Are coupling capacitors always needed at the input stage of a power amp?

And usually, how is there value chosen?

Zc = 1/jwC == |Zc| = 1/wC

Lets take 30 Hz as our low frequency, then |Zc| = 1/2*pi*30*C

Ok, then we get a value |Zc| = 1/kC
What determines the value of C? Is it the input impedance of the amplifier?

Thanks