Hi Mike,
My primary interest is for hand-held receivers for radio direction-finding.
The circuit I described used a cascode RF amplifier with a pair of 2N2222s
followed by SA602, ceramic filter, SA602, balanced RC audio filter to the
LM386. I get plenty of gain using a 6" loop antenna on 80m, and I can
reduce the gain of the cascode stage when I get close to the transmitter
(within about 10 feet of a 1 watt transmitter.)
The cascode stage drives an untuned transformer feeding the SA602
mixer, but I can't remember what turns ratio I used. You can also
control the gain of the SA602 chips - they did this on the K1 rig and got
about 50dB of AGC if I recall. Remember that the SA602 doesn't have
really great strong-signal handling capability, so you'll want to be able
to reduce the gain on strong signals to avoid overloading the BFO stage.
I used single-ended coupling between the two SA602s with a ceramic
filter. Probably could have used balanced configuration with two filters
but that wouldn't have added much. The second SA602 uses balanced
output to drive the LM386 through an LC network that I found in a
Swedish 80m DF receiver. You may want to increase the gain of the
LM386 a bit - there are a couple of ways to do this. I used a resistor
from pin 1 (?) to ground, which can increase the gain to 70dB, but
in practice I didn't need that much, and the capacitor from pin 1 to pin
8 (from memory - check the data sheet) usually gives enough gain
without causing too much noise or other problems.
Mostly I'm using parts from my junkbox, but then it is VERY well
stocked from hamfests and surplus parts sources over the years. The
major design criteria for me are: wide range RF gain control, so I have
good sensitivity while also being able to take bearings on very strong
signals when I get close to a transmitter; small size, physically rugged,
and easy to operate with one hand, since I use it while running through
the woods; and low power consumption so I can run it on a small
battery. My latest version draws about 10ma at 9V and is built into a
length of 1" PVC pipe. The hardest part is getting the circuit into and
out of the pipe, but it makes a sturdy and cheap housing that is just
about right for handheld use. It is a fairly specialized application, but
most of the circuit components aren't that out of the ordinary. Well,
not everyone listens to 80m on a 6" diameter loop with switchable sense
antenna, or intentionally puts the tuning control where it is hard to
reach. One thing I still want to add is an S-meter output, which will
drive a CD4046 VCO chip to give a tone that changes with signal strength
(so I can take bearings by ear while watching where I am running.) Also
I'm considering some 2m circuits, but haven't been able to spend as
much time playing with them as I would like.
Good luck!