Mmmm, I'm afraid I'm going to have send off for the "Ladybird Book of DSP Filtering"
. That is an area which has passed me by, but I'm still at a stage where the options are still completely open at the moment.
The Maplin AF unit (contributed by a guy called Robert Penfold and written up in some detail on the old Maplin magazine) uses five IC's and RC filtering to get the job done; (2) 1458C, CA3080E, 741C and a TBA820M. It was discussed briefly on this Forum back in 2014
https://www.vintage-radio.net/forum/...d.php?t=111583, but otherwise, has passed in to total obscurity as far as Google is concerned. I built it ~20 years ago.
Anyway, I'll continue the re-build leaving the filter aspect until everything else is up and running, which will give me some time to catch up with the times! Many thanks for the inputs, which I'll certainly be looking into.
B