Notice that the opamp circuit has its inputs grounded so it needs a dual polarity supply.
The circuit was part of an Automatic Volume Control circuit and an opamp rectified the output to make the negative control voltage.
This circuit uses an N-channel FET that needs a negative voltage on its gate to turn it off. If you use a P-channel FET (and reverse the polarity of the capacitors and diodes) then it is turned off by a positive voltage on its gate.
The circuit can have as much gain as you need. Its 100k to 3.3k resistors makes it have a max gain of almost 30. Add a preamp if you want.
EDIT: One capacitor (C3) was backwards.