Discrete Instrument Preamp Cable
Having played ukulele and bass in a few ukulele groups, I've noticed that it is common to find ukes and basses with piezo pickups but no pre-amp in the signal chain. Owing to the very low output of a piezo pickup a pre-amp is vital for improving the quality and presence of the instrument's sound. I sought to remedy this for my own instruments by building a simple lightweight pre-amp suitable for a ukulele.
There is a very simple and well known pre-amp design using a single JFET transistor called the Tillman pre-amp. The creator, Don Tillman, has made the plans readily available on his website. It's a simple unassuming common source FET stage that looks and performs somewhat like the first 12AX7 stage in a Fender preamp. The added beauty is that the FET transistor responds not unlike a tube amplifier and the result is a similarly warm pleasant tone. The pre-amp cable is designed to be phantom powered via a XLR connector. In the absence of a suitable mixing desk, a power box is employed.
It is my re-design of the power box that gives this pre-amp some special features. The power box is 9 volt powered and a typical rectangular 9 volt battery can carry some weight. This can become an unnecessary counter balance especially when the instrument is a feather light ukulele. My solution was to employ a small and lightweight 9 volt remote control battery commonly found at Jaycar Electronics (NZ and Australia). This coupled with the fact that most of the pre-amp circuitry is housed inside the actual instrument jack means overall a very light compact product. I also introduced an LED indicator that notifies that the pre-amp is on and operational when plugged in and also gives an approximate indication of battery life.
With such an unusual battery employed in the power box I soon realized that there is no corresponding battery clip. So I came up with a workable solution using the two terminals of a PCB mounted standard 9 volt battery connector at each end of my battery. I also employed some small neodymium magnets at either end of the terminals to hold the battery secure in place. Problem solved.
This project also had some "domestic" aspects to it – use of a clothes iron for making the PCB and a sewing machine to make the velcro and nylon fabric strap holder.
I now use it to great benefit with all my stringed instruments - electric guitars, basses and ukuleles.