I am trying to establish the group delay for single inductor 1st order LP shelf chosen to give me a turn over point at around 200Hz (-3dB). There is no HP section

I understand the first order has a phase shift of 45deg at crossover (?) so presumably 90deg at 400Hz and so on?

Of course when I put this into a group delay calculator, it gives a constant group delay regardless of frequency (around .6ms).

I was under the impression that the GD should vary with frequency?

If this filter was implemented in DSP I am guessing the GD would still be the same as a passive component for IIR filters as they still have to implement this phase shift to achieve the "crossover"?

Layman language answers only please, I am not a mathematician!

Thanks for any help...