Let's see what kind of trouble we can get into wiring some speakers up in Parallel. Back to the analogy first.
If we were to install a T-fitting on the pump outlet, then run a pipe from each output of the tee to each load (remember, a dual voice coil speaker), then we connected the outputs of the loads together, and send the water back to the pump, we have made it easier for water to flow, and reduced the resistance of the system.
Ok, Now let's see the electrical example. Run a wire from the positive terminal of the amplifier to the positive terminal on one of the
voice coils. From that same terminal, connect to other positive terminal
(remember: dual voice coil driver). Connect the two negative terminals together, and then to the negative terminal of the amplifier. There you go, the voice coils are wired in parallel.
So, what is the new resistance we have? This can be quite simple, and can be quite complicated at the same time. If we have two identical loads (6 Ohms), and they are connected in parallel, then the resultant resistance is half of one of the loads. In our case, 3 Ohms. Now, to make this complicated. If we have two different loads, wired in parallel, the resultant resistance equals: R =1 / ( (1/R1) + (1/R2) ), where R1 and R2 are the different resistances. So, if we have two 6 Ohm drivers we get as follows:
R = 1 / ( (1/6) + (1/6) )
R = 1 / (2/6)
R = 1 / (1/3)
R = 1 / 0.3333
R = 3
Hope you caught that! Try it with two different resistances. Say a 4 Ohm speaker, and an 8 Oohm speaker.
R = 1 / ( (1/4) + (1/8) )
R = 1 / ( 0.25 + 0.125 )
R = 1 / ( 0.375 )
R = 2.6666
Neat huh? Works for as many drivers as you want, too!
OK, this is starting to drag, let's tie it up fast!!! The voltage across each load connected in parallel is the same, whereas the current passing through each will differ depending on the load. Kirchoff probably had something to do with that as well.
The problem with paralleled loads is that we need to move a lot of water to get both pumps to spin, or speakers to move just as far. It's no longer a matter of how hard we can push the water, it's how much water we can move.