Hi it’s easier to quote SSLtech (Keith):
“The BIG changes to this card were brought about when SSL was being asked to supply ever-larger consoles. -From 40channels being considered 'big' in the early days, to 64 channels being considered unremarkable... the summing buses (which extend the full length of the console) were struggling to sum the signals quietly in larger frame consoles. The original, asymmetrical (single-bus, also called 'unbalanced') summing system was really being asked to do too much, and couldn't cancel induced noise to which a longer set of buses (32 track uses, six aux/cue buses, four or six master buses), and induced noise was becoming a big worry. There wasn't the ability to go balanced, because that would mean replacing every frame mother board, every routing matrix switch, and every CHANNEL mother board... clearly out of the question.
As a result, in the later 1980's SSL switched from the 'unbalanced' design to a 'quasi-balanced' design, where a convenient set of spare pins (which had been grounded for inter-channel separation) were turned into ground-compensated audio ground returns, which would then allow any induced noise to be canceled. -a few retro-fittable changes to the module, one new mother board per bucket, and some modifications to the channel amplifier card, and the hum was reduced. In smaller frame consoles, there was little need to do this mod, but all consoles from about 1988 onwards were made to the 'quasi-balanced' spec.”
