Naim Audio Power Amp Grounding Question

[martinbls]

Hi Guys!

I have one question about some irritating thing in two different Naim amps, regarding the grounding method.

In pic 1 and 2 you can see the innards of a Nap 160. There are the main amplifier pcb's with the output devices in TO-3 packages, and near these devices are the power supply connections, consisting of positive rail, negative rail, speaker out and ground.
On the other side of the pcb we have signal input, signal ground and ANOTHER connection to common ground! (All the green wires are used for a connection to common ground.)

Then, on pic 3 you can see a Nap 180, where the third wire on the front of the pcb beside signal input and signal ground is NOT another green wire connected to common ground, but a purple wire, which simply ends somewhere in the cable tree, near the LM317 voltage regulator, not being connected to anything!

Can somebody explain this strange kind of wireing, especially the difference from the 160 to the 180?

This is somehow important to me! (I have a little humming-noise problem over here...)

Thanks a lot!
Martin