This is a fixed-bias grounded-Source FET amp. Gate bias is provided by jacking the Source up about 0.6V with the diode. Power is supplied directly by Phantom.
The FET: http://www.fairchildsemi.com/ds/2N/2N5459.pdf
If you look at the Typical curves on page 3, at 0.6V bias the FET will pass a nice 1.5mA. But the 2N5457 is specified for a range of Vgs(off) 0.5V to 6.0V. Biased at 0.6V, the Drain current for any specific 5457 could be anything from 0.4mA to 8mA.
The gain depends on the FET, and also on the mike amp input impedance. Mike amp input impedance tends to be around 2K differential, but I have seen under 1K and up to 12K (you can't go higher with standard Phantom power).
5457 Gm varies a lot with Drain current. Or actually with Vgs(off).... devices with different Vgs(off) have nearly the same Gm if bias is set to give the same current. That suggests a current-source would be better than a fixed bias voltage (but not as simple).
At 0.6V bias, various 5457s will give Gm of 0.2mmho to 4mmho, or effective Source resistance of 5K to 250 ohms.
Picking a 5457 with low Vgd(off), and a mike amp with low (1K) input impedance, voltage gain from gitar to one side of the XLR is 500/5K or about 0.1.
Picking a 5457 with high Vgd(off), and a mike amp with high (2K) input impedance, voltage gain from gitar to one side of the XLR is 1K/250 or about 4.
The average 5457 will give Id about 1.5mA, Gm about 3mmho, Drain resistance about 300 ohms. Gain into one pin of 1K or 2K inputs is about 1.6 to 3.
So it will work with the average 5457, but not with the extreme 5457. A very-low-Vgs(off) part will be very weak.
But I'm not sure it is "safe". Drain-source voltage breakdown
rating for 5457 is 25V. Phantom is 48V (and possibly 52V). We can subtract 0.6V for the diode, whoop-ee. The actual Drain voltage will be less by the drain current flowing in the 6.8K Phantom resistor. For the typical 5457 this is 1.5mA in 6.8K or 10.2V. 48V-0.6V-10.2V= 37.2V across the FET. In real life, most "25V" parts will stand this for a very long time, but there will be some instant-deaths, and possible long-term failures. A low Vgs(off) part will not only have low gain, but could see over 45V in some cases.
If you build it, you might want to select FETs, and you might want to be prepared to replace C4 D1 with a low-voltage current source to stabilize voltages and gain.
Since it has gain (typically 3 to 6), hot guitars may overload mike inputs.
I also suspect it will overload on very hot guitar, though the overload action may be musically interesting.
