I am following this tutorial to build a simple JFET pre-amp to connect a guitar to an iphone.

This is the schematic: The schematic I have soldered the components as per the schematic directly onto the 1/4" mono jack for the guitar.

The issue is it only works when I have one finger touching the 3.5mm jack's mic wire/pad and another finger touching the 1/4" tip connection (blue dots on image). Is this an issue with grounding, with the diodes or ...? I am at a loss here.

  • \$\begingroup\$ The JFET is wired to a fixed-voltage supply. That allows NO OUTPUT VARIATION. I suggest you read up on "phantom power" interfaces. \$\endgroup\$ – analogsystemsrf Jun 16 at 12:22
  • 1
    \$\begingroup\$ @analogsystemsrf: The "2.8VDC" is just sloppy notation. The microphone input of an iPhone (or any phone, really) has a small DC voltage on it, but has a series resistor of a couple of k. The way this circuit is drawn, it works on the assumption that there's that series resistance in the phone. \$\endgroup\$ – JRE Jun 17 at 8:15

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