[Elecraft] Audio Filter mini module

[Darwin, Keith]

I bought one a few weeks ago but preaching duties at church kept me from
having any free time to look at it.  Our new pastor has arrived so I can
return to non-preaching status which puts some free time back into my
life.

Time for that AF mini module!

Last night I put it together.  The entire kit took 2 hours.  I did not
install the battery box or the rubber feet since I plan to eventually
mount the module in an enclosure.

Assembly was easy, as is typical with mini modules and Elecraft stuff in
general.  I hooked it up to my K2, applied power and was rewarded with a
red LED and filtered sound in my headphones.

The low pass filter is very nice.  When activated it cuts some highs
even with the cutoff freq set to the highest setting.  I listened to 20
USB this morning wondering if the amount of cut would affect SSB
usability and it didn't.  The hiss was reduced and signals were still
adequately clear and crisp in the "cans".

The band pass filters however, don't do nice things on SSB.  Too much is
filtered.

On CW, the band pass filters are pretty nice indeed.  I find the first
one to be wonderful for must uses.  It doesn't ring enough to bother me
and the sweepable center freq is nice.  I did notice that the freq range
is rather non-linear.  With a 600 Hz tone peaked, the frequency control
is most of the way "up".

The 2nd band pass filter is noticeably more "focused" than the first.
The additional filtering comes at a price of increased ringing.  It
isn't enough to keep me from using this setting when needed but it won't
be my normal setting for rag chewing under typical band conditions.
Contest conditions may require it however.

What would I change with it?  Glad you asked.  I'd love a blend control
rather than a switch.  Something that can smoothly change from no BPF to
full BPF  I reviewing the schematic I think it would be an simple mod
but with the big switch already mounted, it would not be an easy thing
to actually do.

- Keith KD1E -
- K2 5411 -