The Club 5501 is a mono amplifier for subwoofer use in cars. It can be confusing to see that it has two inputs, marked left and right, as well as two speaker output pairs. But it is still a mono amplifier, with only one channel. The two inputs are there to make it easier to connect to a normal car radio/head unit, where you will often have two normal full-range connectors, left and right. You can insert both into the 5501. However, if you only have one signal lead going to the 5501, you can use either of the inputs. They are combined into one inside the amplifier anyway. Also, the two sets of speaker connections are there to make it easier to connect two subwoofers simultaneously. The two sets are completely identical, so you can use either one for a single subwoofer, or both. If you connect two, they will be in parallel.