Q: “Practical advice, please. I’ve recently started playing live and want to send my keyboard’s output straight to the mixer for the PA, but when it’s plugged in it’s very noisy. Is this common with line outputs?”
A: Line-level signals are fairly quiet in relation to, say, typical electric guitar pickups, but your noise problem most likely stems from a different situation. Almost all electronic keyboard outputs are unbalanced and, depending on the length of the cable you use, pick up interference and noise. There are a couple of ways to deal with this noise before it hits the mixer.
The most common approach is to connect your keyboard output to a Direct Box. This will accomplish two things. First, it will convert the unbalanced signal to a balanced one, providing protection from line noise and permitting you to run a long cable to the mixer. Second, it will reduce the line-level signal to a mic level, which allows your keyboard output to be plugged into a mic input on the mixer.
An alternative is to use a small mixer onstage, which is a useful way to submix multiple keyboards before they hit the PA. Just make certain the mixer offers balanced line-level outputs, which you’d connect to the line ins of the main mixer. The Mackie 1202VLZ Pro and similar small mixers work great for this.
Among the few keyboards with balanced audio outputs are the Kurzweil K2600 series. Note that these are still line-level signals (with 200-ohm impedance) and should be plugged into your mixer’s line ins.