Hi Sam, good to hear from you.
The project is a VLF single frequency receiver, an improved version of
the one at:
http://www.aavso.org/observing/programs/solar/easySID.shtml
About half way down the page is a gif, click on it to see the full
schematic.
I am interested in receiving signals at 17 kilohertz, which are
radiating from a transmitter in the house. The transmitter is low
power, as supplied by the manufacturer, it has a 90 foot range (with
the manufacturers companion (black box) receiver. The transmitting
antennas are 6 inch air coils with hundreds of turns of fine gauge
wire, which is driven by an audio amp chip with the input frequency
fixed at 17 kilohertz.
The manufacturers companion receiver runs on very expensive single
source batteries that don't last long AND it doesn't have enough range
to start with!
I want to use cheap laser pointer batteries, or small nicad button
cells, which are a little pricey initially...but they pay off about
the 4th time you recharge them:>:
I DO NOT think the high input impedance is necessary, but usually
fet's and low input bias are associated with low noise op amps.
Regards,
A