Audio Signal Injector and Tracer in One Circuit

If you are building or troubleshooting audio circuits, both audio signal injector and audio tracer is important tools. tracing audio signal means tapping a signal at certain point, and check if the signal is exist.  Here is the schematic diagram of the  signal injector/tracer circuit:

Tracing an audio signal can be done using an audio amplifier, but using a handheld instrument would be more convenient, since you can make such instrument with special probes. As the opposite of tracing, injecting an audio signal is important too. Injecting an audio signal means connecting a certain point at your circuit with an audio signal source, usually to observe the response of the circuit’s output when the circuit is fed with an input.  The circuit uses two transistors with some passive components. A selector switch is provided to select the injector or the tracer function.