stumbled across this blog then you might want to backtrack for a little background:

Dreaming of building a Piezo Buffer
Breadingboarding the Piezo Buffer
How to test a JFET

Having got my circuit prototype working on a breadboard I began to wonder how representative it really was of how the end buffer was going to perform. You may remember me commenting on how noisy the circuit sounded when fed into an amp. Was this noise the result of the breadboard or something else? There was only one way to find out and this involved building another prototype. This time I pulled out some perfboard.

Above is what I used. I see that there are various types of perfboard available, but I chose this one specifically because it doesn't contain any conductive strips. Each hole stands on its own and as such I felt that I could be as freeform as I liked. My hope was/is that if I can get the circuit working to my satisfaction then I will simply box it and project done.

Here's a picture of the breadboard prototype to remind you of the rat's nest that needs to be tamed.

I wasn't sure of the best way of proceeding. What I decided in the end was to set myself a couple of principles to help guide my decisions.

Principle 1 was to avoid using any wire other than the legs of the components themselves. My thinking here is simply that the shorter the  connections, the better.

Principle 2 came from the realisation that Principle 1 wouldn't always be possible. I wouldn't use jumper wire unless I really, really needed to and where I had a choice, I'd jump the ground wire, not the positive signal wire. I hoped that this would leave me with an easier circuit to debug. Yes, I was planning for the worst-case scenario.

I did a little sketching to map out an optimum circuit layout for the perfboard. Using the power of Tiki, I split the board in half. The right hand side is an idea I had for a switchable gain arrangement which I will explain in a later post. The left hand side is the circuit proper, all centered around the JFET component. And for my sanity, I placed each input/out on a separate side.

In retrospect I think that there is a lot to be said for having all the input and output connections made on the same edge. What I'm finding is that I'm constantly flipping the board over in order to admire my handiwork debug. This would be a lot easier if the connections were all on one side.

You'll see from the sketch above that I haven't drawn out all the holes in the perfboard; it's just a guide. My intention was always to allow a little bit of leeway as I fitted the components for real. 

Here's the board fitted. It looks pretty neat.

Note the excessive use of terminal blocks. I know it might seem a bit crazy, but while I'm still experimenting, I want the ability to try "stuff" without having to desolder. If things go according to plan then these will be ripped out towards the end.

I'll call out that I've used a sort of "star ground" method. Where I've been able, I've soldered close grounds together and then you can see with the black jumper wire that they all reach to a common connection at the centre of the board.

Here's the underside of the perfboard.

When I powered the board up I had all the expected problems with bad solder. These were easy to fix. Eventually I got sound...

I was heart-broken to discover that the circuit was just as noisy on the perfboard as it is on the breadboard. NOOOOOOOOO! What on earth could be wrong? WHY ME????

After much thinking I began to wonder if perhaps the noise was due to my driving the JFET with too high a voltage. It was at this point that I dug out my Dying Battery Simulator. Ha ha! I just KNEW I'd get to use it sometime!

What you'll see in the above video is my discovery that reducing the supply voltage did in fact reduce the noise significantly. As it dropped below 8 volts the circuit started to behave itself to the point where I was doing hand-springs down the hall and high-fiving the dog.


I've since figured out the resistance required to achieve the voltage drop and added this to the perfboard.

I'd like to end on a high, but unfortunately, I can't in this instance. Although the noise has been significantly reduced, there is still a quiet but noticeable background hum being produced that is slowly destroying me. My next post will be about debugging. As I write, I'm all out of ideas. Wish me luck...

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