Andy RW9RN has developed a wee circuit for insertion in the audio line to provide variable bandwidth control for CW and SSB.
He has a video about it here.
VK4PLN audio filter board
Nik VK4PLN has now received his 700Hz CW audio filter boards and built up the board as shown below:
He put these on the spectrum analyser. The spectrum before the filter is included in circuit:
And after:
Seems to work! To Nik’s ears an LM324 gives a better result than the TLC274 op amp. You mileage may vary.