The 100K resistor along with the resistor from the input jack sets the input impedance.
If anything I might make it larger like 220K or 330K. It does not change the gain.
Gain of the first stage is set by the feedback resistor divided by the resistor from
the - input to common.
In this case common is the +12V. The gain of the first stage is 1+47K/2.2K = 22.4
You could change the 100K to 1M but you would want a FET op-amp like TL071 for the first stage
If anything I might make it larger like 220K or 330K. It does not change the gain.
Gain of the first stage is set by the feedback resistor divided by the resistor from
the - input to common.
In this case common is the +12V. The gain of the first stage is 1+47K/2.2K = 22.4
You could change the 100K to 1M but you would want a FET op-amp like TL071 for the first stage