Today I assembled my first preamp based on a MFP-102
FET using a circuit design I got from the Internet (see URLs listed below) and components I bought from Radio Shack last summer. It buffers a pair of Radio Shack piezo pickups to drive a 15 Amp guitar amplifier I have. It worked first time, but the two piezo pickups are very sensitive in spite of wrapping them in foam. I will be experimenting on how to make the piezo pickups less sensitive and where to best locate them on my CBGs. Here is the circuit design I used with all Radio Shack components:
Here's a photo of my "test" circuitry:
Here are 3 URLs related to the circuit design above: http://www.rason.org/Projects/jfetamp/jfetamp.htm http://www.hawestv.com/amp_projects/fet_preamp/fetpreamp1.htm http://www.hawestv.com/amp_projects/fet_preamp/fetpreamp2.htm#schem...
I have also put together a Radio Shack parts list with current prices. Radio Shack has been slowly downsizing the components selection that they offer over the years, so hopefully they still have all the parts when you get around to building this circuit. You can check by going to the Radio Shack website (google: "Radio Shack" + "components"). Here's that parts list:
I'm told the specs for these FETs vary widely and that you should buy several in case the first couple are out of spec for this application. For out of spec FETs, you can still use them if you change the values for R2 and R3, but that whole topic is way beyond the scope of this posting. If you are new to FETs and the MPF102, you need to be sure to know which lead is which. Here's an image file that shows which lead is which for the MPF102:
Maybe this is a better picture:
If I had a working digital camera, I would take a photo and post it, but my solder side of the PCB makes me feel ashamed (it's so messy - workmanship-wise). I think I need to invest in one of those soldering "third-hand" helpers with the two movable alligator clips and magnifying glass to help see what I'm doing. A photo of the component side along with how the battery is connected would be good to have. Also a second photo of how I temporarily connected the circuit board and components inside my CBG would be helpful. So, I'll see what I can do to come up with a digital camera.
As the purpose of the preamp is to strengthen the signal from the piezo pickups so that you can use a long cable between the CBG and the guitar amp, I shortened the two leads on the piezo pickups (to minimize signal degradation); but I did so perhaps a bit too much, as it is difficult to reposition the pickups in different places on the back of the soundboard and have the preamp circuit in the middle. As I have it rigged right now, the two pickups are mounted on the neck inside the box with the preamp board between them. The 9V transistor radio battery is velcroed to the bottom back side of the cigar box. After the thin foam failed to minimize sensitivity, I added thick foam, and find that that seems to isolate it too much such that I barely get any gain out of my guitar amp (but no fingering noise). So, next time, I'll try wrapping it in 2 layers of thin foam. If that fails, I'll try hot glue, but that will make the installation permanent, so I'll likely hold off on that as long as possible.
Additional experiments will be to use this circuit on different instruments I've built to see how well it works with canjos as well as other CBG configurations. That's why I'd like to keep the circuit "portable". It will take some practice to see how I can get a good "electric sound" out of these instruments. Well, that's all for now.