Please help. I am curious about some of the components in this circuit. They are in particular L1 and L2. The circuit description says nothing about them but this, L1 and L1 Diameter: 5mm / Wire: 0.5mm / Spiral: 8. If anybody knows anything about this please inform me. Also, I would like to know which resistor I can swap with a pot to make it a variable gain amp rather than the fixed 20db?? Thanks in advance.