Quote from: Albert H on November 16, 2025, 03:55:07 am

I designed a replacement for the 15kHz filters in the Pro IV, using op-amps with gyrators emulating inductors.  It was published some years ago in EDN.  I'll dig it out and post it here.

is this it?

https://www.edn.com/brick-wall-lowpass-audio-filter-needs-no-tuning/

That's the article.  There was a small error in one of the inductor values, but the values given in the gyrator circuit are substantially correct.  However, there's a slightly simpler circuit I now use which I've added to this thread.