JFET Study

This is in response to occasional inquiries about suitable JFETs for use in battery powered audio preamps such as my own published designs. It appears that the situation for discrete JFETs is dire. A number of makers appear to be abandoning discrete JFET production, starting with through-hole packages such as the convenient TO-92. The situation appears to be worse outside of the US.

Already, the PN4416 JFET that I use in the HPF-Pre is obsolete. I bought a lifetime supply to ensure myself that I could continue production, and to guarantee myself a predictable range of parameters. I’d like to find alternatives that DIY’ers can still get for building these circuits at home, because the HPF-Pre is supposed to be an open design.

Test Circuit

The test circuit is a simple JFET source follower, powered by a 9-V battery (or two 9-V batteries in series for J105 and J111). To measure, simply plug JFETs into the circuit, and read the source-gate voltage with a voltmeter.

Results

All 100 measurements (10 per JFET) are shown here:

Discussion

It's very interesting that the 2N types have much less spread in their parameters than the J types. I don't know how to interpret this. It looks to me liek a certain number of types (2N5486, J106, J109, and J112) would be OK for the simple JFET buffer. J111 and J105 might come in handy for circuits running on a higher voltage rail, but that need to handle a large input swing, such as a general purpose preamp that can handle "active" electric bass signals. Parts on the low end of the 2N5486 or high end of 5485 spread would be suitable for building an HPF-Pre. These parts are so cheap that you can afford to buy a pile of them and choose ones that have a desired value of VGS using the test circuit shown.

The situation in Europe

From what I can tell, discrete JFETs in through-hole packages are no longer available in Europe, and I can guess that it's going to be the same in Australia. I suspect that the US has more of a legacy of old designs to support, thus more of a reason to supply older package types. You could buy a big bag of JFETs from the US, and share them with your friends. Or, take the plunge and start using surface mount. It's not too hard! I have used small numbers of surface mount parts in prototyping by laying pieces of self adhesive copper tape (left over from an electric bass cavity shielding project of course) on bare circuit board, and soldering the JFET to the board. You only need to succeed once.