This article explains how to connect your subwoofer to your Focusrite interface in order to use it alongside a pair of monitor speakers to listen or mix stereo audio. The user guides for some of our larger interfaces will instruct you on how to set up in 2.1, 5.1 and 7.1 for using multi-channel audio, but it is perhaps more common for customers to wish to mix in stereo with the aid of a subwoofer.
If you wish to use a subwoofer with stereo audio you should not connect it to a separate line output on your interface. Instead, you should connect the main line outputs you would otherwise be connecting to your pair of speakers, directly to the inputs on your subwoofer. You should then connect the left and right outputs of the subwoofer to your monitor speakers. This means that the amplifier in the subwoofer receives the audio with the full frequency range and you can use the crossover dial/switch to control the frequency that is output from the subwoofer and the speakers. For example, if the crossover is set to 80Hz, you can expect the subwoofer to output any frequencies below 80Hz and the monitor speakers to output anything above 80Hz.
Here is a diagram for setting up a common commercially available powered subwoofer.
If you are using multi-channel audio where the audio being sent to the Sub or LFE is not already at full frequency range you will use the setup described in your unit's Manual.
If you have a passive subwoofer/speakers you would generally also be using an amplifier or home receiver to provide power to the speakers, so the stereo line outputs would go to the amplifier/receiver's L/R inputs. The amplifier/receiver would then act as the crossover/amplifier to the sub/speakers, splitting the audio properly.
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