My experiences with external filters ( NIR-10, Autek Q-1, and MFJ 752)
are a lot like John describes. I gave up on all of them. The DSP approach
in the NIR-10 may work for some, but, the weird noises really bothered me
on phone and on CW the unit was just about worthless ( I sold the NIR-10
and the buyer loved it and uses it all the time -- so probably a matter of
My approach now is as I used to do before. Try to let my hearing do the
filtering and switch in the 250 Hz CW filter when my ears and the 500 Hz
filter don't do the job.
> 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
> had it.
> 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.
> John G3JAG
> On 18-Feb-99 ARDUJENSKI@aol.com wrote:
> > With a lot of message traffic on FILTERS (and some quite costly) I
> > was
> > 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
> > is
> > 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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> Date: 18-Feb-99
> Time: 10:18:36
> John Crux
> Consultant in product forgery - Asia and
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