AVR Freaks

Long story: I have a design that has a 3.5mm audio jack and was meant to interface with dynamic microphones. The requirements changed after the board was designed and has to work with both electret microphones and dynamic microphones. The input signal is coming in from the 3.5 mm jack, to an active filter with gain, and to an audio codec. Unfortunately the audio codec does not have software control of the input microphone level, so I am left to do this in hardware. The problem is that the dynamic microphones are 11-12 mV peak to peak and the electret microphones are 300-500 mV peak to peak. Also the dynamic microphones do not need phantom power whereas the electret need"phantom power. There's also no microcontroller on board.

My solution to this problem was that I was going to put a 2k pullup resistor to the 5V rail and connect that to the tip/ring of the 3.5 mm jack. This will work well for the phantom powered electret and find an optimal gain for the electret microphone with the active filter.

With the dynamic microphone, I was going to have a special cable harness built (which they said was ok) and add a JFET pre amplifier inside the cable, with the dynamic microphone connected to the gate of the amplifier. I was hoping to get a gain equal to the electret microphone (a gain of about 40) so that I could get away with using the one active filter on board.

When I tried adding a JFET, I went out to Radio Shack and purchased the only one they had, the MPF102.

I used this exact schematic as a test, but was only able to achieve a gain of about 1.2 to 1.4 when adjusting the source and drain resistors from the schematic after trying the exact values in the schematic. I did use a 12V supply even though I only have 5V available on board, I figured I could order a JFET off Digikey once I prove that this works.

Short story: Is there any way to get a gain of about 40 from a JFET? Is there a more scientific way to self bias the JFET or is it sort of guess and check (I browsed a bunch of non-engineer tutorials I think). Any help on how to get some nice gain out of this JFET from a dynamic microphone would be greatly appreciated.

Thanks!