1186 Chapter 32
conductors is essential for its operation. Like many
others, it describes a balanced interface in terms of
signal symmetry but never mentions impedances! Even
the BBC test for output balance is actually a test for
signal symmetry.^4 The idea that balanced interface is
somehow defined by signal symmetry is simply wrong!
It has apparently led some designers, mostly of exotic
audiophile gear, to dispense with a differential amplifier
input stage in their push-pull amplifiers. They simply
amplify the (assumed) symmetrical input signals in two
identical, ground-referenced amplifier chains. No mech-
anism exists to reject common-mode voltage (noise and
interference) and it is actually amplified along with the
signal, creating potentially serious problems. Rejection
of common-mode voltages is the single most important
function of a balanced receiver.
In an unbalanced circuit, one signal conductor is
grounded (near-zero impedance) and the other has some
higher impedance. As we will discuss in Section 32.5.4,
the fact that not only signal but ground noise currents
flow and cause voltage drops in the grounded conductor
makes an unbalanced interface inherently susceptible to
a variety of noise problems.
32.3.2 Voltage Dividers and Impedance Matching
Every driver has an internal impedance, measured in
ohms, called its output impedance. Actual output imped-
ance is important, as we discuss below, but often absent
from equipment specifications. Sometimes, especially
for consumer gear, the only impedance associated with
an output is listed as recommended load impedance.
While useful if listed in addition to output impedance, it
is not what we need to know! A perfect driver would
have a zero output impedance but, in practical circuit
designs, it’s neither possible nor necessary. Every
receiver has an internal impedance, measured in ohms,
called its input impedance. A perfect receiver would
have an infinite input impedance but again, in practical
circuit designs, it’s neither possible nor necessary.
Figs. 32-8 and 32-9 illustrate ideal interfaces. The
triangles represent ideal amplifiers having infinite
impedance input—i.e., draw no current—and zero
impedance output—i.e., deliver unlimited current—and
the line conductors have no resistance, capacitance, or
inductance. The signal voltage from the driver amplifier
causes current flow through the driver output imped-
ance(s) Zo, the line, and receiver input impedance Zi.
Note that the output impedance of the balanced driver is
split into two equal parts. Because current is the same in
all parts of a series circuit and voltage drops are propor-
tional to impedances, this circuit is called a voltage
divider.
The goal of an interface is, with rare exception, to
deliver maximum signal voltage from the output of one
device to the input of another. Making Zi much larger
than Zo assures that most of the signal voltage is deliv-
ered to the receiver and very little is lost in the driver. In
typical devices, Zo ranges from 30: to 1 k: and Zi
ranges from 10 k: to 100 k:, which transfers
90–99.9% of the available—i.e., unloaded or open
circuit—signal voltage.Matching is a term that often causes confusion. A
little math and Ohm’s Law will prove that when Zo and
Zi are equal, maximum power is transferred from source
to load, although half the signal voltage is lost. If trans-
mission line effects apply, Zo and Zi must terminate or
match the characteristic impedance of the line to
prevent reflection artifacts. Although modern audio
systems seldom use cables long enough for transmission
line effects to apply or benefit from maximum power
transfer, early telephone systems did both. Telephone
systems began by using miles of existing open wire
telegraph lines that, due to their wire size and spacing,
had a characteristic impedance of 600:. Since ampli-
fiers didn’t yet exist, the system was entirely passive
and needed to transfer maximum power from one phone
to another. Therefore, transformers, filters, and other
components were designed for 600: impedances to
match the lines. These components were eventually
incorporated into early sound reinforcement, radio, and
recording systems. And the 600: legacy still lives on,
even though modern requirements for it are all but
extinct.
Sometimes, instead of meaning equal, matching is
used to mean optimizing some aspect of circuit perfor-
mance. For example, the output transformer in a
vacuum-tube power amplifier is used to optimize power
output by converting or impedance matching the
low-impedance loudspeaker to a higher impedance that
suits the characteristics of the tubes. Similarly, the
modern technique of making Zi much larger than Zo to
transfer maximum voltage in signal interfaces is often
referred to as voltage matching. It uses 10 k: or higher
input impedances, called bridging because many inputsFigure 32-9. Basic unbalanced interface.Device A Device BSignal Out In SignalZo
Zi