http://www.multiupload.com/SHCZFGJA5H
In the test from yesterday I layed the ring on the other side of a Din A5 cartboard and neared it to the coil, so I definitivly can tell you that it was not the hand, because of the vanishing expotential amplitude of the signal the very closer the ring came.
In the video it's more the hand but it also works with rings only or iron, as example tin-cans (from a half meter so far).
This experimental circuit isn't adjusted to any frequency at all - perhaps this is much better - and it HAS to detect metal at this level if it should find it out in the wild!
I just have some problems to keep the "hum" alive.
If the circuit stands at a very good calibrated position (sometimes mm-work) the static remains and the speaker outputs it all the time, but sometimes I have to touch the FETs gate with my finger until the "field" is up again.
Anyway its possible to get this good audible signal all the time.
Also recognised there is some "phase-shift" involved, sometimes noise raises, sometimes vanishes if metal comes near to coil or even to FET wires. But don't focus too much on this few trafo-windings - they mean nothing. Those thing was just near my hand so I attached it.
Next I have to find out how to calibrate the peak-gain by tunable condenser or variable resistor and how to make it stable and sensitive. If you have some FET left you can built this circuit in 15min. and make your own experiences, verify my discoveries and support the task!