[sdiy] Hum Problem Kaput

[Ingo Debus]

synthos at xs4all.nl wrote:
> The problem is that you shouldn't confuse 2 concepts which are widely confused...
> There is safety grounding, and there is signal grounding.
> In theory, all your audio gear should be grounded for safety.

Double insulated gear doesn't need safety ground. IMHO this is even
safer, because it doesn't rely on the quality of the safety ground.

> But then you will run into problems when you're are building a complex network of
> signals, as you will make groundloops in the process of connecting signal grounds.
> (these loops can be considered as coils, in which the magnetism of the current in the
> wires around you generates electricity flow, audible...)

In most cases the magnetically induced voltage isn't the problem (if it
were, taping the audio and mains cords together would help, making the
area small). Problem is that the audio ground path is shared by other currents.

> If you decide to unground some gear that is still connected to the mains, there are
> two issues to take into account:
> - Your mixer, or central ground, might not be suitable for handling safety ground
> well enough.

This was my first thought too reading Harry's comment. If there's a
ground fault interrupter then you're fine, it will trip at some 30 mA.
But if not then the fault current is really high, and goes through the
thin shield of audio cables or even through PCB traces. Who guarantees
that these won't burn long before the breaker/fuse notices?

> - If you unplug something from the mixer or central ground (say, if you decide to
> patch into another channel, you are leaving unpatched gear ungrounded, making
> it unsafe! Or you might get a few sparks while repatching, as you only had a few
> volts difference....

Another valid point.

Ingo