This filter was found in Exar documentation from the 1980s. The circuit has an attenuation of 60 dB at the selected frequency. The power supply must be symmetrical.
Found in old documentation, this simple test checks both the polarity and the existence of voltage...
Found in Radio Electronics from the 90s, this circuit can vary the frequency in small values...
This circuit can be made usng the TIP32 transistor but must use carbon microphone (not very common these...