Why use ground wire from preamp to amp???

I see no reason for the ground wire.

In the case of a piece of audio equipment that does not use the wall receptacle safety equipment ground, if the equipment was designed properly the signal ground of the piece of equipment is connected to the metal chassis of the equipment.

A piece of audio equipment where the equipment is designed to use the wall receptacle safety equipment ground the designer of the equipment may have or not have connected the circuit signal ground to the metal chassis of the equipment. Some designers incorporate a switch so the user can lift the signal ground from the wall safety equipment grounded chassis.

When an interconnect is connected between two pieces of equipment the signal grounds of the equipment is connected together by the signal ground return wire of the interconnect.

Any time a piece of equipment that uses the wall receptacle safety equipment ground, where the circuit signal ground of the equipment is connected to the metal chassis, and then connected to a piece of equipment that does not use the wall receptacle safety equipment ground, that  piece of equipment also then becomes grounded to the wall receptacle safety equipment ground by the signal ground wire of the interconnect.

I suggest you look for another interconnect manufacture.

jazzonthehudson said:

The interconnect RCA are mostly only grounded on the source side as the manufacturers know there may be a ground differential so they use use this workaround for, again from the ground differential point of view, possible deficient systems.

What you speak of is the shield of a shielded interconnect cable.

My post refers to the signal ground return wire of an interconnect.

Now in the case of a coax cable the shield and the signal ground return wire are one in the same and the shield must be connected to the connectors at both ends.

infection said:
My Tannoy DC10T's have a ground/earth input...

http://www.tannoymarketing.com/WebServices/dh.ashx?t=qv&v=9309

"Tannoy’s unique fifth

speaker terminal electrically grounds the driver

chassis, reducing potential sound degradation form

radio frequency interference. Eliminating potential

RF ‘noise’ enhances midrange clarity and allows

Definition to deliver an even more expressive

musical presentation."

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Are you using that? Where did you connect it to?

Since most components have grounded ac power cords and most interconnects have grounds, every component has two earth ground and thus ground loops. The third wire would add additional ground loops. All of this is to satisfy Underwriters Labs on the off chance that you have a loose power wire within the component and are grounded when you touch the component. All of this ruins your sound.

Not sure "most" of audio and or video equipment designed and manufactured today use the wall receptacle safety equipment ground on all their equipment.

Just to name a few that do not use the safety equipment ground on all their audio equipment.

Ayre 

Arcam

Cambridge Audio

Marantz

Denon 

Nad

Sony

I am sure there are others.

It has nothing to do with whether the outer case is metal or plastic. It has to do with the insulation used on the AC internal power wiring. Equipment that is designed and Listed for only 2 wire power cords that does not use/need the third wire safety equipment ground uses double insulated AC power wiring. It does cost more than regular insulated wire.

In time, jmho, all audio equipment as well as video equipment designed and manufactured for the home consumer use will use 2 wire power cords.

There is no need for the safety equipment ground wire today and it can/does degrade the sound  of audio as well as video equipment. Manufactures just need to spend the extra money and use double insulated AC power wiring in their equipment.