Handbook for Sound Engineers

1208 Chapter 32

the CATV drop or antenna and its lightning ground
connection (see Section 32.4.1). Isolators will not pass
dc operating power to the dish in DBS TV systems.

Since most unbalanced interfaces are made to
consumer devices that have two-prong ac plugs,
isolating the signal interfaces may leave one or more
pieces of equipment with no ground reference whatso-
ever. This could allow the voltage between an isolator’s
input and output to reach 50 Vac or more. While this
isn’t dangerous (leakage current is limited in UL-listed
devices), it would require unrealistically high (CMRR
over 140 dB) performance by the isolator to reject it!
The problem is solved by grounding any floating gear
as shown in Fig. 32-50. This is best done by replacing

the two-prong ac plug with a three-prong type and

adding a wire (green preferred) wire connected between

the safety ground pin of the new ac plug and a chassis

ground point.

A screw may be convenient as the chassis ground

point. Use an ohmmeter to check for continuity between

the screw and the outer contact of an RCA connector,

which itself can be used if no other point is available.

Although, in the example above, an added ground at

either the preamp or the power amp would suffice,

grounding the device with the highest leakage

current—usually those devices with the highest ac

power consumption rating—will generally result in the

lowest noise floor.

32.6.3.3 Unbalanced to Balanced and Vice Versa

The reader is referred to Chapter 11, Section 11.2.2,

Audio Transformers, for a more detailed discussion of

these applications.

Beware of RCA to XLR adapters! Fig. 32-51 shows

how using this adapter to connect an unbalanced output

to a balanced input reduces the interface to an unbal-

anced one having absolutely no ground noise rejection!

The potential noise reduction benefit of the balanced

input is completely lost.

Proper wiring for this interface, shown in Fig. 32-52,

results in at least 20 dB of noise rejection even if the

balanced input is one of typically mediocre perfor-

mance. The key difference is that, by using shielded

twisted pair cable, the ground noise current flows in a

separate conductor that is not part of the signal path.

Driving an unbalanced input from a balanced output

is not quite as straightforward. Balanced equipment

outputs use a wide variety of circuits. Some, such as the

one in Fig. 32-53, might be damaged when one output is

grounded. Others, including most popular servobalanced

output stages, can become unstable unless the output is

Figure 32-43. Ground isolator stops noise current in shield of unbalanced cable.

Device A Device B

Driver

Signal

Isolator Receiver

Signal

Chassis Noise voltage Chassis

Figure 32-44. Stereo unbalanced audio isolator. Courtesy
of Jensen Transformers, Inc.

Figure 32-45. Stereo balanced audio isolator. Courtesy of
Jensen Transformers, Inc.