Audio Precision AUX Series User Manual And Specifications page 12

Chapter : Introduction
input to reduce amplitude of the switching carrier before the
signal is ranged. The Audio Precision AUX-0025, AUX-
0040 and AUX-0100 family of switching amplifier mea-
surement filters fulfills this requirement.
A switching amplifier by any name...
In this document we will refer to audio amplifiers with
modulated switching carrier outputs as switching amplifi-
ers; 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 amplifi-
ers."
Generally, switching amplifiers impose the audio signal on
the carrier by pulse width modulation (PWM). (Class T is a
variation on this, adding a dynamic modulation of the car-
rier frequency and other signal processing.)
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 subwoofer amplifiers) may use a carrier frequency
as low as 80 kHz. Full-range amplifiers have higher carri-
ers, up to 1.5 MHz or more.
2
Some switching amplifiers provide no filtering at their out-
put and depend upon the inductance and mass of the loud-
speaker to integrate the signal, 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 accurate measurement by an external analyzer.
Features of the filters
Each channel of an AUX-0025/0040/0100 is a passive low-
pass filter specifically designed to minimize switching
amplifier carrier components while passing a broad audio
spectrum. The filter provides the signal preconditioning
necessary to accurately measure switching amplifier out-
puts using a wide-range audio analyzer.
Passive design
For this application, a passive filter was determined to be
the best approach. An active filter would require input
attenuation and variable gain to accommodate the wide
range of signal amplitudes that might be applied, adding
noise and distortion to the signal.
Inductors
Custom inductors were specified with an emphasis on
power handling and minimizing low-frequency distortion
while satisfying the filter response requirements.
AUX-0025 / 0040 / 0100 Measurement Filters: Introduction