Hi,
This is a response for FRIST44 who wrote:
"The other thing is, in the above DIY summing mixer, would I need to always connect 16 inputs? Would disconnecting any inputs or sending no signal to some inputs affect the impedance/loading or stuff? Sorry I am an electronics idiot......"
You're definitely not an idot. This is critical to the design. The sort of summing design you are discussing likes to see a low impedance source. If you have nothing hooked up to an input the impedance is infinite=not good. You want something low. I build the Folcrom with Ulysses and in our design it is important for unused channels to be disengaged for this very reason. Thats one of the main reasons we have all those switches.
If you are doing a 1/4" patchbay design you can make dummy plugs out of TRS connectors to solve this issue. No wire, just the connector. Just solder the tip to the sleeve inside the connector. This yields a 0 ohm source which the overall summing network agrees with just fine. You can also connect the tip and sleeve to the shell which ties the signal to ground. Experiment with both and see if you hear a difference.
Jon Erickson
www.helsingaudio.com