Originally Posted by chileboy
. . . why does one need to use the grounding cable on turntables, when I don't need it on any of my other equipment?
Not a dumb question. It's mainly because of the low level of the signal, which requires substantial amplification by the phono-preamp. It doesn't take much to couple 60Hz hum into such a low-level signal, and the hum is then amplified by the preamp.
Here is the situation with my setup: Internal to the turntable, the cables connect to the cartridge and nothing else. There is no
grounding to the audio wiring, and because the turntable has a two-wire plug, the metal of the turntable, like the tone-arm, is not chassis-grounded. If I don't hook up the ground wire, I hear a slight 60Hz hum, from the AC power coupling into the audio. If I then touch
any metal part of the turntable, the hum gets load enough to shake the house. If I hook the ground-wire to my preamp, all hum goes away, as the metal of the turntable is now grounded.
The only way the metal parts of my turntable are grounded is through that ground wire. If the turntable had a three-prong plug, then a separate ground-wire might not be needed, but then you would have the possibility of a ground-loop, which would be fixed by removing the third prong and adding a ground-wire. So that's the best way to do it.
You could also ground the metal of the turntable using the audio-cable shield, but then, if there is a ground-potential difference, an AC current would flow through the shield, which would again induce hum into the signal.