This circuit is from an American transistor book from the 60's. The transistor is germanium of the time.
Found in the March 1974 Radio Electronics magazine, this circuit is designed to measure the level...
Adafruit Breakout PDM Microphone This Adafruit tested microphone card is available from Mouser...
This circuit was obtained from a 90s Radio Electronics. The value of R1 determines the scale...