Worked on getting the amplification stage done after a brief stab at getting the correct spice models working in LTSpice and NGSpice. I will pursue getting the LM358 spice model working after I complete the rest of the circuit.

The amplification stage is using half of an LM358 in a non-inverting configuration to give a minimum gain of 4.125 V/V and a max of 10.375 V/V. I may change the values if I have to increase the rail voltages.
The other half of the LM358 is being used a indicator to make sure that the signal is being amplified before it makes its way to the filter stages.

The next part that I work on is going to be the input and output buffers. I'll leave that design a surprise.