Or look at it another way.
The adapter is rated for 9v output with a 400mA load.
You measured it with no load at 11.5v.
So, what's the internal resistance of the adapter?
And, what's the output voltage of the adapter when a 150mA load is on it?
Hint: apply Ohm's Law
[eta]
You are trying to make this much more complicated than it really is.
The adapter is rated for 9v output with a 400mA load.
You measured it with no load at 11.5v.
So, what's the internal resistance of the adapter?
And, what's the output voltage of the adapter when a 150mA load is on it?
Hint: apply Ohm's Law
[eta]
You are trying to make this much more complicated than it really is.