Found in an 1868 Mullard documentation this circuit has a dynamic range of 10 Hz to 25,000 Hz. Modern equivalent transistors can be used.
This circuit, from an old ON Semiconductor publication, suggests a shield to excite an LED from an...
This circuit was obtained from an old American documentation having a limited adjustment range....
Microchip Continues Expansion of AVR Microcontroller Product Line with Addition of Three New...