Handbook for Sound Engineers

Tubes, Discrete Solid State Devices, and Integrated Circuits 357

resistors. In fact, while these parts are usually promoted

as input stages, they have applications to many circuits

where precise resistor ratios are required. The typical

90 dB common-mode rejection advertised by many of

these manufacturers requires ratio matching to within

0.005%.

Any resistance external to the line receiver input

appears in series with the highly matched internal resis-

tors. A basic line receiver connected to an imbalanced

circuit is shown in Fig. 12-64. Even a slight imbalance,

one as low as 10Ÿ from connector oxidation or poor

contact, can degrade common-mode rejection. Fig.

12-65 compares the reduction in CMR for low

common-mode impedance line receivers versus the

THAT1200 series or a transformer.

The degradation of common-mode rejection from

impedance imbalance comes from the relatively

low-impedance load of simple line receivers interacting

with external impedance imbalances. Since unwanted

hum and noise appear in common-mode (as the same

signal in both inputs), common-mode loading by

common-mode input impedance is often a significant

source of error. (The differential input impedance is the

load seen by differential signals; the common-mode

input impedances is the load seen by common-mode

signals.) To reduce the effect of impedance imbalance,

the common-mode input impedance, but not the differ-

ential impedance, must be made very high.

12.3.6.5 Balanced Line Receivers with the

Common-Mode Performance of a Transformer

The transformer input stage has one major advantage

over most active input stages: its common-mode input

impedance is extremely high regardless of its differen-

tial input impedance. This is because transformers offer

floating connections without any connection to ground.

Active stages, especially those made with the simple

SSM2141-type IC have common-mode input imped-

ances of approximately the same value as their differen-

tial input impedance. (Note that for simple differential

stages such as these, the common-mode and differential

input impedances are not always the same.) Op-amp

input bias current considerations generally make it diffi-

cult to use very high impedances for these simple

stages. A bigger problem is that the noise of these stages

increases with the square root of the impedances

chosen, so large input impedances inevitably cause

higher noise.

Noise and op-amp requirements led designers to

choose relatively low impedances (10 k~25 k:). Unfor-

tunately, this means these stages have relatively low

common-mode input impedance as well (20 k~50 k:).

This interacts with the common-mode output imped-

ance (also relative to ground) of the driving stage, and

added cable or connector resistance. If the driver, cable,

or connectors provide an unequal, nonzero

common-mode output impedance, the input stage

loading will upset the natural balance of any

Figure 12-63. THAT 1246 with 6 dB gain. Courtesy THAT
Corporation.

Figure 12-64. Balanced circuit with imbalance. Courtesy
THAT Corporation.

6

3 1

2

+In

–In

12k 6k 5

12k 6k

U1

1246

Output

7

4

Sense

Vout

Ref

VCC

In+ VEE

In–

Rimbalance

+

• Vout

Vin+

Vin–

R 1 R 2

R 3 R 4

Vdiff

2

Vdiff

2

+

Figure 12-65. CMR imbalance versus source. Courtesy

THAT Corporation.

90

100

80

70

60

Common mode rejection ratio–dB

50

40

30

20

0 100 200

Differential source resistance error–Ohms

300 400 500 600

Common Mode Rejection versus Imbalance

InGenius Line Receiver

Conventional Line Receiver