Handbook for Sound Engineers

Sound System Design 1297

the output impedance of the mixer/amplifier can be
called the source impedance for the loudspeaker.
These four terms—source, input, output, and
load—and their relative nature are important to an
understanding of impedance and level watching. As an
example, consider a microphone whose impedance is
200 :. That impedance is actually the internal imped-
ance of the microphone and should be called the source
or output impedance of the microphone. (The micro-
phone is a source from the viewpoint of the
mixer/amplifier.)
That same microphone should probably be loaded
with an impedance of 2 k: or higher. That load imped-
ance is actually the input impedance of the mixer/ampli-
fier (the input of the mixer/amplifier is a load to the
microphone).

34.5.1.7.2 Impedance Compatibility

Impedance watching just means making sure that when
two devices are connected, they are compatible from an
impedance viewpoint, Fig. 34-61. Here are some rules
to help watch impedances.

34.5.1.7.3 Passive Devices

In the special case of a passive filter, like a loudspeaker
crossover network or a passive graphic equalizer, input
and output impedances must be matched. These devices
are the origin of the familiar term impedance matching.
Impedance matching means that if the device is a loud-
speaker crossover network and it has an 8: low-fre-
quency output impedance and an 8: high-frequency
output impedance, then it must be connected to an 8:
low-frequency loudspeaker and an 8: high-frequency
loudspeaker. Any other impedance, either higher or
lower, will degrade the performance of the crossover
network. (The input to a modern loudspeaker crossover
network is designed for the very low actual output
impedance of a modern power amplifier.)
An older passive device such as a passive graphic
equalizer has similar requirements. If such a device has

a 600: input impedance, then it must be connected to a

source impedance of exactly 600:. The same goes for

the output. If the passive graphic has a 600: output

impedance, then it must be connected to a load imped-

ance of exactly 600: to insure proper operation of the

graphic equalizer. In many cases, build-out and termina-

tion resistors must be added to match these impedances

(see Chapter 23).

34.5.1.7.4 Passive Sources

Impedance matching for a passive source, like a

dynamic microphone or guitar pickup, simply means

supplying a compatible load impedance for that device.

The device specifications should be a reasonably accu-

rate guide to the proper load impedance. A good rule of

thumb for dynamic microphones is that the microphone

load impedance (which is probably the input imped-

ance of a mixer or preamplifier) should be at least five

to ten times the microphone’s rated source impedance.

Thus, for a 150: (source impedance) microphone, the

optimum load impedance would be 750–1500: or

higher. This requirement is satisfied by the input of

almost all low-impedance mixer inputs. Note that the

Figure 34-60. Origin of terms. Courtesy Fender Musical

Instruments.

Source

Microphone

Input Output Load

Loudspeaker

Mixer/

amplifier

Figure 34-61. Impedance watching. Courtesy Fender

Musical Instruments.

B. Do not overload the outputs of an active device

like a power amplifier (impedance watching).

A. The passive crossover must be loaded with correct

impedance loudspeakers (impedance matching).

Amp Input

HF

LF

8 7High-frequency

horn driver

8 7Low-

frequency woofer

8 7Passive

loudspeaker

crossover

Power

amplifier

inputs outputs

L

R

L

R

LR

4 7 load

seen by

power

amplifier 7loudspeaker

system

7

loudspeaker

system