Designed for the audio track, this circuit was found in a Popular Electronics from the 90's. Other operational can be used and R7 can be 470 k. The frequency is 1 kHz.
Suitable for battery powered microcontrolled applications this Texas Instruments circuit from its...
This amplifier with the LM1098 and LM101 by Motorola from a 1990 manual can be designed with more...
Found in a 1967 Ferranti documentation, this circuit uses a LDR from that time as a sensor....