The Mint-box Piezo Buffer

A small public service interruption: Here's a chance to give something back
for this free open-source electronics project. This summer (2014) I'll be participating
in the Canary Challenge, a bicycle ride to raise funds for breast cancer research.
Please click HERE to donate. I lost my mother to breast cancer, and I'd
greatly appreciate your help preventing other deaths. Many thanks - Scott.

Piezo pickups are definitely useful, but they have one really serious problem: Impedance matching. Having an impedance mismatch is like having your car in the wrong gear, and piezos really need to see a very high input impedance to sound good. If a piezo has to drive a low impedance the sound will lose a lot of low end and also sound bad in the mids and highs. There are a variety of good "acoustic" preamps available to help with this, including built-in electronics and floor units. Trouble is, they're expensive and usually include EQ and effects you may not need. Not all piezo pickups need this kind of buffer, but I've run across a decent number that do.

Often enough I've done sound for musicians with just this problem, so I finally ended up building a few very simple, very cheap buffer boxes that I could carry in my bag and even sell cheaply if a musician wanted to have one. It runs off a 9 volt battery, uses a single FET (field effect transistor) and a few other parts, and can be mounted inside one of those little tin mint boxes.

Here's the schematic:

Changes: The 20pF capacitor is now 1pF - really just a tiny little value to shunt RF to ground.

For the FET I use a 2N5457, which is a fairly common part. It's the same FET used in the Scott Hampton "J-FET Mic Preamp" kit. The resistors can be 1/8 or 1/4 watt. The input and output capacitors (0.033uF and 0.1uF) should be something good for audio - I use metallized polyester caps. The 20pF capacitor can be just a ceramic disc. The 4.7uF capacitor can be tantalum or electrolytic, doesn't have to be anything fancy.

Courtesy of Lorenzo Neri: If you can't find the 2N5457, a BF245C can be used with the following changes:
R=220K, change to 150 ohms
R=10M, could be increased to 22M.

I'm not going to write too much about the construction, other than to give a few tips.

Perfboard layout

A couple of scans of my usual perfboard layout.

Here I've flipped the component scan over and superimposed it with the bottom. Hopefully this shows the layout more easily.

Suggested parts list

You can buy all these parts from any decent supplier, but here's a list of Mouser part numbers.
140-PF1H303K0.033uF polyester
140-PF1H104K0.1uF polyester
74-199D10V4.7A4.7uF tantalum
140-50N5-200J20pF ceramic
291-4.7M4M7 resistor
291-10M 10M resistor
291-220K220K resistor
291-10K10K resistor
502-112BStereo jack
123-5006-GR9v battery snap

Many thanks to:
Walter Harley for providing the original circuit and helping me tweak it for my upright bass pickup a few years ago.
Dave Latchaw for providing a wealth of DIY microphone inspiration including the use of mint boxes for prototyping.