Use a `delwrite~` / `delread~` delay line
Set the delay time to a small value
Feed the input directly to a `+~` node
Feed the delayed signal to a `-~` node
Multiply the delayed signal by the allpass feedback gain
Route the result back into the delay line
Use the standard allpass structure: `y = -g*x + x_delayed + g*y_delayed`
In PlugData, build it with `inlet~`, `delwrite~`, `delread~`, `*~`, `+~`, and `-~`
Keep the feedback gain between `-1` and `1`
Use a `send~` / `receive~` pair if needed for the feedback path
Adjust delay time and feedback gain to change the allpass effect