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U5A is a flip flop set up in the toggle mode. Its outputs at pin 6, called 'Q', and
pin 7, called 'Q not' indicated by a line over the Q symbol, are opposite DC
levels of either +5 volts or 0 volts. R27 is a pull up resistor for the flip flop's
set line. C32 delays the +5 volts to U5A's set line ensuring it comes up in the
set condition, which is +5V on output 'Q' at pin 6 and 0V on pin 7, 'Q not',
when power is first turned on. When S4 is pressed it applies a ground to C31
(sheet 2) causing C31 to discharge. When the button is released the ground
is removed from C31 and it charges through R38 to +5 volts. This positive
transition causes U5A to change states, (flip flop). An interesting thing to
know is when a switch opens or closes it usually has problems and may
actually open and close many times before finally ending up in it's final state.
This is called switch bounce and is actually 'seen' by logic circuits like U5 and
causes unpredictable results. C31 and R38 help prevent these 'bounces' from
getting to U5 and is called de-bouncing.
U6 is called an analog switch. It contains 4 electronic switches, we'll only be
using 2 of them that are turned on and off by signals applied to pins 7 and 13.
With +5 volts on these pins it's associated switch is turned on and 0 volts
turns it off. With U5 Pin 6 at +5 volts U6 pin 13 is also at +5 volts. This
causes the switch between U6 pins 1 and 2 to close. At the same time U5 pin
7 is at 0V. This applies 0V to U6 pin 5 which causes the switch between U6
pin 3 and 4 to open. This turns off the Loudness function as mentioned
above. When S4 is pressed and released, U5A toggles causing pin 6 to
change to 0V and pin 7 to change to +5V. This places 0V on U6 pin 13
opening the switch between pins 1 and 2. It also places +5 V on U6 pin 5
closing the switch between pins 3 and 4 and turns off the loudness function.
Since pin 7 is +5V, power is applied to the loudness LED on the front panel
(sheet 3) through current limiting resistor R28 which lights the LED indicating
the UAM4SYS is muted.
Line Output Buffer
With headers H1 and H2 the user can select either processed (from U3) or
unprocessed (directly from input buffers U1A and U1B) audio to be sent to the
line outputs. R4 and R30 set the bias to ½ supply voltage (+5 volts) while
isolating the bias voltage from the power supply. R32 and R33 are pull-up
resistors which prevent oscillations that can be caused by external load
capacitance. C13, C14, C17, and C30 are coupling capacitors. The selected
right channel audio signal enters through the header pin 2 and is applied to
the op amp buffer through C14. The output at U1 pin 8 is applied to the right
line output RCA connector through C17. The left channel is identical to the
right.
Notice that the signal to the line output buffer is before the mute function
which will be discussed in the 'Power Amp' description. This means that even
though the amplifier is muted the input signal will be present on the line output
UAM4SYS • 7
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