An electronic mixer is a device that combines two or more electrical or electronic signals into one or two composite output signals. There are two basic circuits that both use the term mixer, but they are very different types of circuits: additive mixers and multiplicative mixers. Additive mixers are also known as analog adders to distinguish from the related digital adder circuits.
A simple three-channel passive additive mixer. More channels can be added by simply adding more input jacks and mix resistors.
A "virtual ground" active additive mixer. The buffer amplifiers serve to reduce crosstalk and distortion.
An adder, or summer, is a digital circuit that performs addition of numbers. In many computers and other kinds of processors, adders are used in the arithmetic logic units (ALUs). They are also used in other parts of the processor, where they are used to calculate addresses, table indices, increment and decrement operators and similar operations.
Schematic of half adder implemented with five NAND gates.