This low power bass booster is from the 1977 National Semiconductor Audio Handbook. The circuit is powered by 6 to 9 V voltages and the feedback determines the enhanced bass range.
This TTL circuit for digital signals up to a few dozen megahertz was found in old documentation, but...
This circuit with an operational amplifier was found in the magazine Radio Electronics of...
This detector is from the 1990 Motorola Dataar Databook, but can be adapted to work with more...