This circuit controls the channel separation as described in the March 1973 Radio Electronis magazine article. The circuit was proposed by Mullard (Philips) and consists of two channels as shown.
We found this circuit in a 1990 documentation. The operational amplifiers are type 741 and the LED is...
We found this circuit on an Elektor from the 90s. The frequency is given by capacitor C1. The...
Found in National Semiconductor publication from the 1980s. This gain-100 circuit can be...