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.