Service manual

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.

FETs Q18, Q19, Q22, Q23, Q25 and Q26 are used to connect pre fade and post fade stereo

signals to the solo busses. The FETs are controlled from the VCA fader block via the solo line

and transistor Q24. The pre fade signal is mono and derived from the left and right signals via

summing amplifier IC1.

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 LEDs 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

FETs 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.

7. Group Module (and mic input option).

The group module is divided in to 3 independent sections; aux return, aux send and audio groups.

For XL411 group modules:-

The aux return input is fed via T filters in the frame to a differential amplifier IC1. Pre-set VR1

provides CMR adjustment and capacitors C1, C2, C3 and C4 provide additional high frequency

filtering; C56 and C82 provide AC coupling.

The signal then passes to a simple shelving EQ formed by amplifier IC2 and potentiometers RV1

and RV2 before entering the VCA IC3. The VCA gain is controlled by potentiometer RV3 via

amplifier IC5. The 0dB calibration is set by pre-set VR3 and the distortion is trimmed to a mini-

mum by pre-set VR2. The VCA can also be controlled from the master vca fader busses which

are connected via switches SW4 to SW13.

For XL414 group modules:-

The aux return input is fed via T filters in the frame to a special 2 stage differential mic/line ampli-

fier which has a continuously variable gain control and is capable of boosting by up to 50dB or

attenuating by up to 30dB. Switch SW33 connects 48 volts to the input signal while C1 and C2

provide AC coupling to the low noise amplifier transistors Q37 and Q38. The common mode gain

of the transistor pre-amplifier circuit is set very low to allow high signal levels to pass without