These basic bass controls were obtained from an article in Electronics Now magazine in February 1995. They are basic passive circuits in (a) basic tone, (b) boost, (c) cut, and (d) flat response.
This detector with ultra-low deviation was found in an American documentation from the 90s....
This circuit is from an old Popular Electronics, but it can be easily implemented nowadays, even...
This is the configuration of this type of oscillator given in an article in Electronics Now...