Handbook for Sound Engineers

1210 Chapter 32

directly grounded at the driver end, which reduces the
interface to an unbalanced one with no noise rejection.^41
Unless a balanced output already utilizes a built-in trans-
former, using an external ground isolator such as the one
shown in Fig. 32-53 is the only method that will simulta-
neously avoid weird or damaging behavior and minimize
ground noise when used with virtually any output stage.
This approach is used in the ISO-MAX® Pro model
PC-2XR pro-to-consumer interface.

32.6.3.4 RF Interference

As mentioned earlier, immunity to RF interference or
RFI is part of good equipment design. Testing for RFI
susceptibility is now mandated in Europe. Unfortu-
nately, much of the equipment available today may still
have very poor immunity. Under unfavorable condi-
tions, external measures may be needed to achieve
adequate immunity.^42
For RF interference over about 20 MHz, ferrite
clamshell cores shown in Fig. 32-54, which are easily

installed over the outside of a cable, can be very effec-

tive. Some typical products are Fair-Rite #0431164281

and Steward #28A0640-0A.43,44 In most cases, they

work best when placed on the cable at or near the

receive end. Often they are more effective if the cable is

looped through the core several times.

If this is inadequate, or the frequency is lower (such

as AM radio) you may have to add a low-pass—i.e.,

high-frequency reject—RFI filter on the signal line.

Fig. 32-55 shows sample 50 kHz cutoff, 12 dB per

octave low-pass RFI filters for unbalanced or balanced

audio applications. For best performance and audio

quality, use NP0 (also called C0G)-type ceramic capaci-

tors keeping leads as short as possible, under ¼ inch

preferred. For stubborn AM radio interference, it may

help to increase the value of C up to about 1000 pF

maximum. The 680μH inductors are small ferrite core

types such as J.W. Miller 78F681J or Mouser

434-22-681. If the only interference is above about

50 MHz, a ferrite bead may be used for L. For the

balanced filter, inductors and capacitors should be ±5%

Figure 32-49. Loop created by two ground connections.

Figure 32-50. Grounding floating equipment when isolators are installed. From Jensen AN004.

TV receiver

CD player

Cassette player

Preamplifier

Crossover and

subwoofer

power amplifier

Main

power

amplifier

ac power safety ground

Ground noise

voltage

Entry

lightning

ground

CATV 3' 20' 15'

3'

3'

TV

CATV

Preamplifier

Power

amplifier

Powered

subwoofer

Added

ground

Added

ground

Isolator

Isolator