Hello,
In order to say what resistor you need we need to know three things. 1- Transistor type 2- desired gain 3-source impedance. Since we already know the supply voltage. One thing I will say is the cable fet idea works great with high impedance loads like a guitar anywhere from 8k to 1meg but not well with low impedance of 1k typical of a electret mic capsule. You might want to rethink using this as a means of getting more gain I think it will just increase your noise floor.
It is a 2N5087, gain is 1 or less, source impedance is 3K. Less than one gain requires extra resistor to GND I assume.
Thanks for the info about the FET case. Won't need to try that one for my mic.
My first impression is *why* are you doing this? Most standard mics have a FET in them, and this is sufficient to drive a short cable to a recorder.
But there are a few reasons for putting a transistor "buffer" in between the FET and the mic cable. One is to drive long cables, and possibly low impedance inputs, like transformer input preamps, or any professional preamp for that matter. (eg., the Presonus Firepod has an input impedance of 1.7k or something). Driving a low impedance input from a FET will still work, but you might lose some of the signal (maybe 10dB, I don't know). Another reason is to drive a balanced cable. If you're interested in this, check out the balanced line driver (by Scott Helmke, adapted from the original Scheops design) at the "Micbuilders" Yahoo group. This circuit actually uses Phantom power to supply the FET, so no battery box is needed at all. Finally, another use for buffering might be to drive a transformer (low impedance) to step up the voltage without using a preamp.
Anyway, electronics is like programming: You should think of what you're trying to do before starting. What is the problem right now? How do you plan to fix it? And finally, the kicker: how can you judge/measure that your fix has actually worked!
Richard