The 3124mb+ utilizes the same microphone preamp circuit that is used in all API consoles. It uses the RE-115 K mic input transformer along with the same output transformer that is used in API equalizers. The unit has an internal 48-volt phantom power supply, which is front panel switchable for each channel. Also provided is a front panel -20 dB pad switch that affects both the mic and line in. A 1/4" phone jack is available on the rear panel for each channel as an insert tip-send/ ring-return placed between the mic pre amp output and the mixer input.

Like the 3124+, the 3124mb+ provides up to 65 dB of gain to an output clip level of +28 dBu. The Hi-Z front panel input goes directly to the op-amp, allowing a low level input such as a guitar or bass to be amplified without a matching transformer or direct box. This Hi-Z input can accept input levels as high as +20 dBu, making it perfect for keyboards and other high level devices. The rear output is an XLR connector.

Additionally, the 3124mb+ adds a stereo panner with a level control, and a post "fader" aux send. The stereo panners and the aux sends are bussed to master output controls and then to the rear panel jacks. A stereo AUX return is provided to return effects or cascade additional mixers for more inputs. The unique stereo aux return control acts to balance the left/right signal, yet staying stereo. It can also be used as a mono return.