Service manual

7
INPUT MODULE
Diagram XL301_1.DGM
The input signal enters via the relevant XLR connector on the console back panel at pins 7 and
8 on connector 'A' (ST1), where pin 7 is hot [pole 3 of the XLR in standard format], and pin
8 is cold [pole 2 of the XLR in standard format]. Input ground [pole 1 of the XLR] is on pin
9 on module connector 'A', which, on the single input version, is grounded only through to the
EARTH LIFT switch SW2. [See later section for details of the dual input version]. SW1 supplies
+48V phantom power for condenser microphones etc. via current limiting and decoupling com-
ponents R10, R11, R12, and C7 to pins 2 and 3 of the XLR connector with reference to pin 1.
When SW1 is depressed, pin 1 of the xlr connector is grounded regardless of the position of SW2
( ground lift) The MIC/LINE switch SW3 offers a balanced attenuator and impedance changing
network before the signal arrives at the balanced input stage.
In standard format, this is of an electronically balanced design, where RF decoupling is achieved
by C10, C11 and C12, input gain is adjusted by RV1, and a typical Common Mode Rejection
Ratio of 85dB at 100Hz is set by VR1. DC blocking for the input amplifiers Q1 and Q2
is provided by C8, C9, C19 and C20, and the diode bridge network D4, D5, D6 and D7 blunts
any spikes in excess of rail voltages. IC1 debalances the input signal and feeds it to the phase
reverse switch and high pass filter via C24.
If input balancing is by transformers, these will be fitted to a bracket which runs the length of
the console beneath the motherboards. They are wired via pins on connector 'A': 19 [hot,
primary], 20 [cold, primary], 21 [cold, secondary], and 22 [hot, secondary]. The transformer
secondary is RF decoupled by C15, C16 and C17. It is then fed into the same input stage as the
electronic balanced input stage built around IC1.
When the module is equipped for electronic balanced input, splash links are used in the four
E locations on the pcb. When a transformer balanced input is used, these links are in the four
T locations to select the alternative route and also adjust the gain of the input stage to match
the transformer.