Handbook for Sound Engineers

Tubes, Discrete Solid State Devices, and Integrated Circuits 353

Modern IC microphone preamplifiers provide a
simple building block with performance equaling
discrete solutions without a costly input transformer.

12.3.6 Balanced Line Interfaces

In professional audio, interconnections between devices
frequently use balanced lines. These are especially
important when analog audio signals are sent over long
distances, where the ground references for the send and
receive ends are different or where noise and interfer-
ence may be picked up in the interconnection cables.
Differences in signal ground potentials arise as a
result of current flowing into power-line safety grounds.
These currents, flowing through finite ground imped-
ances between equipment, can produce up to several
volts potential difference between the ground references
within a single building. These currents, usually at the
power line frequency and its harmonics, produce the
all-too-familiar hum and buzz known to every sound
engineer.
Two other forms of interference, electrostatic and
magnetic, also create difficulty. Cable shielding reduces
electrostatic interference from fields, typically using
braided copper, foil wrap, or both. Magnetic interference
from fields is much harder to prevent via shielding. The
impact of magnetic fields in signal cables is reduced by
balanced cable construction using twisted pair cable.
Balanced circuits benefit from the pair’s twist by
ensuring that magnetic fields cut each conductor equally.
This in turn ensures that the currents produced by these
fields appear in common mode, wherein the voltages
produced appear equally in both inputs.
The balanced line approach comes out of telephony,
in which voice communications are transmitted over

many miles of unshielded twisted pair cables with

reasonable fidelity and freedom from hum and interfer-

ence pickup. Two principles allow balanced lines to

work. First, interference—whether magnetic or electro-

static—is induced equally in both wires in the twisted

paired-conductor cable, and second, the circuits formed

by the source and receiver, plus the two wires

connecting them form a balanced bridge,^32 Fig. 12-59.

Interfering signals appear identically (in

common-mode) at the two (+ and ) inputs, while the

desired audio signal appears as a difference (the differ-

ential signal) between the two inputs.

Figure 12-58. THAT preamp circuit with phantom power. Courtesy THAT Corporation.

RG

CG

+In

Out

In

RG 2

RG 1

U1THAT1512

–In

+In

R 1

10k

R 2

10k

C 1

470p

C 2

470p

C 3

47p

Out

C 4

47u

C 5

47u

D 3

1N4004

D 4

1N4004

D 1

1N4004

D 2

1N4004

+15 –15

R 6

6k8

R 5

6k8

+48 V

R 3

4R7

R 4

4R7

–15

+15 V

C 6

100n

C 9

100n

Ref

V+

V

R 9

2k7

Figure 12-59. Balanced bridge. Courtesy THAT

Corporation.

Rcm- Rcm+

Rcm- Rcm+

Vcm (differential)Signal

Receiver

Driver

+

+

_

_