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also applied to control the oscillation via resistor R128. This is shorted out by FET Q4 if the
oscillation amplitude is too low but once the oscillation amplitude is high enough diode D23
charges capacitor C89 negative which feeds the gate of the FET, raising its impedance and allow-
ing enough negative feedback to provide controlled oscillation. Pre-set VR9 matches the circuit
voltages to the FET gate off voltage so as to set the oscillator amplitude to +14dBu. Switch SW9
is connects or disconnects the oscillator to the talk system and stops oscillation in the oscillator
when it is disconnected by charging capacitor C89 negative.
White noise is provided by IC9 and filtered by resistors R101 to R104 and C72 to C76 to produce
pink noise. This is amplified to produce 0dBu peak at the output of amplifier IC18. The signal is
connected to the talk system via switch SW8.
The external input is fed via T filters in the frame to the differential amplifier IC16. Pre-set VR8
provides CMR adjustment and capacitors C64, C65, C66 and C67 provide additional high fre-
quency filtering; C68 and C69 provide AC coupling. The external input is connected to the talk
system via the external on switch SW7. This switch also connects the talk system to the external
output via differential output amplifier IC38. Output symmetry is adjusted by pre-set VR16 and
high frequency stability into any load is assured by filter capacitors C273, C274, C275 and C277.
Further filtering is achieved by T filters in the console frame and AC coupling is via C272 and
C276.
The talk system can also be selected to route to the master left, right and mono busses and the
matrix, group and aux busses via switches SW11 to SW16. The buffer for this is formed by ampli-
fier IC3 and transistors Q5 and Q6.
The solo summing amplifiers IC24 and IC26 are used to recover voltage from the solo left, right
and PFL busses. They are referenced to a clean 0VA or noise reference which is sent from the
master module. Capacitors C103, C106 and C121 provide AC coupling and capacitor/resistor
networks C102, C105, C120, R156, R160 and R178 ensure maximum stability and high fre-
quency rejection at all gains.
The solo bus inject signals are fed via T filters in the frame to differential amplifiers IC22, IC23
and IC25. Pre-sets VR10, VR11 and VR12 provide CMR adjustment and capacitors C96, C97,
C98, C99, C108, C109, C110, C111, C114, C115, C116 and C117 provide additional high fre-
quency filtering. C100, C101, C112, C113, C118 and C119 provide AC coupling.
The amplified solo signals route to the head phone and local outputs via CMOS switch IC6. They
also route to the solo bus outputs via differential output amplifiers IC27, IC28 and IC29. Output
symmetry is adjusted by pre-sets VR13, VR14 and VR15 while high frequency stability into any
load is assured by filter capacitors C128, C129, C130, C132, C134, C135, C136, C138, C140,
C141, C142 and C144. Further filtering is achieved by T filters in the console frame and AC
coupling is via C127, C131, C133, C137, C139 and C143.
The solo switching system can operate in two ways, automatic cancelling or additive. The default
is cancelling in which case any active solo will cancel all previously active solos. When a solo is
first enabled it sends a DC pulse on the solo clear line which is received by all other solo circuits
which switches them off. A local timer inhibits input from the clear line for a short time to stop
self cancelling. The hold off time is long enough to allow both sides of stereo signals to be moni-
tored providing both solos are enabled at the same time.
Summary of Contents for XL4
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