I've researched all manner of potential ways of doing the buffering, but invariably, I keep coming back to one deceptively simple design that has been posted by Scott Helmke.
The solution I talk about in this post owes a lot to Scott’s inspired work. I'm calling it the Mark 1 because I am pretty certain that once I start breadboarding that this will evolve into something else.
As is the case with Scott's, the heart of my buffer is a component called a "2N5457" transistor. It's what's referred to as an "N-Channel General Purpose Amplifier" or more generically an "N-Channel Junction Field Effect Transistor" (JFET). The strength of this particular transistor lies in its ability to handle extremely high impedance input from devices that are unable to supply much current. If you've been paying attention then you'll be pointing at the screen right now saying, "but surely, that's exactly what piezo pickups are!"
Pretty much the whole Piezo Buffer circuit is as it is because of the needs of the 2N5457.
I stumbled across this great little app the other day called iCircuit. The picture above was created in it. It's been a bit of a revelation for me that I can experiment with circuits without actually having to build them for real. Just as well really, because I'm still waiting for parts to arrive in the post. iCircuit is a circuit-building app with a cool simulator mode. You can switch the circuit on and see what happens!
Now, I'm not saying that this App is perfect or even if it is an accurate simulator, but it has allowed me to explore ideas and that's keeping me happy at the moment. I've spent hours and hours playing with it.
Let me give you a quick run-down of the highlights of the Mark 1 Piezo Buffer circuit. I hope to be experimenting with it soon:
The signal generator top left is purely to simulate current generated by a piezo. It won't appear on the finished board.
Rp is a pull-down resistor. This sets the input state. I've read that this should be 10 times the input impedance. I'd like to experiment with a pot here to see what effect different values has on the quality of the sound. In Scott Helmke's design he is using this pull-down resistor to effect a sort of "gain". This is definitely worth trying out.
Rd, Rg, Rs, Cin and Cout are a set of resistors and capacitors used to calibrate the 2N5457. I won't explain the values I am intending to start with just yet (though you can see them on the schematic), Suffice to say that there is some science behind them that I will explain in a later post, once I've had a chance to validate the circuit.
The two capacitors marked 800MHz and 1900MHz is a part of the circuit that I'm not totally sold on yet. I may drop it altogether. What it is meant to do is to provide a degree of protection from RF interference by shunting part of the signal to ground. The issue I have with this is that I don't think that I'll need it and I don't like that it is removing from the signal. Before I commit, I'll experiment with it and see if my ears can tell the difference. Maybe I could make it switchable? This is the one thing in this circuit that I don't think iCircuit is modelling properly, but what do I know?
The only other thing of note is the 9V power supply to run the 2N5457.
I'll call out the grounding issues I've been having with iCircuit. I seem to remember reading that you need to be specific with ground connections to give iCircuit the best chance of simulating the circuit properly. What I've found is what seems like trivial differences (to me at least) in ground connections can have significant difference in how the circuit simulation plays out. What I have above was achieved through some pretty delicate trial and error. I can't wait to find out whether my real circuit needs to be put together with such precision.
Okay, there's the groundwork laid for this project. I'm still a little confused as to how to go about turning the circuit diagram above into a functioning pedal. Let's hope I can figure all that out! Rest assured, you'll get all the highs and lows here on the blog. Until next time...