I have tried an outboard DSP box (JPS NIR-10) but like everyone says,
it does not do a lot for you if you have adjacent strong signals. But
I also noticed that it added its own layer of high pitch audio noise.
I suspect this is mostly from the speaker amplifier chip.
My Omni-V is noisy enough in the audio section - I keep the built-in
audio filter in line most of the time, even for SSB.
I also now have an MFJ DSP box to test. Its specs are excellent but I
guess it will also add its own noise level.
The bottom line is that these box are clever but they are at least as
expensive as a good crystal filter. And they have to work on the
audio output from the rig, not on the signal inside the rig.
When the going is rough I use the -20dB attenuator. I used it last
night to work E44DX on 80m SSB, when the adjacent channel QRM was
heavy enough to almost destroy the S9 signals from E44.
The cheapest add-on filter I have found useful was a military surplus
gizmo in a metal can, like a potted transformer. It was beginning to
rust; it was chunky and it cost about a dollar (nobody wanted the
thing ...) but I managed to decipher the rather cryptic label. It was
a 400-800 Hz audio bandpass filter ! I got the other one they had as
well - at that price it was worth a risk. It helps sometimes, but if
the QRM gets past your front end mixer/filters, you have basically
The wartime military "FL8" audio filters (made in USA) used to be very
popular over here, but they are pretty useless. The CW peak frequency
is too high for comfort; the things work best in wide (AM) mode, to
cut off some of the higher audio hiss. The older I get the more I
prefer CW in the 300-800 Hz range.
I guess a cheap and cheerful SCAF filter does OK as long as you can
set the response to a comfortable range.
On 18-Feb-99 ARDUJENSKI@aol.com wrote:
> With a lot of message traffic on FILTERS (and some quite costly) I
> wondering if any one has installed a SCAF filter say 100-2400 hz
> range as an
> alternative? I have one installed in the INDEX LABS QRP PLUS that
> phenominal. I will mention that I have found todate the AUTEK QF-1
> to perform
> admirably with my OMNI-D in cw pileups during QRP contests and only
> cost $25.
> Anyone else using these alternative filters?
> Alan KB7MBI
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