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 transistor used in this circuit is uncommon today. AC124 or equivalent such as AC188K and...
Found in a Popular Electronics from 1992, this circuit uses a PLL567 in an audio meter bridge that...
Hughes suggests this circuit in a 1968 documentation. The circuit uses old transistors that can be...