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Summing mixers are usually very basic analog mixers with left / right stereo outputs.  Audio is sent to them from DAW d/a outputs to be “summed” (combined). Typically a Summing mixer will have 8 stereo analog inputs (16 mono inputs) and two analog outputs  These inputs can be referred to as stems. Some people prefer the sound of mixing ( summing) to take place on analog hardware rather than within a DAW’s digital mixer.