Disconnecting NetworkWithin audio equipment there are two grounds. These are signal and power ground. Audio designers usually like to keep these as separate as possible as noise from the power ground can pollute the signal ground. However, when these are connected directly to Safety Earth the result can be ground loops.
So what is a ground loop? Consider a CDP and an amp. Each are connected to earth in the wall via their mains plug. When you run electrical interconnects between the CDP and amp a local loop is created eg one can trace a connected path from the CDP through to the amp via the interconnect, through the signal ground in the amp to the Safety Earth, down the mains cable to the wall socket, along the earth wire to the CDP mains socket, up the mains cable to the CDP Safety Earth and into the signal ground.
What travels around this loop? Current created by, for instance, stray magnetic fields. These fields are generated by the transformers in the connected audio equipment. This circulating current gives rise to the characteristic hum in audio equipment and hum bars in video.
To overcome ground loops, a number of strategies can be adopted.
Firstly, something basic. You can disconnect the earth wire in the plug of either the CDP or amp. Alternatively, one can use a plug with a dummy (plastic) pin. People do actually do this and it does work. But what you are left with is a real risk of electrocution or fire.
Let's say a stray wire in the equipment with the removed earth comes detached and contacts the case. A sudden, massive spike in current will rush into the power/signal circuit and attempt to run to earth through the interconnects and into the equipment which is still connected to mains earth. These interconnects, which are not designed to handle the current involved, are highly likely to cause a fire. If you are unlucky, somewhere a component may fry breaking the path through the signal ground leaving you with a live case waiting to (possibly) kill you.
Another strategy is to "lift" the signal ground above the Safety Earth. To do this one connects all of the grounds in the equipment to a single point (star ground) and then place a resistor between ths point and the Safety Earth. This resistor serves to resist the circulating current travelling around the local loop and prevents the hum.
Again, however, should a fault occur on the PCB, the current will rush through the resistor which is likely to fail. We are then back in a similar situation as before. So, this approach is not generally recommended although you will see it being used in the DIY community.
The disconnecting network is similar to the resistor approach but uses more robust components which are ensured to survive the currents involved during a fault. Consider the picture below.
Disconnecting Network
The Safety Earth attaches the earth wire from the mains cable to the case. The zero volt line connects the network to the star ground. During normal operation the resistor lifts the internal ground and prevents the loop. The cap helps reduce noise. The diode bridge provides two paths to earth in the event of a fault. So, if one path fails there is another as backup. Moreover, the bridge contains very solid conductors making failure unlikely. The switch allows the disconnecting network to be bypassed (if a ground loop is absent) but still provides a route for the fault current to Safety Earth via the bridge.
This latter option seemed to be a pretty good to me and if it's good enough for Bryston (I've seen Bryston schematics which include exactly the same components as the disconnecting network), it's good enough for me!
My useful(?) threads can be found
here.