@steve Thanks a million for getting me started with hum remover. Once you know how to use it, it is a fantastic tool. I have managed to get quite close to removing the hum in this sample file (there are six more on the album).
With the hum, the spectrogram of a low level passage (where the hum is clearly audible) looks like this:
I noticed that there is not much in the spectrogram at 50Hz, so it looks to me that the notch at 50 Hz does not do much except cut out signal. Do you have any suggestions how I could improve the result?
In 2017 I had already taken a shot at removing the noise (on Gaspard, another track of this album). Digging through my archives, I found the spectrogram from back then. It is very similar to the one of the Debussy above. I have no idea though how had the spectorgram settings to get the dark lines. I find they show the spectral lines better than the light on dark.
I had used notch filters starting with Q=20 @100Hz to Q=40 at 1000Hz.