16
INPUT SOLO LOGIC
At any input, depressing the solo switch connects 5V to the solo LED and to the solo
control bus
SOLO-CNTL-IP
via a 4148 diode. The cathode of the solo LED connects to the
flash bus
FLASH
via a 220-ohm current-limiting resistor. The flash bus is a current sink and
only operates when an input has been soloed.
U1A is an LM339 comparator wired as an inverting buffer. Its output is wire-OR connected
to the output of U1C, another LM339 comparator, allowing U1C to short U1A to ground.
(note: unlike a standard op-amp used as a comparator the LM339 has a open collector
output and switches only to its negative supply. In this case, ground.) When
SOLO-CNTL-
IP
is higher than the solo reference voltage (1V, ref), U1C goes high, allowing U1A to pass
the solo clock. R70 and C157 form a lowpass filter which removes any extraneous “junk”
from the signal.
U1A drives power darlington transistor Q1202 through a shaping network consisting of
R1286, C1228, and R1287. Q1202’s collector is distributed to all input solo switches via the
FLASH
bus, acting as a current sink for all input solo LEDs.
Comparator U1D’s inverting input is wired in parallel with U1C. U1D’s output goes low
whenever
SOLO-CNTL-IP
is high, controlling analog switches U1211D, U1211C, U3C, U3D,
and U3A. U1211 delivers input solo signals to the headphone system. U3 delivers input
solo signals to the LCR meters.
OUTPUT SOLO LOGIC
At any output, depressing the solo switch connects 5V to the solo LED and to the solo
control bus
SOLO-CNTL-OP
via a 4148 diode. The cathode of the solo LED connects to
the flash bus
FLASH-OP
via a 220-ohm current-limiting resistor. The flash bus is a current
sink and only operates when an output has been soloed.
U1B is an LM339 comparator wired as an inverting buffer. Its output is wire-OR connected
to U2B, another LM339 comparator, allowing U2B to short U1B to ground. When
SOLO-
CNTL-OP
is higher than the solo reference voltage (1V, ref), U2B goes high allowing U1B to
pass the solo clock. R75 and C159 form a lowpass filter which removes any extraneous
“junk” from the signal.
U1B drives power darlington transistor Q1 through a shaping network consisting of R43,
C150, and R44. Q1’s collector is distributed to all output solo switches via the
FLASH-OP
bus, acting as a current sink for all output solo LED’s.
Comparator U2C’s non-inverting input is wired in parallel with U2B’s. U2C’s output goes
high whenever is high. U2C’s output controls analog switches U1211A, U1211B, U4A, U4B,
and U4C. U1211 delivers output solo signals to the headphone system. U4 delivers output
solo signals to the LCR meters.
