26
gain of the circuits by 55dB. It does this by effectively shorting out the negative current feedback
to greater or lesser extents. High frequency stability and constant bandwidth at all gains is assured
by capacitors C10, C14, C15, C28, C32, C33 and C34.
The nominal differential gain of the mic amp is 15dB to 70dB. If the transformer option is fitted
the transformer (1:5) provides 14 dB of gain and the active circuit needs to be adjusted to provide
the remaining 1dB to 56dB of gain and present a much higher impedance for the transformer
output. This is achieved by component changes as marked on the schematic.
The mic amplifier outputs feed the balance pan circuits formed by amplifier IC2 and
potentiometer RV2. This provides up to 10dB of reciprocal boost and cut.
The balance pan outputs are then fed to the frequency adjustable hi pass filters. These are second
order active networks formed around amplifier IC3 and adjusted by potentiometer RV3.
The filter outputs feed the phase change circuits which are formed by amplifier IC4. These oper-
ate in non inverting mode until switch SW4 or SW5 are activated. When this is done FET Q14 or
Q15 are switched on causing the appropriate amplifier to operate in inverting mode.
CMOS switch IC6 selects which signals are feed to the 11 led meters in conjunction with transis-
tors Q16, Q17 and Q18. If the meter pre change over line is logic high the meters monitor the mic
amp output and if it is logic is low the meters monitors the input module signals via the direct
output lines.
The meter circuits are formed by amplifier IC8 which is configured as two peak responding half
wave rectifiers and meter drive IC9 and IC10. 0dB adjustment is via pre-sets VR6 and VR9.
Comparitor IC11 is used in conjunction with transistors Q22 and Q23 to provide a signal present
led drive at -25dBu
The direct output signals from the input module fed to the differential output amplifiers formed by
IC5 and IC7 via links on socket SK7, SK8, SK9 and SK10. These can select the direct output
source from the input module or direct from the mic amplifier outputs. Output amplifier symmetry
is adjusted by pre-sets VR5 and VR8 while high frequency stability into any load is assured by
filter capacitors C53, C54, C55, C58, C71, C72, C73 and C76. Further filtering is achieved by T
filters L5, L6, L7, L8 and AC coupling is via C56, C57, C74 and C75.
6. Stereo Input Module (option).
The stereo input module houses the analogue insert, EQ, VCA, mute, pan and bus feed circuits
along with logic circuits to control routing and other major functions.
Audio signals are received from the stereo input pod and fed to the insert and EQ circuits via
sockets ST1 and ST7. These can be used to connects the module to the test input bus: this is for
factory or service use only and both sockets should have their links set to the normal position at
all other times.
The insert send circuits are formed by the differential output amplifiers IC2 and IC4. Output
symmetry is adjusted by pre-sets VR1 and VR6 whilst high frequency stability into any load is
assured by filter capacitors C3, C4, C5, C6, C17, C18, C19 and C20. Further filtering is achieved
by T filters in the console frame and AC coupling is via C7, C8, C21 and C22.
Summary of Contents for XL4
Page 2: ...2 ...
