Good evening, or morning, or afternoon.

I'm trying to calculate de input impedance of a simple microphone pre-amplifier circuit, the circuit has a -20dB PAD attenuator and I just want to be sure if I'm calculating things right. The aim is to get as close as possible impedances with and without the PAD.

Hope the drawing is not to bad.

Thank you in advance!

 

Doing the math in my head (a very empty place) it looks good to me.