User Guide
23
Channel Module [Diagram XL88_1,2 & 4]
Eight identical channel modules are fitted. These each consist of two pcbs inter-
connected by a 20 way IDC cable and a single screened channel assignment jumper.
The lower (main) pcb carries all the input and output circuitry with the smaller top
pcb holding the matrix mix controls and pfl routing.
The channel input signal appears at XLR socket ST1 pin 3 [hot], 2 [cold] and 1
[gnd] on the main channel pcb (B2700). If no input transformer is fitted, links L1
& L2 are used to bypass the transformer option and the transformers Zobel net-
work components, R9, R10 & C8 are omitted. First order R.F. filtering is provided
by R1/C3 and R6/C6, and over-voltage protection by D1/D2 and D3/D4. IC1
operates as a differential amplifier whose common mode rejection is set by VR1
at the factory for maximum rejection [typically 85dB] at 100Hz. The signal then
passes through C10 to the Direct output level control and, via the connector ST6
pins 3&4 to the matrix mix level controls on the second pcb.
From the direct output level potentiometer RV1 the signal goes to the non-invert-
ing buffer IC2b, and then to the Main mute switch SW1 as well as to the meter
changeover switch SW3. After leaving the mute switch the signal is again buffered
and fed to IC3 the balancing network for the direct output which appears at
connector ST3 pins 3 [hot] and 2 [cold],the output is protected from excess
voltage by the diode bridge D5-8, and set for maximum common mode rejection
at 100Hz by VR2 [typically 85dB]. A pin header (ST2) is provided to allow the
retrofit of an output balancing transformer. When no transformer is used links are
inserted between pins 1&2 and 5&6.
The Zobel network R43/C47 is only fitted when a transformer is incorporated.
The matrix mix level controls on the second pcb (B2701) are each preceded by an
on/off switch with associated led indicator. The wipers of the level controls feed
the eight common mixing busses via resistors. One buss is jumped from the channel
assignment connector ST3/4 back to the main channel board to allocate the group
matrix section. The mix group selected will be according to the module location
within the main frame.
Group matrix output.
The jumpered input from the matrix mix board feeds a virtual earth summing stage
[IC4b] before being passed to the main matrix level fader. After buffering [IC5b],
the signal is fed via ST7 to the Matrix mute switch SW2 as well as to the meter
changeover switch SW3. After leaving the mute switch the signal is again buffered
and fed to IC6 the balancing network for the matrix output which appears at
connector ST5 pins 3 [hot] and 2 [cold],the output is protected from excess
voltage by the diode bridge D9-12, and set for maximum A pin header (ST4) is
provided to allow the retrofit of an output balancing transformer. When no trans-
former is used links are inserted between pins 1&2 and 5&6.