Don Pearce wrote in message news:498105de.430631890@localhost...

Circuit diagram here - it doesn't matter how many inputs you plug in
as long as the op amps are all there. The gain will stay constant.

http://81.174.169.10/odds/multi_cable.pdf

The best solution to the OPs problem so far. But he does need to know in
advance how many inputs he wants. I got the impression he wanted to be able
to add, or subtract, inputs at will without causing any change in gain,
which, of course, he can't do with that topology.

As usual, ignore the Australian.

Absolutely!

David.