I designed this circuit which will provide a reference for PWM comparators based on both a manual control and an external control voltage. The only way I could think of to do it is more complicated than I'd like, and I wonder if anyone knows a simpler way.

The point labeled "SIG_REF" will be -1V to +9V based on the pot position. The control voltage coming in the TRS jack will be 0 to +5V. The output labeled "PULSE_REF" needs to be at least -1V to +9V. When no connector is plugged into the TRS jack, the pot should cover the full range, and should sit at the midpoint +4V when set to 50%.

So the problem is that the control voltage needs to be amplified by 2 and offset before summing with the SIG_REF, but PWM_EXT needs to be 0V when there's nothing plugged into the jack. That means the + input of U13A needs to be grounded, and the inverting input can't be offset when there's nothing plugged in. Neutrik makes a jack that has a normally-closed tip switch to cover grounding the + input, and a normally-open sleeve switch which could be used to ground the gate of a PMOS transistor to apply the offset voltage when the control voltage is plugged in.

Is there any simpler way to do this?

 

You could simply mix the two voltages through two resistors. They way you have it now is the "proper" way -no guesswork necessary and everything stays nice and linear.

 

Well I want both the external and the manual controls to each be able to cover the full 10V range, so the control voltage needs to be amplified and offset.

Part of my concern is that I've only found one connector model that has this NO switch, found in a Neutrik PDF showing their different jack/plug circuits. I see that a few different places carry them, but Neutrik's website doesn't seem to know what that model is when searching for it (NMJ6HCD2-SM), which makes me concerned about the future availability of the thing that is necessary for the external control voltage to work properly.