This circuit was found in a 1966 EEE. What it does is it converts the logic levels of 0 V and +4 V into -6 V and +6 V. Some older type transistors can be replaced by modern equivalents.
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...