This circuit with RIAA equalization is from 1985 US documentation. The circuit has a flat response of 150 Hz to 40 kHz. The power supply must be symmetrical.
In the figure we have the trip circuit for a relay with audio signals. The circuit is from an 80's...
This circuit was found in an American documentation, being able to multiply the input frequency by 10 or...
The PNP transistor can be a BC548 and the NPN any common phototransistor. The circuit is from 1968...