Handbook for Sound Engineers

Grounding and Interfacing 1195

it flows through a person’s body. More relevant is the

fact that these noisy leakage currents will flow in any

wire connecting such a floating device to safety ground,

or connecting two floating devices to each other as

shown in Fig. 32-25.

32.5 Interface Problems in Systems

If properly designed balanced interfaces were used

throughout an audio system, it would theoretically be

noise-free. Until about 1970, equipment designs

allowed real-world system to come very close to this

ideal. But since then, balanced interfaces have fallen

victim to two major design problems—and both can

properly be blamed on equipment manufacturers. Even

careful examination of manufacturers’ specifications

and data sheets will not reveal either problem—the

devil is in the details. These problems are effectively

concealed because the marketing departments of most

manufacturers have succeeded in dumbing down their

so-called specifications over the same time period.

Figure 32-21. Voltage difference is magnetically induced over length of safety-ground premises wiring.

Device A Device B

Driver Receiver Power transformerparasitic capacitances

Internal ground

Chassis

*Optional power line EMI filter

Ground voltage

difference

Parasitic

transformer Ground wire

resistance

Outlet

White

Black

Load current

Outlet Load current

* Optional power line filter

Chassis

Power transformer

parasitic capacitances Signal

Internal ground

Figure 32-22. Magnetic fields surrounding line and neutral
can induce voltage into safety ground.

Current in L and N are equal but in opposite directions

Safety ground “sees”

zero magnetic field at

exact midpoint

(cross-section view)

Instantaneous L and N currents are flowing into

drawing and out on drawing Figure 32-23. Lamp current (upper) versus induced voltage

(lower) for phase-controlled dimmer.

On On

On

Figure 32-24. For grounded equipment, interconnect cables complete a wired loop.

G G

R

Ground voltage difference

Device A Device B

Driver

Signal

Chassis

Receiver

Chassis

Signal

+ hum