This circuit can generate signals of different frequencies, switched by a DC voltage. The frequencies are determined by the crystals. The circuit is from a 90s American documentation.
This configuration was found in the Texas Instruments optoelectronics manual. PNP transistors...
These basic bass controls were obtained from an article in Electronics Now magazine in February 1995....
This circuit was obtained in a 1991 Electronics Handbook. The publication no longer exists, but...