Handbook for Sound Engineers

1194 Chapter 32

32.4.4.2 Parasitic Capacitances and Leakage Current
Noise

In every ac-powered device, parasitic capacitances
(never shown in schematic diagrams!) always exist
between the power line and the internal circuit ground
and/or chassis because of the unavoidable interwinding
capacitances of power transformers and other line
connected components. Especially if the device contains
anything digital, there may also be intentional capaci-
tances in the form of power line interference filters.
These capacitances cause small but significant 60 Hz
leakage currents to flow between power line and chassis
or circuit ground in each device. Because the coupling
is capacitive, current flow increases at higher noise
frequencies. Fig. 32-20 shows the frequency spectrum
of current flow in 3 nF of capacitance connected
between line and safety ground at an ac outlet in a
typical office.

This tiny current, although it poses no shock hazard,
causes hum, buzz, pops, clicks, and other symptoms
when it couples into the audio signal path. This capaci-
tive coupling favors higher frequencies, making buzz a
more common symptom than pure hum. We must
accept noisy leakage currents as a fact of life.

32.4.4.3 Parasitic Transformers and Inter-Outlet Ground
Voltage Noise

Substantial voltages are magnetically induced in prem-
ises safety ground wiring when load current flows in the
circuit conductors as shown in Fig. 32-21. The magnetic
fields that surround the line and neutral conductors,
which carry load current, magnetically induce a small

voltage over the length of the safety ground conductor,

effectively forming a parasitic transformer. The closer

the safety ground conductor is to either the line or

neutral conductor, the higher the induced voltage.

Because, at any instant in time, line and neutral currents

are equal but flow in opposite directions, there is a plane

of zero magnetic field exactly midway between the line

and neutral conductors as shown in Fig. 32-22. There-

fore, Romex® and similar bonded cables generally

generate significantly lower induced voltages than indi-

vidual wires in conduit, where the relative positioning

of the wires is uncontrolled.

The voltage induced in any transformer is directly

proportional to the rate of change of load current in the

circuit. With an ordinary phase-control light dimmer the

peak voltages induced can become quite high. When the

dimmer triggers current on 120 times per second, it

switches on very quickly (a few microseconds) as shown

in Fig. 32-23. Since the magnetic induction into safety

ground favors high frequencies, noise coupling prob-

lems in a system will likely become most evident when

a light dimmer is involved. The problems are usually

worst at about half-brightness setting of the dimmer.

This parasitic transformer action generates small

ground voltage differences, generally under 1 V,

between ac outlets. The voltage differences tend to be

higher between two outlets on different branch circuits,

and higher still if a device on the branch circuit is also

connected to a remote or alien ground such as a CATV

feed, satellite dish, or an interbuilding tie line. We m u s t

accept interoutlet ground noise voltage as a fact of life.

32.4.4.4 Ground Loops

For our purposes, a ground loop is formed when a signal

cable connects two pieces of equipment whose connec-

tions to the power line or other equipment causes a

power-line-derived current to flow in the signal cable.

The first, and usually worst, kind of ground loop

occurs between grounded devices—those with

three-prong ac plugs. Current flow in signal cables, as

shown in Fig. 32-24, can easily reach 100 mA or more.

The second kind of ground loop occurs between

floating devices—those with two-prong ac plugs. Each

pair of capacitances CF (for EMI filter) and CP (for

power transformer parasitic) in the schematic form a

capacitive voltage divider between line and neutral,

causing some fraction of 120 Vac to appear between

chassis and ground. For UL-listed ungrounded equip-

ment, this leakage current must be under 0.75 mA

(0.5 mA for office equipment). This small current can

cause an unpleasant, but harmless, tingling sensation as

Figure 32-20. Typical leakage current from line to safety
ground coupled via 3000 pF capacitance into a 75: spec-
trum analyzer input.

100 MA

10 MA

1 M!

100 nA

10 nA

1 nA

50 100 1k 10k 100k 1M

Frequency—Hz