well, feeding the output into the input, and then routing the feedback out to an amplifier (possibly through effects)
you can get a nice range of feedback sounds… low rumbles, hissing, motor-like sounds, piercing, high freq. feedback etc. it can sound massive, or very slight.
you can do this with pretty much anything (effects box, mixer, amp, etc.) that has at least a couple outputs. I’ve used a couple of Ibanez DM 1000 digital delays (rackmount) for this purpose - it’s great for raw feedback, though it’s difficult to manipulate precisely. I tried it once, successfully with just a solid state amplifier.