The HF card is biased. If you look at the caps and resistor values and the Pin Numbers 3 and 9 they are +/- 18 volts. That's why they are not on the original schematic. On a standard non biased card you would have a single cap for each pair and the 4.7 Megs R107, R108 wouldn't be there. Just think of a Charge Coupled network.
Rob