Handbook for Sound Engineers

Grounding and Interfacing 1211

tolerance parts or better to maintain impedance balance.
The balanced filter can be used for low-level micro-
phone lines, but miniature toroidal inductors are recom-
mended to minimize potential hum pickup from stray
magnetic fields. These filters, too, are generally most
effective at the receive end of the cable.
When possible, the best way to deal with RF inter-
ference is to control it at its source. Fig. 32-56 is a sche-

matic of a simple interference filter for solid-state

120 Vac light dimmers rated up to 600 W. It begins

attenuating at about 50 kHz and is quite effective at

suppressing AM radio interference. It must be installed

within a few inches of the dimmer and, unfortunately,

the components are large enough that it usually requires

an extra-deep or a double knock-out box to accommo-

date both dimmer and filter. Parts cost is under $10.

A speaker cable can also become an antenna. In a

strong RF field, enough voltage can be delivered to the

semiconductors in the power amplifier that they become

a detector and interference can be heard in the speaker

even though the amplifier may be unpowered! More

commonly, this problem occurs when the amplifier is

powered and RF enters its feedback loop. In either case,

the solution depends on the frequency of the interfer-

ence. Ferrite cores on the cable near the amplifier may

Figure 32-51. Incorrect connection of an unbalanced output to a balanced input.

Figure 32-52. Correct connection of an unbalanced output to a balanced input.

Figure 32-53. Bullet-proof connection of balanced output to unbalanced input. Use the shortest possible cable on the RCA

side to avoid common-impedance coupling.

Typical consumer output

470

100k

10 MF

SH

HI

Adapter

Typical Pro input 150 pF

10k

10k

10k

10k

150 pF

2

3

1

2

3

1

LO

SH

HI

Typical consumer output

470

100k

10 MF

SH

HI

Typical Pro input 150 pF

10k

10k

10k

10k

150 pF

2

3

1

2

3

1

SH

LO

HI

LO

100k

100k

100 100 MF

SH

YEL

ORG

WHIBLK

RED

BRN

47K

Typical consumer input

100 100 MF

HI

Typical Pro output

2k37

2

3

1

2

3

1

LO SH

HI

LO

JT-10JB-D

4.7 MF

Figure 32-54. Ferrite cores.

Figure 32-55. RF interference filters for audio lines.

2

3

1

In Out

L

C

Balanced

1

2

In L Out LC^3

C

Unbalanced L = 680 MH

C = 220 pF