Found in a 1964 publication, this circuit makes use of transistors from the time. It operates on frequencies up to 3 kHz.
Found in a Popular Electronics from 1992, this circuit uses a PLL567 in an audio meter bridge that...
Found in an EDN from the 90s, this circuit uses an electret microphone. The output can be used to...
We found this circuit in an old documentation, but the circuit is current due to the components it uses and...