This simple audio signal tracer or follower can be made with a BC548 instead of the original transistor. The output is made in a small crystal phone. The power can be from 5 to 9V.
This application is not modern since few computers have parallel ports. But you can use this...
Using a 741 op-amp this circuit high-pass to 10 kHz was found in the February 1995 issue of...
Found in a 1969 EEE, this circuit is based on an operational amplifier of the time and on old type...