A Switching Amplifier By Any Name - Audio Precision AUX-0025 User Manual

Chapter 1: Introduction
audio signal, the ranging functions of an audio analyzer may respond to
the carrier rather than to the audio signal, reducing the accuracy of the
measurements.
accurately measure the output of such an amplifier is to insert a care-
fully-designed low-pass filter between the output of the device under test
(DUT) and the analyzer input to reduce amplitude of the switching carrier
before the signal is ranged. The Audio Precision AUX-0025 Switching Am-
plifier Measurement Filter fulfills this requirement.
ing carrier outputs as switching amplifiers; in other literature the term
switchmode amplifier may be used. These devices include Class D, Class I
and Class S amplifiers and also Class T amplifiers and "digital amplifiers."
pulse width modulation (PWM). (Class T is a variation on this, adding a dy-
namic modulation of the carrier frequency and other signal processing).
subwoofer amplifiers) may use a carrier frequency as low as 80 kHz.
Full-range amplifiers have higher carriers, up to 1.5 MHz or more.
pend upon the inductance and mass of the loudspeaker to integrate the sig-
nal, reproducing the audio but not the inaudible carrier. Other amplifiers
include an output low-pass filter, which reduces EMI and aids the loud-
speaker in integrating the signal, but which is generally not sufficient for ac-
curate measurement by an external analyzer.
2
The best solution in using a broad range, broad bandwidth analyzer to

A Switching Amplifier by any name...

In this document we will refer to audio amplifiers with modulated switch-
Generally, switching amplifiers impose the audio signal on the carrier by
Figure 2. A diagram of a sine wave and a pulse width modulated (PWM) switching carrier
modulated by the same wave.
Switching amplifiers designed for a limited bandwidth (such as
Some switching amplifiers provide no filtering at their output and de-
A Switching Amplifier by any name...
AUX-0025 User's Manual