"Tim Wescott"
But while it was mentioned that the best S/N ratio isn't to be had at a
perfect impedance match, no one said where it _can_ be found.
Amplifiers -- most specifically RF amplifiers, but audio ones, too --
have an optimal impedance for the best noise performance. It varies by
the amplifier, but it's basically the the noise voltage of the amplifier
reflected to the input, divided by the noise current reflected to the
input. If you can present the amplifier with that impedance without
losing any power in the coupling stages, then you're doing pretty good.
** Equal source/load impedance matching is virtually never used in audio -
with the exception of long cable runs where the characteristic impedance of
the cable may be matched with a resistive load at the receiving end to
neutralise the undesired effects of cable inductance and/or capacitance.
With low noise sources (ie mics and other passive transducers) the practice
is to make the load 5 to 10 times the source impedance. This is most easily
done with FET and tube inputs and also BJT stages where local or loop
feedback makes the actual load impedance quite high.
But as Tim said, it is highly desirable to match the "noise impendence" of
the amplifier to the source impedance.
FETs and tubes have optimum noise impedances in the megohms range while BJT
stages can be tailored to give much lower values - down to a few ohms for
MC pickups and ribbon mics.
But, you get back to the fact that you're working at MF, and your signal
is dominated by atmospheric noise, so it doesn't matter so much.
** Not so quickly - a ferrite loop antenna is VERY inefficient so the
noise generated by the loop can dominate over atmospheric sources. A low
noise FET makes an excellent ferrite loop pre-amp as it adds only about 2dB
to the theoretical noise for source impedances in the 10k to 100 kohms
range.
.... Phil