Introduction To The Uam4; How It Works - Ramsey Electronics UAM4 Manual

INTRODUCTION
The UAM4 is a state of the art stereo audio amplifier with a maximum output
of 40 watts, 20 watts per channel in stereo mode, or 40 watts in mono mode.
The UAM4 is designed to be configurable in a number of different ways to
suit your requirements and allow you to experiment and learn about the
capabilities of the Maxim MAX9708 amplifier integrated circuit. After you
finish this kit, all you need to do is supply power, an audio signal and
speakers and you are ready to go!

HOW IT WORKS

The UAM4 is a class D amplifier. The letter D is simply the next letter after
C, and does not stand for digital. Class D and Class E amplifiers are
sometimes mistakenly described as "digital" because the output waveform
superficially resembles a pulse-train of a digital signal, but a Class D
amplifier merely converts an input waveform into a continuously pulse-width
modulated (square wave) analog signal. (A digital waveform would be pulse-
code modulated.)
Class D amplifiers are much more efficient than Class AB power amplifiers.
As such, Class D amplifiers do not need large transformers and heavy heat
sinks, which means that they are smaller and lighter in weight than an
equivalent Class AB amplifier. All power devices in a Class D amplifier are
operated in on/off mode. The term usually applies to devices intended to
reproduce signals with a bandwidth well below the pulse switching frequency
which is the case of the UAM4 is about 200kHz..
The heart of the UAM4 is the Maxim MAX9708 amplifier IC. The MAX9708
converts the input signal to a sequence of pulses whose averaged value is
directly proportional to the instantaneous amplitude of the signal. The
frequency of the pulses is typically ten or more times the highest frequency of
interest in the input signal. The pulse frequency in the UAM4 is around
200kHz or 10 times 20kHz. The output of such an amplifier contains
unwanted spectral components (that is, the pulse frequency and its
harmonics) which must be removed by a passive filter. The resulting filtered
signal is then an amplified replica of the input. For the UAM4 the filtering is
handled by the patented technology of the Maxim MAX9807 amplifier chip
and the speaker. The design uses a unique filterless modulation and spread-
spectrum switching mode. This eliminates the need for large inductors and
capacitors on the output.
Typical class D amplifiers use a simple modulation method called 'Fixed-
Frequency Modulation' (FFM). This simply means that a switching frequency
is selected and that is the frequency at which the output is switched. The
MAX9708 can use this method, but it also has a special method called
'Spread-Spectrum Modulation' (SSM). With this method the switching
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