Separate PSU grounding
I built recently a headphone amp where I packed the transformer in a one box and the PSU and amp boards in the other.
In the first box I packed a bit of a filtering before and aftere the trafo, a fuse and connected the central tap of the trafo to the mains ground by using a ground breaker. Box is also connected to the other side of the breaker (mains side). The output voltage ~25AC was taken by a cable to the second box. There, the ground from the trafo box was connected directly to the PSU ground pin and I linked PSU ground to the chassis over another ground breaker (this time only a 100ohm resistor in parallel to a filter cap 0.1uF).
I cannot say that I have any hum issues. A bit of a barely audible hum is present on the maximum volume. I wonder if the connection to the 2nd box is relevant at all or should I leave the second box floating without any ground connection?
To expand my question - what is the proper way of grounding devices that have PSU and/or transformer in a separate box.
That's an interesting question.
I would do it like this:
Ground the transformer box case directly at and from the mains inlet ground. Then use a small value resistor (say 20 Ohm / 5 W, with maybe a 100 nF cap in parallel) from the box case ground to the centre tap of the transformer.
Wire the centre-tap from the transformer through to the junction of the smooting capacitors (in box 2) and use that junction as a star point for the audio circuits. I would not ground that star point to box 2 case.
I would then take a second ground wire, from the transformer box case ground point and run that directly to the case of box 2.
In this way both box cases are at mains ground potential, and all of the audio circuits are lifted away from mains ground at a single point.
Make sure that the audio signal input and output connectors on box 2 are not connected to the case.
Use heavy gauge wire for the two ground wires between the boxes.
If you want to be flash use metal shell connectors for the umbilical cord, and then use an overall shield / screen on the umbilical wiring between the two boxes, connect the shield / screen to the metal shell of the connectors at both ends of the umbilical and ensure that the connectors are firmly grounded to the cases. In this way the shield keeps the cases at the same potential and replaces the non-audio ground wire between the boxes.
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