Found in Mallory documentation from 1968 this circuit uses an integrated circuit from that time. The circuit has a flat response from 10 Hz to 700 kHz.
Found in American documentation from the 1980s, this circuit uses transistors from that time. It...
This circuit of old documentation can be implemented with modern operational amplifiers. The...
This 80's circuit uses a small light bulb as an indicator, but an LED with the appropriate resistor...