How to hook up a gemini amplifier?

I agree fully with expert .

Switch input connection pairs and if the 'fault' doesn't changes from one channel to the other it is almost certainly an amplifier or output fault.
Likewise switching the speakers. If after swapping the speaker connections the fault changes to the other speaker it is an amplifier fault. If not a speaker fault.

I have a Gemini xga 4000 that only channel 2 works fine but channel 1 is very very low .......

What kind of preamp and how are you using the xga, I assume on PA, and you want to use maybe a guitar preamp and amp head directly to it? Your preamp could go straight line in to the xga, the only thing the (whatever kind) of head is for power of onstage monitoring of your guitar. That is optional since the PA should have monitors. I personally use my full rig onstage to hear myself but, I mic it to the PA which always sounds better than direct. If you could be more specific to these models and uses or ideas, I could help you more.

A large amp like this likely has protection circuit (probably a relay) that doesn't connect the speakers until the amp is stable and balanced. You relly need the schematics for the amp.

It may NOT have a fuse. If too much power was sent or an amplifier drove DC to the speaker, the voice coil may have fried. The specs do NOT brag aboput protection, so it is likely either the speaker is blown or the internal wiring is broken. Open the unit and use an ophmmeter to tesst the speakers.

You connect POWERED speakers to the left and right outputs... You MUST use POWERED speakers or an external amplifier. This unit has a small headphone amp, but that CANNOT power passive speakers.

A simple test is to connect internal speakers across an AA battery (for about half a second) & listen for a click as battery is connected & another as battery is disconnected.

If this does not happen, check the internal jack plug connection & its connection to the speakers.

If OK, then you can test each speaker individually using the same battery across each in turn.

If this advice is helpful, I would appreciate a rating.

You'll have to take it in to get it repaired. From what you described it sounds like you blew the output stage out. You can't run from your speaker output jack into the input jack of another amp. You should go out of the recording out, line out, effects out or in the least case your headphone out. What happens usually is your output transformer short circuits due to a mismatch of impedances. It sounds like you still have a transformer though so you might have gotten lucky. You will need to take it in though. Sorry.

try placing your ear on the speakers, adjust the volume control up and down several times, you should hear a humming sound that goes up and down. if not problem with the stage before the amplifier, that is the sound processor. if you dont hear the hum. problem with the amplifier and speakers, including the sound ports or jack. dissassemble the unit. look for loose connections from speakers to amplifier.