A question about AC Ground and Shielding

Question

I see this question on EE but reading those I am not sure they answer what I am asking here.

This is the problem. I am designing an audio device that is powered by an external 15VDC adapter. This external adapter wall plug have two pins. No ground connection to the AC outlet. The output of this adapter has two wires, positive and negative.

Internally, my device is receiving this 15VDC and creating a virtual ground, that is 7.5V, 0V and -7.5V.

This is an audio device and it has an amplifier, so I will have shield cables all over the place, from the volume potentiometer to the PCB, from the input/output connections to the PCB and so one.

Like I said, this device has no ground wire connected to the AC power outlet but I need the metal case to act as a RF shield, to prevent hum.

How should I proceed in terms of wiring? I mean, I have all these cable shields. Should I connect all the cable shields to the metal case? Should the metal case be connected to the internal 0V?

Answer 1

You seem to be lucky. Many power adapters have protective earth pin of the mains plug in use and it's directly connected to the DC output. The result: horrible ground loop hum if it's a part of an audio system which uses non-balanced signals. The phenomena really stretches the nerves of people who try to use a PC as a part of a bigger audio system.

Your audio equipment probably has an audio input. Connect your metal case to the signal ground of that input and do not allow any other circuit points in your equipment make any electric contact with the case.

Shielded internal cables: If a shield is not a signal path, it can well be connected to the local ground of the carried signal. Let the other end of the shield be free to avoid local ground loops. If the shield carries signal, it shouldn't be connected to anything else. if you have several uninsulated shields, which do not carry signals, your best bet is to insulate them and connect to the groundpoint of the carried signal. If you cannot insulate them and they make contact with the case, connect them firmly to the case to avoid microphonicity.

If possible, do not allow the case make any contact with concrete floor or other audio equipment.