Handbook for Sound Engineers

354 Chapter 12

A common misconception in the design of balanced
interfaces is that the audio signals must be transmitted
as equal and opposite polarity on both lines. While this
is desirable to maximize headroom in many situations,
it is unnecessary to preserve fidelity and avoid noise
pickup. It is enough if the bridge formed by the combi-
nation of the circuit’s two common-mode source imped-
ances (not the signals) working against the two
common-mode load impedances remains balanced in all
circumstances.
In telephony, and in early professional audio
systems, transformers were used at both the inputs and
outputs of audio gear to maintain bridge balance. Well-
made output transformers have closely matched
common-mode source impedances and very high
common-mode impedance. (Common-mode imped-
ance is the equivalent impedance from one or both
conductors to ground.) The floating connections of most
transformers—whether used for inputs or
outputs—naturally offer very large common-mode
impedance. Both of these factors, matched source
impedances for output transformers, and high common-
mode impedance (to ground) for both input and output
transformers, work together to maintain the balance of
the source/load impedance bridge across a wide range
of circumstances. In addition, transformers offer
galvanic isolation, which is sometimes helpful when
faced with particularly difficult grounding situations.
On the other hand, as noted previously in the section
on preamplifiers, transformers have drawbacks of high
cost, limited bandwidth, distortion at high signal levels,
and magnetic pickup.

12.3.6.1 Balanced Line Inputs

Transformers were used in early balanced line input
stages, particularly in the days before inexpensive
op-amps made it attractive to replace them. The advent
of inexpensive op-amps, especially compared to the cost
of transformers, motivated the development of active
transformerless inputs. As the state of the art in op-amps
improved, transformer-coupled inputs were replaced by
less expensive, high-performance active stages based on
general-purpose parts like the Texas Instruments TL070
and TL080 series, the National Semiconductor LF351
series, and the Signetics NE5534.
As with microphone preamplifiers, common-mode
rejection is an important specification for line receiver
inputs. The most common configuration for active
balanced line input stages used in professional audio is
the simple circuit shown in Fig. 12-60. To maintain high
common-mode rejection (CMR), the four resistors used

must match very closely. To maintain a 90 dB CMR, for

example, the resistor ratio R 1 /R 2 must match that of

R 3 /R 4 within 0.005%. The requirement for preci-

sion-matched resistors to provide high CMR drove the

development of specialized line receiver ICs.

To maintain the high CMR potential of precision

balanced line receivers, the interconnections between

stages must be made through low-resistance connec-

tions, and the impedances in both lines of the circuit

must be very nearly identical. A few ohms of contact

resistance external to the line driver and receiver (due,

for example, to oxidation or poor contact) or any imbal-

ance in the driving circuit, can significantly reduce CMR

by unbalancing the bridge circuit. The imbalance can be

at the source, in the middle at a cable junction, or near

the input of the receiving equipment. Although many

balanced line receivers provide excellent CMR under

ideal conditions, few provide the performance of a trans-

former under less-than-ideal real world circumstances.

12.3.6.2 Balanced Line Outputs

Transformers were also used in early balanced output

stages, for the same reasons as they are used in inputs.

However, to drive 600: loads, an output transformer

must have more current capacity than an input trans-

former that supports the same voltage levels. This

increased the cost of output transformers, requiring

more copper and steel than input-side transformers and

putting pressure on designers to find alternative outputs.

Early active stages were either discrete or used discrete

output transistors to boost the current available from

Figure 12-60. 1240 basic circuit. Courtesy THAT

Corporation.

Sense

Vout

Ref

Vcc

Vee

In+

In

NC

R 1 R 2

R 3 R 4

Part no. Gain R 1 , R 3 R 2 , R 4

THAT1240

THAT1243

THAT1246

0 dB

–3 dB

–6 dB

9 k 7 9 k 7

10.5 k 7 7.5 k^7

12 k 7 6 k 7