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UAM4SYS Theory of Operation
Note: There are four pages of schematics included in this manual. Please
refer to them as directed as you read the following.
Power Supply
Since electronic equipment doesn't work without power, lets start off by
describing the UAM4SYS power supply. For reference when reading this
section, check schematic sheets 1 and 2 (pages 38 & 39). The UAM4SYS
requires a 12 to 18 volt DC input at 3.3 amps applied to Power jack J5. From
there the power goes two directions. First, to VR1, a 7810 positive 10 volt 1
amp regulator. C28 filters the regulator's output. 10 Volts is used to power
most of the circuitry you see in schematic 1, as well as the 5 volt power
supply on schematic sheet 2. Since the regulator is before the power switch,
all circuits powered by the 10 volt are powered as soon as the unit is plugged
in. On schematic sheet 2, regulator VR2, a 7805 positive 5 volt 1 amp
regulator that powers circuitry on schematic sheets 1, 2, and 4. C39 filters
VR2's 5 volt output.
Turn back to schematic sheet 1. The second place the input power is routed
to the power switch, S1. When this switch is turned on, power is applied to
the power amp IC, U7 on schematic page 4. It is also applied to R29 through
J6 pin 10 to J40 pin 1 on the front panel (schematic sheet 3). This causes
D4, the power indicator LED, to light.
An interesting note is that because the 10 volt and 5 volt circuits are active
whenever the power supply is plugged in, you can listen to the input signal
on the headphones and the line outputs will be active even though the power
switch is off. The operation of these sections is discussed in detail in the
following sections. I'm sure you can think of some interesting applications for
this 'feature!'
Input Buffer
IC U1 contains 4 operational amplifiers, (op amp). U1A and U1B are set up
as a buffer with unity gain. C5, C10, C11, and C12 are input coupling
capacitors. They provide DC isolation between the input audio device and
the UAM4SYS. R11 and R14 provide a 5 volt bias to the op amp buffers.
Because the input can be a balanced signal and there is negative feedback
applied to the '-' input of U1A and U1B, R1, R15, R23 and R24 are
necessary to isolate that feedback signal from the applied input signal. A
resistor is needed on both the '+' and '-' inputs because the signal is
balanced. C47 provides filtering of any signal that might try to sneak thru to
the power supply and therefore assures a stable reference bias on the op
amp input.
UAM4SYS • 4
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