Another option is the external 4StatesQRP HI-PER-MITE audio CW filter kit . ?This filter ?is 200 Hz wide and has no ringing - a truly ?amazing 4 pole filter with 3 low pass filters and 1 high pass filter cascaded together. ??
All background hiss is essentially eliminated when the filter is engaged. ?
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Google ‘HI-PER-MITE’ for more info including an audio demo of this CW audio filter in action.
Ham Radio