We found this active crossover in a Radio Electronics magazine from June 1961. Note the different adjustment ranges. The signal must be applied to the amplifier input.
This circuit appears in this section in configurations that use other op amps. This is from 1999...
Found in a 1977 documentation, this circuit can use any general-purpose transistor. The signal is...
This circuit was found in the December 1992 Radio Electronis magazine. R2 is an NTC, there is a...