Here it is. Very simple. I used an obsolete 40673 dual gate MOSFET. I didn't need both gates so I just soldered them together. You could probably substitute an easily obtained MPF-102 JFET. The capacitors and the coil in in the gate circuit form a parallel LC filter circuit that resonates in the 40 meter ham band. The 1 Megohm resistor maintains a very high impedance for the LC circuit, helping to maintain circuit Q (sharp response). It also drains off any charge that builds up on the gates. For the antenna input I just put 2 turns on the 4.5uH coil -- this too helps maintain LC filter response. The 470 ohm resistor serves as the load and limits quiescent current through the MOSFET. It should pull about 19 milliamps -- with a 9V battery that should give you around 26 hours of listening time between battery changes. That's fine with me -- I don't use this thing that much.
Realize that I'm using this with an RTL-SDR dongle that has been given the familiar modification that allows it to use direct sampling in the HF bands. Some ideas here: