o--------o------+9V
         |        |
         _        5
         ^        K
         |        |   10uf
         o--10M---O--+|]--o--out
         |        |       |  
  0.15uf |     |--d       |
 0-||-o--0----]| BS170    1
      |  |     |--s       0
      |  |        |       0
      |  1        5       K
      |  0        K       |
      _  M        P       |
      ^  |        o       |
      |  |        t       |
      |  |        |       |
 o----o--o--------o-------o---Gnd

2  1N914 diodes
2  10M resistors 1/4w
1  5.1k resistor 1/4w
1  100k resistor 1/4w
1  5k pot linear
1  0.15uF capacitor 16v min.
1  10uF electrolytic cap. 16v min.
1  IFR510 (Radio Shack), BS170 MOSFET or similar

The wiper of the pot is connected to one end of the pot making it a variable resistor. The lower the value, the higher the gain.
1