Hi Piercarlo,
Neither the ZED6 or ZEDi8 feature the Aux output which would generally be used to send a mix to a monitor, allowing different relative levels of the input to what’s being sent to the main speakers.
e.g. You might want the vocals to be louder in the monitor than in the main speakers, or the other way around.
For this you would be looking at the ZEDi-10 or ZEDi-10FX really.
The ZED6 and ZEDi8 feed a copy of the main signal to the headphone output until you press the headphone switch on individual input channels, then the headphones will be fed from any input where this switch is pressed, and the full level of this signal (the main level control has no effect on the level going to the headphones).
Monitors are generally mono, but if they have a stereo input, you could connect the headphone output to this using a stereo jack to 2x mono connectors (maybe jack or RCA depending on the monitor).
What you shouldn’t do, is to connect to a monitor using a mono connection from the headphone output as balanced jack and stereo jack connectors are the physically the same but work differently.
If you do only have a mono input on the monitor, the easiest thing would be to connect just left or right signal (with a stereo jack to 2x mono cable) and don’t pan anything.
Note that you will need a powered monitor or separate amp.
But in short, the best mixer for this setup would be one with an aux output.
Thanks,
Keith.