Using an operational amplifier of the time, this circuit was obtained from a 90s Radio Electronics. The circuit covers the frequency range from 2 Hz to 20 kHz.
The circuit presented may use any general purpose transistor. The indicator is an LED and...
This source was found in a 1982 American publication, but can be easily assembled using 2N2995 (PNP...
Found in a 1980s documentation, this circuit uses as base a component of the era. The formula for...