The Y cable earned its name from the way it looks. There is a single connector on one end with a cable that splits, either as a zip cable or with a small box that conceals the split, and finishes with two connectors at the other end. A stereo breakout meets this same visual description and yet does not perform the same way as the typical Y cable. Let’s take a look at the difference between the two.
A Y cable is used when there is one signal that must go to two different places. For example, when you have one 1/4-inch TRS headphone output and you want two people to listen to the same thing on their own headphones with 3.5 mm connectors. The quickest solution to this problem would be a Y cable with a single 1/4-inch TRS male to dual 3.5 mm TRS female jacks. Each of the two headphones will get exactly the same signal. The cable is wired so each contact on the single end connects to the equivalent contact of each connector on the dual end. Be advised that a traditional Y cable is designed specifically to split a signal. Using a Y cable to combine two signals is not recommended because you have no control over the way the signal is combined.
A stereo breakout, on the other hand, is designed to do what the name states, it breaks out a stereo signal into two discrete points. A common situation requiring a stereo breakout is playing music from an iPod® through an audio mixer with 1/4-inch TS inputs. The solution is a 3.5 mm TRS to dual 1/4-inch TS stereo breakout cable. You connect the 3.5 mm end to the iPod and on the other end you’ll have the left signal (tip of the connector) on one of the 1/4-inch connectors and the right signal (ring of the connector) on the other, allowing you to connect each to its own input on the mixer. When used properly, a stereo breakout can be used in either direction. You could use the same breakout from the previous example to take the left and right outputs of a mixer and connect them to a laptop equipped with a 3.5 mm stereo input.
It’s important to know what you need to accomplish in order to select the right cable. Hopefully this has shed some light on these two popular audio problem solvers so that you can always make the right choice.