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increase current drive capabilities of the buffers when required. Crosstalk and noise are kept to a
minimum within the module and the console bus system by use of an active ground reference
system. A clean 0VA or noise reference is sent from the master module to all the input modules.
This is buffered by IC27 which provides the active ground reference which is summed into all the
bus send circuits and provides a clean earth for all the aux output potentiometers. VR5 adjusts the
DC offset on the active earth to a minimum. This is necessary to reduce transient noise as signals
are switched off or on to a bus.
FETs Q18, Q19, Q22, Q23, Q25 and Q26 are used to connect pre fade and post fade stereo
signals to the solo busses. The FETs are controlled from the VCA fader block via the solo line
and transistor Q24.
Switches SW7 to SW21 are used to connect pre fade or post fade signals to the aux send
potentiometers RV15 to RV34. These provide adjustment of the current fed on to the bus via fet
hybrids HA1, HA2 and HA3. The hybrids provide signal on of switching and are located directly
above the buss connectors so as to minimise the bus length and crosstalk. They are controlled by
automation system via shift registers IC29, IC30 and IC31, and voltage translating hybrids HB3
to HB7. Status LEDs for the aux busses and other automated functions are arranged in constant
current chains and are controlled from the shift registers via led hybrids HC1 to HC6.
The group busses are controlled in a similar way from shift register IC28 via hybrids HB1, HB2,
HA4 and HA5. The master buss is controlled from shift register IC31 via Transistor Q33 and
FETs Q29, Q30, Q31 and Q32.
All bus switching is directly controlled from the automation system in the fader tray via data,
strobe and clock lines 1 and 2. The status of switch SW27 to SW49 is monitored by the automa-
tion system via resistor ladders R250 to R272 and analogue to digital converter lines 1,2 and 3.
Positive and negative references for the resistor ladders are provided directly from the fader tray
via +5V switch and 0V switch lines.
5. Stereo Input Pod (option).
The stereo input pod has two mic/line level inputs, a gain balance control, hi pass filters, direct
outputs and a two 11 led meters.
The mic input signals pass through T filters L1, L2, L3 and L4, to the switchable 25dB pads
formed by R9, R10, R11, R42, R43 , R44 and switch SW2. Pre-sets VR1 and VR3 set the pad
CMR to a maximum. Switch SW1 connects 48 volt phantom power to the inputs and AC cou-
pling capacitors C3, C4, C21 and C22 connect the input signals on to the bases of low noise
transistors Q2, Q3, Q8 and Q9. Further high frequency filtering is provided by capacitors C5, C8,
C9, C23, C26 and C27.
Transistors Q2, Q3, Q8 and Q9 provide amplification of the signals and their collectors form an
output to IC1 which functions as a comparitor. This gives buffered outputs to the following
circuits and ensures the amplification transistors are operating at constant current by providing
feedback via transistors Q4, Q5, Q10 and Q11. These transistors form a voltage to current con-
verters and phase splitters in conjunction with the constant current sources formed by Q6, Q7,
Q12 and Q13. By providing this balanced current feedback to the amplifier transistors the com-
mon mode load resistance seen by their emitters is infinity and the common mode gains of the
overall circuits are zero. The differential loads are set by the potentiometer VR1 which adjusts the
Summary of Contents for XL4
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