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From: pault <pau...@gmail.com>
Newsgroups: sci.electronics.design
Subject: Re: 5kHz receiver
Date: Tue, 24 Apr 2012 04:18:10 -0700 (PDT)
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5khz is in the VLF band (also in the human hearable audio band)

This looks like a suitable VLF receiver circuit:

(Magnetic Pickup) at
http://www.spiro.fisica.unipd.it/~mbarbisa/espfis/VLFwhistle.htm#Magnetic

you probably dont need the second op amp and should feed the output
into
a tone decoder pll like the lm567

an even simpler circuit on the same page might work too
(Super-Tiny VLF Receiver)
http://www.spiro.fisica.unipd.it/~mbarbisa/espfis/VLFwhistle.htm#Super-Tiny

see http://www.scary-terry.com/more_stuff/tonedet/tonedet.htm
for an example lm567 circuit