done more reading, on analogue mixing circuits...
turns out that professional analogue mixers, ( mega expensive mixing desks from the 70's in large studios etc ) use balanced mix buses. they don't go near single ended passive mixing ( a bunch of resistors dividing the input channels up equally and pulled back up to the normal level with an op amp...this passive mixing idea seams to be everywhere on the internet and passed off as THE mixer example and yet it was never really used in any mixing desks ( even the cheap rubbish ones !! )
why? because its really really rubbish !!! turns out the input mix effects the overall output volume.. if you had 8 channels and suddenly mixed down channels 2 - 7 then channel 1 gets louder on its own !!! rubbish :) why ? because the resistors just divide the voltages up and the OPAMP is at a fixed recovery gain..
So, I need to use an active mixer design with a virtual earth or even better a BALANCED MIX BUSS !! since i'm trying to make
a high quality mixer then easy , download a schematic of a balanced mix bus from the 70's and make it again with better new parts?
BUT , i can't find one !! and a virtual earth mixer in a +/-15v opamp with balanced audio ? how does that work ?
I have some +/- 15v power supplies in the post and some really nice differential line driver op amps.
I will need these regardless for professional balanced inputs / output
starting to think the 8 - 1 mix down in analogue was a BAD idea !! I feel like I'm trying to re-invent the steam engine and theirs nobody left alive that knows how they were built. or more likely, I'm just not clever enough to know where to start !
further reading here ! http://sound.westhost.com/articles/audio-mixing.htm#s3