First let's examine
why they must be tied together. We'll cover
where and
how in a moment. There are at least two reasons why one should connect signal ground and chassis ground together in a unit.
One reason is to decrease the effects of coupling electrostatic charge on the chassis and the internal circuitry. External noise sources can induce noise currents and electrostatic charge on a unit's chassis. Noise currents induced into the cable shields also flow through the chassis -- since the shields terminate (or should terminate) on the chassis. Since there is also coupling between the chassis and the internal circuitry, noise on the chassis can couple into the internal audio. This noise coupling can be minimized by connecting the signal ground to the chassis. This allows the entire grounding system to fluctuate with the noise, surprisingly providing a quiet system. Further coupling reduction is gained when the chassis is solidly bonded to a good earth ground -- either through the line cord, through the rack rails or with an independent technical or protective ground conductor. This provides a
non-audio return path for any externally induced noise."
