At our school we connect our qu-24 to the iMac using the headphone jack on the iMac via a 3.5mm jack to the st3 connections on the mixer.
I am wondering if this is best solution as I need to replace the cable so wanted to check if this was the most effective way to connect a computer to a mixer to play audio from the pc.
That will work, just to check are you using a 3.5mm break out cable that goes to
two 1/4 inch TS mono plugs?
You could the USB B connection on the back of the mixer with a cable plugged into the iMac USB jack.
The QU24 will show up as a sound card to the iMac.
In the IO patch menu the USB can be assigned to a channel on the mixer.