CMOS switches U103B and U103C, also working in tandem, route the Ax
Sidechain signal to Summing Stage whenever the Ax IN/OUT button is
pressed IN. Logic ground is routed via SW402 (see Control Board schematic)
to H101 pin 6 causing U103C's switch to open and U103B's switch to close
through the use of logic invertor U301 A. When Ax IN/OUT is OUT, the
above logic states flip and the Ax Sidechain signal is disconnected from
U104A.
6.1.5 Summing Stage
U104A is a summing amplifier, while U104B is a unity-gain inverting ampli
fier to provide differential output for the servo balanced output stage. The
audio signal is coupled to this last stage via capacitors 0 1 6 and 0 1 7 .
6.1.6 Servo Balanced Output Stage
The Servo Balanced Output Stage has two servos, a Cross-Coupling Servo
and a DC Servo, within the output circuitry. U106A and U106B are balanced
high pass output amplifiers, having a time constant (R129/C120 or R137/
0 2 3 ) that defines a pass frequency of less than 1 Hz.
The differential signal from the switching stage is routed as two equal,
opposite-polarity, signals to respective minus and plus input terminals of
U106A and U106B. This configuration creates two equal, opposite-polarity
signals as the final balanced output signal. In addition, the outputs from the
differential amplifiers are cross-coupled back to each others'open input
terminals, to provide AC servo protection for shorted or unbalanced connec
tions present at an output XLR. VR102 is a trimming resistor used to adjust
the AC voltage leaving the differential output stage. TP101 is provided as a
test point for this balancing procedure (see Section 7.0).
U105A and U105B are output monitors that provide a DC servo error signal
back to U106A and U106B inputs, to correct for any DC offset from the
output amplifiers. This circuit keeps the balanced output signal at 0 VDC
without the use of any output coupling capacitors.
6.1.7 Relay Bypass
Relay K101 is a 5 VDC relay that switches in a hard wire bypass between the
input and output XLR's whenever the relay coil is not powered. When the
RELAY Button, SW405 (shown on the Control Board schematic), is pressed to
indicate off (button out), a ground path through H101-7 and D103 activates
the relay coil, causing its switches to connect the differential output stage to
the output XLR connector. When a properly designed remote RELAY
switch is connected to pin 3 of the J301 REMOTE CONTROL port (located on
back panel), a ground path through D102 provides a similar action. Q103
controls the LED indicator (Control Board) to provide visual confirmation of
the BYPASS mode. D301 and D302 provide voltage regulation for proper
shutoff of Q103 whenever a remote RELAY switch is used.
6-4
Circuit Description
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