On audio mixers, a bus which has an independent feed from each individual channel. Each channel has a control to adjust the level being sent to the auxiliary master output, which in turn has a control to adjust the overall level at the output bus. The auxiliary channel may be a simple output (to feed a device such as a tape machine or monitor), or it may be a "loop". An auxiliary loop sends a signal from the auxiliary output bus to a signal processing device such as a reverb generator, then brings the output of that device into an "auxiliary return" bus (thus creating a loop from the desk to the device, back to the desk). This return bus will have a level control pot, which is used to mix the incoming signal into the mixer's master output bus.