Lostgallifreyan wrote in
:

I also wonder if fully balanced feed is needed in either case, dynamic
or electret. If one wire is firmly grounded, and is part of a twisted
pair with the wire that carries the DC feed and the AC signal out, then
it ought to cancel out any incoming HF anyway. It's possible that trying
to make it fully balanced might make it more vulnerable, not less,
because it has no firm ground on either pole. The main thing is to get
the gain up as close to the electret as possible, and that will want
very small parts. Transistor rather than transformer..


Bit more thought on that... If the circit to be fed by this has two grounds,
one for frame, the other for local AF input signals, then instead of CAT5
pairs, use a twisted pair cheap 3mm thick cable with a screen. Screen to
frame ground, twisted pair for AF line and signal ground. That should shunt
HF pickup to ground in the equipment where it is likely already done, and
allow at least a metre of signal line to work well.