Impedance - Toa A-503A Manual

Impedance

Impedance refers to the way a device reacts to the application of electric current. The device will
exhibit varying amounts of resistance and either capacitance or inductance. For our purposes, the
resistance is most important. In keeping with common practice, when we say "impedance" we will
mean resistance.
Impedance, in this sense, refers to how much resistance
the device presents to the free flow of electricity
through it. At a given drive voltage, the lower the
impedance of the receiving device, the higher will be
the current flow through it. This is important to know
when working with amplifiers, because if the load
impedance presented by the speakers is too low, it may
draw so much current that the amplifier will overwork
itself and deliver distorted sound, overheat — perhaps
even burn out.
Impedance is measured in ohms, named for Georg
Ohm, who first described the set of electrical relation-
ships now known as Ohm's Law (see fig. 6). Every device
will have both an input impedance (also called the load
impedance) and an output impedance (also called the
source impedance). The input impedance of an amplifier
could range from 600 ohms to 10,000 ohms, or even
higher (see side bar). A typical speaker impedance may range from 4 to 16 ohms.
R = Resistance in Ohms
W = Power in Watts
V = Electromotive Force in Volts
I = Current in Amperes
Figure 6
2 2
V V
R
W
W
2
I
V
I
W =
R =
V = I =
W x R
W
I
W
I x R
R
Ohm's Law
10
TOA Electronics Amplifier Guide
Impedance "Matching"
A common point of confusion is the concept of
"impedance matching." Transmission line theory
states that the load impedance and source
impedance should be equal, to avoid reflec-
tions in the line. But this requirement holds
only when the line is longer than the shortest
wavelength of the signal. For audio frequen-
cies, the line would need to be over 9 miles long
for transmission line theory to apply. When
using solid-state equipment and typical cable
runs of several hundred feet or less, the best
performance is obtained when the load imped-
ance is about 5 to 20 times greater than the
source impedance. So, for example, a 10,000
ohm input is a good "match" for a 600 ohm out-
put.
2
I x R
V x I
V
R
W
V