Handbook for Sound Engineers

1200 Chapter 32

Fig. 32-33 shows three devices whose design does
not allow shield current to flow in signal reference
conductors. The first uses a star connection of input
pin 1, output pin 1, power cord safety ground, and
power supply common. This technique is the most
effective prevention. Noise currents still flow, but not
through internal signal reference conductors. Before
there were printed circuit boards, a metal chassis served
as a very low-impedance connection (effectively a
ground plane) connecting all pins 1 to each other and to
safety ground. Pin 1 problems were virtually unknown
in those vintage designs. Modern printed circuit board–
mounted connectors demand that proper attention be
paid to the routes taken by ground noise currents. Of
course, this same kind of problem can and does exist
with RCA connectors in unbalanced consumer equip-
ment, too.
Fortunately, tests to reveal such common impedance
coupling problems are not complex. Comprehensive
tests using lab equipment covering a wide frequency
range have been described by Cal Perkins^31 and simple
tests using an inexpensively built tester called the
hummer have been described by John Windt.^32 Jensen
Transformers, Inc. variant of the Hummer is shown in
Fig. 32-34. It passes a rectified ac current of 60–80 mA
through the potentially troublesome shield connections
in the device under test to determine if they cause the
coupling.
The glow of the automotive test lamp shows that a
good connection has been made and that test current is
indeed flowing. The procedure:

1. Disconnect all input and output cables, except the
   output to be monitored, as well as any chassis
   connections (due to rack mounting, for example)
   from the device under test.


2. Power up the device.
   3. Meter and, if possible, listen to the device output.
   Hopefully, the output will simply be random noise.
   Try various settings of operator controls to famil-
   iarize yourself with the noise characteristics of the
   device under test without the hummer connected.
   4. Connect the hummer clip lead to the device chassis
   and touch the probe tip to pin 1 of each input or
   output connector. If the device is properly
   designed, there will be no output hum or change in
   the noise floor.
   5. Test other potentially troublesome paths, such as
   from an input pin 1 to an output pin 1 or from the
   safety ground pin of the power cord to the chassis
   (a three-to-two-prong ac adapter is handy to make
   this connection).
   Note: Pin 1 might not be connected directly to
   ground in some equipment—hopefully, this will be
   at inputs only! In this case, the hummer’s lamp
   may not glow—this is OK.

32.5.3 Balanced Cable Issues

At audio frequencies, even up to about 1 MHz, cable

shields should be grounded at one end only, where the

signal is ground referenced. At higher frequencies,

where typical system cables become a small fraction of

a wavelength, it’s necessary to ground it at more than

Figure 32-33. Equipment with proper internal grounding.

Signal circuitry Signal circuitry Signal circuitry

Power

supply

Power

supply

Power

supply

Sig Ref Sig Ref Sig Ref

Safety ground Noise Safety ground Safety ground

voltage Noisevoltage

2

3

1

2

3

1

2

3

1

2

3

1

2

3

1

2

3

1

Figure 32-34. The Hummer II. Courtesy Jensen Trans-

formers, Inc.

“Wall Wart”

transformer

12 V automotive “Test Light”

12–24 Vac > 100 mA 1N4001 rectifier