[Lucien Nunes]

If you are full-wave rectifying two different voltages from the same secondary, then the two DC circuits can never be interconnected as the voltage between them is unavoidable. The circuits must either run from the same rectifier or have independent secondaries isolated from one another.

To get multiple voltages from the same secondary that can share a DC common terminal, one must either use full-wave rectification with a centre-tapped winding (which is also the DC common), or resort to just half-wave which has its own problems.

Even so, I wouldn't make a monitor amplifier that shared a power source with anything else. There's too much scope for interaction which reduces its usefulness for troubleshooting. You want your testing tools to be beyond reproach so that any abnormality must be in the device under test.