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jacks. Therefore if you are using the line outputs to record a program you can
mute the speakers but continue to record the input. If you are feeding another
amplifier the mute function on the UAM4SYS will silence only the local
amplifier output.
Headphone Amplifier
U11 and U12 (sheet 2) are 750mW headphone amplifiers. C45 is a power
supply filter capacitor. C33, C37, C34, and C38 are coupling capacitors. C35
and C36 are bypass capacitors. U10 is a digital potentiometer that provides a
DC level output at pin 6 that controls the gain of U11 and U12. S5 and S6
(sheet 3) are used to increase or decrease this DC level.
To increase the gain of the headphone amps S6 is pressed. This places a
ground on pin 2 of U10. This high to low transition tells the IC to increase the
output voltage (pin 6) 1/64 of the +V to –V voltage (5 volts). If S6 is held down
for more that one second the output voltage will go another 1/64 step every
100 milliseconds until it is released or the full +5 volt level is reached. The
output voltage is applied to the volume control, pin 4, of U11 and U12. The
down button S5 functions just like S6 except the voltage is decreased. The
really neat thing about U10 is that it remembers where it is set even when the
power is removed. So if you turn off the UAM4SYS and then turn it back on
the headphone level will be where you left it. Also notice that the signal to the
headphones is before the mute function, just like the line output signals, which
will be discussed in the 'Power Amp' description. This means that even
though the amplifier is muted you can still hear the input signal. Therefore you
can make sure the correct program is ready to go before you apply it to the
speakers for everybody else to hear.
The right channel audio from the Tone/Volume/Balance IC, U3, is applied to
U12 through C38. The signal is amplified and fed to headphone jack J20
through C34.
Power Amp
The Power Amp U7 a MAX9708, 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 the pulse-train of a digital signal, but a Class
D amplifier merely converts an input waveform into a continuously pulse-width
modulated (square wave) 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 mean that they are smaller and lighter in weight than an equivalent
UAM4SYS • 8
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