Handbook for Sound Engineers

1196 Chapter 32

First is degraded noise rejection, which appeared
when solid-state differential amplifiers started replacing
input transformers. Second is the pin 1 problem that
appeared in large numbers when PC boards and plastic
connectors replaced their metal counterparts. Both prob-
lems can be avoided through proper design, of course,
but in this author’s opinion, part of the problem is that
the number of analog design engineers who truly under-
stand the underlying issues is dwindling and engi-
neering schools are steering most students into the
digital future where analog issues are largely neglected.
Other less serious problems with balanced interfaces are
caused by balanced cable construction and choices of
cable shield connections.
On the other hand, unbalanced interfaces have an
intrinsic problem that effectively limits their use to only
the most electrically benign environments. Of course,
even this problem can be solved by adding external
ground-isolation devices, but the best advice is to avoid
them whenever possible in professional systems!

32.5.1 Degraded Common-Mode Rejection

Balanced interfaces have traditionally been the hallmark
of professional sound equipment. In theory, systems
comprised of such equipment are completely noise-free.
However, an often overlooked fact is that the
common-mode rejection of a complete signal interface
does not depend solely on the receiver, but on how the
receiver interacts with the driver and the line
performing as a subsystem.
In the basic balanced interface of Fig. 32-26, the
output impedances of the driver Zo/2 and the input
impedances of the receiver Zcm effectively form the
Wheatstone bridge shown in Fig. 32-27. If the bridge is
not balanced or nulled, a portion of the ground noise
Vcm will be converted to a differential signal on the line.
This nulling of the common-mode voltage is critically

dependent on the ratio matching of the pairs of

driver/receiver common-mode impedances Rcm in the –

and + circuit branches. The balancing or nulling is unaf-

fected by impedance across the two lines, such as the

signal input impedance Zi in Fig. 32-28 or the signal

output impedance of the driver. It is the common-mode

impedances that matter!

The bridge is most sensitive to small fractional

impedance changes in one of its arms when all arms

have the same impedance.^23 It is least sensitive when the

Figure 32-25. For ungrounded equipment, interconnect cables complete a capacitive loop.

Device A Device B

Driver

Signal

Chassis

Receiver

Chassis

Signal

+ hum

Parasitic

capacitance

R

Up to 750 MA

120 Vac

N L 120 V 0 V

Neither device is grounded N L

Parasitic

capacitance

0 Vac

Figure 32-26. Simplified balanced interface.

Figure 32-27. The balanced interface is a Wheatstone

bridge.

Device A Device B

Driver

Hi Hi

Lo

Diff

amp

Ground noise

Differential

signal

Zcm

Zi

Zo /2

Lo

+

Zcm

Zo /2

Driver

Vcm

Rcm – Rcm +

Rcm – Rcm^ +

Signal

Receiver

+

+