Audio Engineering

Noise and Grounding 397

chassis connection, however, has no relevance to the basic effect, which would still occur
even if the equipment enclosure was completely nonconducting.

The magnitude of ground current varies with the details of transformer construction and
increases as the size of the transformer grows. Therefore, the more power a unit draws,
the larger the ground current it can sustain. This is why many systems are subjectively
hum free until the connection of a powered subwoofer, which is likely to have a larger
transformer than other components of the system.

Equipment type Power consumption Ground current

Turntable, CD, cassette deck

Tuners, amplifi ers, small TVs

Big amplifi ers, subwoofers, large TVs

20 W or less

20–100 W

More than 100 W

5 μ A

100 μ A

1 m A

13.3.4 Ground Currents Inside Equipment

Once ground currents have been set fl owing, they can degrade system performance in two
locations: outside the system units, by fl owing in the interconnect grounds, or inside the
units, by fl owing through internal PCB tracks, etc. The fi rst problem can be dealt with
effectively by the use of balanced inputs, but the internal effects of ground currents can be
much more severe if the equipment is poorly designed.

Figure 13.5 shows the situation. There is, for whatever reason, ground current fl owing
through the ground conductor C–D, causing an interfering current to fl ow round the loop

Unit 1

Chassis

L

G

D

F

N

E

C

Audio cable

Unit 2

Chassis

F

B

G

Figure 13.5 : If ground current fl ows through the path F  FGG  , then the relatively high

resistance of the PCB tracks produces voltage drops between the internal circuit blocks.