Section one: Three sounds playing dry.

Section two: Three sounds playing with the FX as a send/return.

Not sure if this is truly all-pass, but it definitely sounds cool.

Signal is split into Mid Side, each is processed separately before merging together into L and R.

The all pass is created with a pre-delay set at around 20ms, then into a feedback loop. This loop has four delays with different lengths and feedback amounts. Each of those delays is fed into its own slope set to notch out a specific frequency. Each of the four slops is nothcing a different frequency, so in theory we are hearing every possible frequency at each pass, just at different times. This is what creates the hollow effect. These are then merged with a dry signal via the crossfader.