Steve's Mods for PA0RDT Audio Filter

On ndblist, Roelof Bakker, PA0RDT, modified the filter originally done by
PA0LQ, for narrower bandwidth. That's the version I and most others are
using. Note you'll need an audio amp after it, as these opamps won't drive
headphones. I make my own version that uses opamps that will drive
headphones directly. I've attached Roelof's drawing that was sent to ndblist
last year. http://we0h.us/Amateur_Radio_stuff/paolq_lf_filter_roelof_mods.pdf

Note that you'll need a precision audio generator, and some means of
monitoring the output. Each of the 4 pots is tuned to the center frequency.
I match each pair of 0.027uF caps, to the 4th digit on my cap meter, and
pick the 4 pairs of caps as close as I can to each other. Otherwise the
bandwidth will be wider. Bandwidth depends somewhat on center frequency,
too, with higher pitches giving wider bandwidth. For my 440 Hz pitch, my
filter is about 6 Hz bandwidth at the -3dB points. It does not ring. I use
my HP3586C SLM as the precision generator, which makes an ideal generator
with its tracking generator output. I use a true-rms voltmeter with analog
dB scale for tuneup and bandwidth measurements. Setting the SLM to 0.1Hz
step makes for easy bandwidth measurement!

By the way, I make my version of this filter for sale, for those who don't
do homebrew. I've sold about 20 of them in the past 8 months.

Steve AA7U
NE Oregon