Building a Tiny FET Impedance Converter

[JHanko]

I'm pretty sure Paul means to breadboard the circuit with the values on his schematic and measure the source voltage referenced to ground. Try all the fets in the breadboard circuit and any fet that measures 2.8V to 3.5V will be happy in the circuit.

Thanks for explaining that. Is the source the leg connected to the output cap and 4.7K resistor? Should the test be done with the tip of the phone plug grounded?

[Groovenut]

I'm pretty sure Paul means to breadboard the circuit with the values on his schematic and measure the source voltage referenced to ground. Try all the fets in the breadboard circuit and any fet that measures 2.8V to 3.5V will be happy in the circuit.

Thanks for explaining that. Is the source the leg connected to the output cap and 4.7K resistor? Should the test be done with the tip of the phone plug grounded?

Yes the source is connected to the 4k7 and output cap. No, the tip of the phone plug does not need to be grounded. Essentially you'll want to bread board the circuit with all the caps and resistors on the breadboard and pins where the jfet would go. Then use alligator clips to connect the smd jfet to the correct nodes in the circuit and measure the source voltage. Find the one that is in the happy voltage range and use that to build the circuit.

[J0K3RX]

Thank you for the board layout. So, all of the component values that PRR posted would still be correct with the MMBF201?

If you haven't figured it out already the MMBFJ201 is the SMD version of the standard J201... No need to compare datasheets as they will be exactly the same other than one is a TO-92 through hole package and the other is a SO-23 SMD package...
http://www.farnell.com/datasheets/162741.pdf