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Old 7th January 2020
  #3
Gear Addict
 

To reduce noise, I would use 12k input and feedback resistors instead of 100k. Then you can use 50k linear pots, which loaded with 12k will give a pretty reasonable approximation of a log taper and will have much better tracking between channels if you use stereo pots, while keeping the minimum input impedance at ~10k. There's no limit to the number of inputs you can add, but noise will increase with the number of inputs. Since we're talking line level and no gain, this may not be a problem. Try and see.

A 100R resistor at the output will help with driving capacitive loads (long cables). Arrange it as in the attached schematic and don't forget those 22p caps for stability (use C0G / NP0 ceramics or polystyrene here). Use DC blocking caps at the inputs and outputs as shown, I like Nichicon Muse ES bipolar caps for this, they are tiny and very linear.

Edit: yes, you can put a switch after each pot. If you feed the aux output to the inst. inputs of the RC505, which are high impedance, add a 12k resistor to ground at the aux output position of the switch to keep the same taper.
Attached Thumbnails
Questions on Building a TL074 OPAmp Line Mixer-tenorjazz.png  

Last edited by Cabirio; 7th January 2020 at 04:43 PM.. Reason: Answer to switch question