I'm interested in analog summing - and knowing me - I'd prefer to DIY something simple than spend thousands on a brand name box with bells and whistles I don't need and can't afford. I realize an active gain stage will be needed for make-up gain - and I have plenty of nice Preamps that will easily fit the bill.
I'm looking to make a simple DB25-to-L/R XLR passive (resistive) summing cable that will take the 8 Outputs from a DA converter (RME ADI-8 or SSL AlphaLink) and sum them to the stereo pair of XLR's - the XLR's will then feed a high-quality Mic Preamp for make-up gain and return to the AD Converter to be re-recorded as the Main Mix. I won't need any Mono/Stereo switches or expensive trim-pot controls - just a fixed level summing network for 1+3+5+7 to Left XLR, and 2+4+6+8 to Right XLR.
I'd also like to see how much more difficult it would be to expand to 16 channels (2x DB25's). I'm assuming the resistor values would change, but that's probably all that would be different aside from more soldering.
So - what is a good starting point (and ballpark resistor values for a professional Line Level DAC)? Is it as simple as Line Level Inputs > Resistors in series > tie resistor outputs together and done? Or would this involve something a bit more involved than that (like multiple H-Pads summed together)?
I'm betting this can be crammed into an adapter cable opposed to building something in a project box / rack chassis to save space. I don't see why this would sound any different from any of the $,$$$ summing boxes that have nice gain-stages built in?
Any bright ideas on where to start? Thanks!