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Op amp output resistor value

I’m designing a circuit with a simple op-amp summer as part of it.

I’ve noticed a weird behavior of the output voltage, if I increase the resistor value at the output, the output voltage drops. This is undesirable behavior to me. I’m curious why it happens, and what design choices I can make to remedy the situation. The resistor change is something like changing R19 to 1M instead of 1k.
The output is on the circuit’s output, so I’d like to protect against a user plugging into a large load or something.


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