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The values for R7 and R4 don't matter anywhere near as much as R6. Any value of about 10M or greater for R7 or R4 puts the low-pass filter below a few Hz.The "best" value for R6: depends. If you consider a "modern" 50pF to 80pF capacitance for the capsule, then if you want bass response to be "unimpeded" you need the zero in the filter between the capsule and R6 to be somewhere around 40Hz. If we consider 50pF to be "worst case", then R6 needs to be 80M or larger. Larger R's give more noise, so you want to balance noise with frequency response. 60M to 100M appears to be typical in these designs, however that may be due to components that were commonly available at the time. Many modern transistor mikes use 1G in these positions.
The values for R7 and R4 don't matter anywhere near as much as R6. Any value of about 10M or greater for R7 or R4 puts the low-pass filter below a few Hz.
The "best" value for R6: depends. If you consider a "modern" 50pF to 80pF capacitance for the capsule, then if you want bass response to be "unimpeded" you need the zero in the filter between the capsule and R6 to be somewhere around 40Hz. If we consider 50pF to be "worst case", then R6 needs to be 80M or larger. Larger R's give more noise, so you want to balance noise with frequency response. 60M to 100M appears to be typical in these designs, however that may be due to components that were commonly available at the time. Many modern transistor mikes use 1G in these positions.
