Ramsey Electronics UAM4 Manual page 5

frequency is actually changed around a center frequency, in this case +- 4%
of 200kHz. Using this method the MAX9708 is able to reduce the noise
generated by typical class D amplifiers and as such, the need for large filters
described above is minimized. How this is accomplished is beyond the
scope of this manual, but interested readers can find information on the web
by searching for 'spread spectrum' and checking the Maxim web site at
"www.maxim-ic.com" for the MAX9708 data sheet.
The main advantage of a class D amplifier is power efficiency. Because the
output pulses have a fixed amplitude, output devices (MOSFETS, in the case
of the MAX9708) are switched either on or off, rather than operated in linear
mode. This means that very little power is dissipated by the transistors,
except during the very short interval between the on and off states. The
MAX9708 is capable of achieving up to 87% efficiency. The wasted power,
the remaining 13%, is low because the instantaneous power dissipated in the
transistor is the product of voltage and current, and one or the other is almost
always close to zero. The lower losses permit the use of a smaller heat sink
while the power supply requirements are lessened too.
A TOUR OF THE UAM4
Now let's take a quick tour of the signal path from input to output. Looking at
the schematic (pg. 18) you will notice a large rectangle in the middle. This
represents the MAX9708 and aside from a transistor, LED and zener diode, it
is the major active component in the UAM4. The input signal(s) are applied
to the left and right inputs at the right side of the schematic. These inputs
have the advantage of being able operate in either balanced or un-balanced
modes. Read on for explanations of unbalanced vs. balanced signals.
Unbalanced Signals
Most home entertainment audio devices (at least the ones that don't use
optical connections) are using unbalanced signals. Unbalanced means there
is a signal line for each signal (or channel), and a common ground for them
all. This is fine for consumer audio, but sometimes problems with noise on
the signal and/or ground loops can occur. There is only one signal line
available, so any noise that manages to get onto that line is amplified and
passed to the output, in this case the speakers.
Balanced Signals
In the case of a balanced input signal, each input actually has two signal lines
which are opposite in phase or polarity. That means that when one line
increases the other decreases. The amplifier input uses both of these signals
by amplifying the difference between them to produce an output. If the signal
on both lines is the same the amplifier essentially doesn't have anything to
amplify. The advantage of this is that it is very unlikely that any noise on both
lines would be out of phase and therefore the amplifier will essentially reject
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