Michael Black wrote in
news:alpine.LNX.2.02.1407271413320.23911@darkstar. example.org:

An earphone may not offer the same level of sound collection that a
speaker with a larger cone allows, you may have to play with things. I
remember taking cheap dynamic earphones and taking the bit that went in
your ear off, and using that as a contact microphone for various things.


Could be so. When I asswered I overlooked the detail of headphone type. The
little bud types might not be so good, but the small on-ear types that came
out when cheap Walkmans werre new, are a very good candidate, because the
construction of those is almost identical with that of many cheap dynamic
mics, and the sound fidelity is also very good with a diaphram about 0.75''
to 1'' wide. Mylar too, so no degrading with humidity.

I haven't tried to work out the implications of matching impedance for gain,
but these days it is likely easier not to do it, just use a high resistance
input with low noise and high gain. Cheap op-amps that will do it are easily
had. Dynamic mics with transformer matching might work but even if immune to
RF pickup they will catch magnetic fields as if intended to do so!

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..

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.