Airport express crowding out other audio source?

I'm trying to set up a sound system centered around a set of Harman Kardon Soundsticks. There are two inputs: 1) AirPort Express (older, brick generation) for streaming from iOS and OS devices, and 2) a turntable with a line output. Since the Soundsticks only take one input via a 3.5mm aux jack, I need to use a splitter jack to get both inputs to the speakers. Here's the problem:

1) AirPort alone in splitter jack = system works
2) Turntable alone in splitter jack = system works
3) Turntable and AirPort both in splitter jack = AirPort works and Turntable does NOT work

In scenario 3) I can very faintly hear the turntable output as if it were not plugged into anything, just the unamplified vibrations of the needles. If I pull out the AirPort the turntable comes in full to the speakers.

If in scenario 3) I replace the Turntable with, say, my iPhone, the iPhone works. So it is not a problem with the hardware anywhere along the line, as far as I can tell.

I'm baffled, honestly. Could it be something like the AirPort is continusouly providing an output that is crowding out the analog signal into the line? But then why would the iPhone overcome that?

More to the point, does anyone have a solution?

[Also posted in Apple help forums]