Second order active voice filter (CB7568E)

This filter for the 300 Hz to 3 kHz range was found in an American documentation from the 90's. The operational amplifier is a double 741.

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Bistable Multivibrator (CB9925E)
This circuit using an operational amplifier was found in old American documentation. It can be...
Simple transistor tester (CB23746E)
If the circuit oscillates, emitting sound, it is because the transistor is good. This simple...
Telephone Amplifier with TDA7052 (CB990E)
Like the previous circuit, this is used in the analog part of the telephone lines. The same...