Anyone might wonder how this is possible. Do you need two sets of ears to listen to headphones and loudspeakers simultaneously?
Well of course, if you have two people in the studio, you have two sets of ears, so the question is perfectly valid.
I can imagine some scenarios...
There are probably a dozen other good reasons, but I won't go on.
I imagine that the problem is that plugging the headphones into the monitor system automatically switches the loudspeakers off. There are times when you might want this, but it would be nice if it were an option.
I could suggest that you delve inside your equipment and replace the switched headphone socket with an unswitched one. But it might also be that the same volume control handles both the loudspeaker and headphone outputs, so you might find yourself adjusting it every time you swap. (You could change the level setting on the loudspeakers' power amplifier to correct this.)
A better way would be to use a separate headphone amplifier, connected to your system's monitor output, via a splitter cable if necessary.
To wrap up, I would say that if this is causing you a problem, then you should solve it as quickly as possible. There is no good reason for putting up with irritations in the studio - recording to a professional standard is difficult enough already.
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