I have found Dallas' LP fitle to be a very usefull addition to my
system.
When I first posted a link to his filter I took some hits because some
people think that active is either easeier or better. An active fitler
would
clearly lend it self to easier adjustments. The active circuit at the
link I
posted has a slight advantage for those with radios will less then
ideal IF
filters. The 5KHz, 9KHz, and 10KHz notches will be very usefull. I
built a
set for my DX398 for use on mini-dxpeditions.

For normal adjsutments I like the Tone Tilt that I first experienced in
a Kiwa
MAP SAD.

One simple circuit for which I do not have a link is a "ear protector".
I listen with earphones a lot of the time. My wife is very supporting
but doesn't understand my fascination with utility reception. Nothing
worse then trying to dig out a weak signal, with the AF gain cranked
up,
and a loud static "pop" rolls in. Or even worse, another signal comes
on frequency.

OUCH!

My hearing has suffered enough and have no desire to damage them
any more!

So I built a device that consists of a series resistor, a pair of two
1N400X
wired as a clamp to limit the voltage to a maximum of ~1.4V. Since my
two choices of earhphones are a 40 year old set of Sennheiser HD424, 2K
each side, and a set of Byer's with ~600 Ohms each side. The Sennheiser
are of the "opne air" desing and I can wear them for hours.The Beyers
are
of the clsoe design and I were them when there is too much back ground
noise. They also have a built in boom mic so on those odd occasions I
actually use my ham gear, they are nice for hands free. A foot PTT or
VOX are very usefull.

The earphone protector has a Lo-Z to high Z (8:600 ohms) transformer,
but
it will work great with every radio I have tried it with without
thetransformer.
I use sereis resistors of 470 Ohms, and have a 1K pot across the diodes
to
allow the level to be adjusted. This pot is a screwdriver and even
though the
Beyers and the Sennheisers have over a 2:1 difference, the sound levels
are matched to within a few dB. I can't measure earphone output better
then
about +/- 2dB.

Terry

Ok on the phones. You had mentioned them in your email to me in
September. You had also sent me a diagram of an op-amp with a pair of
emitter followers that I could use to drive the filter. Like I said,
I'm probably going to design the filter to run into a hundred or so
ohms. I'll probably stick an op-amp on the output of the filter to act
as a buffer - although 2k probably wouldn't move the filter response
much. Then of course I'll have to have a baxandall /bandaxall style
tone control so the project grows some. Yes, active filters would
probably be ok and LT has some very nice filter chips that would work
too - and I haven't ruled them out.

More later.
NEO