This sensitive 5-band circuit was found in a September 1995 issue of Electronics Now magazine. The headphones, for greater sensitivity, should be high impedance.
The circuit shown in the figure is suggested by Maxim (www.maxim-ic.com) and makes use of the...
We found this circuit in a Radio Electronics magazine from January 1975. The transistors are...
Found in documentation from the 1980s, this circuit can have several adaptations to use modern...