This disclosure describes electrical stimulators that include some electrical sources (e.g., current sources, voltage sources) that are directly connected to a plurality of electrode nodes, and other electrical sources that may be selectively connected to selected ones of the plurality of electrode nodes via a switching unit, such as a multiplexer. One example stimulator comprises a processor, a plurality of electrode nodes, and a stimulation generator that is coupled to the processor and to the plurality of electrode nodes. The stimulation generator comprises a plurality of negative electrical sources, a switching unit, and at least one positive electrical source. The negative electrical sources are each directly connected to a different one of the plurality of electrode nodes. The switching unit is connected to each of the plurality of electrode nodes. The at least one positive electrical source is connected to the switching unit. The switching unit is configured to connect the at least one positive electrical source to a selected one or more of the plurality of electrode nodes.