The gate of J3 will see changes applied thru C3 and its source current will vary. R13 sets the DC bias of J3. C7 couples the AC component to the load. Varying the load resistance will vary the current thru the load. It is much like a transformer. The voltage drop across R13 sets the maximum output so I can use a 10k headset or a 2k headset with the same voltage output. If I increase the load the voltage will drop as the current increases (as in a transformer). I plan to use a 60 Ohm earbud. The earbud is loud enough with 1 mv.
I vary the load from 100 Ohm to 10K Ohm. Once the resistance exceeds 1K Ohm the source resistor draws more (AC) current away from the load.
This sweep varies the load. Time for a build.