Well, your first mistake is to adjust the vocal in an other location. You should do that live (at least the gain staging, EQ and compression)
There is many reasons for feedbacks to occur but only one way it does happen.

A feedback is only one thing ; the signal from the PA or Monitors is picked up buy the mic and being amplified again and again.

The reasons :

1. The loud speakers or monitors are too loud or the mic gain is too high
2. The mic position in relation with the speakers favors feedback
3. the polar pattern of the mic is not cardioid, so it picks up too much from the sides and back
4. the Room shape emphases some frequencies : tuning the monitor and front PA with 31 band EQ can fix this
5. Too much compression is being used : compressors lower peaks but could lower the whole signal if the threshold is too low. If you then set a higher gain, when you stop signing and the compression stop the gain is too loud and feedbacks occur
   There are many more reasons, but start with that and experiment.

   It's not because it's named ableton live that it is the best choice for using on stage ! ;)

Thanks for your reply. I tried to adjust compression and EQ in the club, so I guess it works better now. I am playing now with gate effect. Many peoples say that it can help alot to avoid the feedback, but I can see almost the same things with or without gate. What do you think? (n)

The gate act on a thresold basis, as soon as it opens, you risk feedbacks.. It can help, but that's not the best way to avoid them.

When I prepare a stage I make sure that all my speakers have a 31band EQ on them.
For a band that will play loud and will need extra power from the monitors, I often make controlled feedbacks on purpose.
A monitor at the time, I make it feed and find the frequency on the eq and reduce that frequency. Then I make a feedback again and adjust the other frequency. At some point I can't remove more of one of the frequencies so the maximum correction as been achieve. I can start over with another monitor.
This whole process lowers all the frequencies susceptible to feedback and allow you to run the monitors at higher levels.
Something similar could be done with the front speakers but I try not to do that because the assistance should have a more balanced sound. I prefer doing the front by ears while playing a well known CD.

CHoc, post: 421232, member: 48605 wrote: Thanks for your reply. I tried to adjust compression and EQ in the club, so I guess it works better now. I am playing now with gate effect. Many peoples say that it can help alot to avoid the feedback, but I can see almost the same things with or without gate. What do you think? (n)

You're better off using EQ than you are to rely on a noise gate to control feedback in a live mixing situation. Gates can come in handy for keeping certain channels "closed" - until they reach a certain db threshold in which they "open", letting sound through - but volumes - especially stage volumes - can change drastically in a live scenario, so gates can become a bit unpredictable. EQ is your main culprit in terms of controlling a PA, both in adding pleasant factors, and in getting rid of unwanted things as well... like feedback.

pcrecord, post: 421235, member: 46460 wrote: For a band that will play loud and will need extra power from the monitors, I often make controlled feedbacks on purpose.
A monitor at the time, I make it feed and find the frequency on the eq and reduce that frequency. Then I make a feedback again and adjust the other frequency. At some point I can't remove more of one of the frequencies so the maximum correction as been achieve. I can start over with another monitor.
This whole process lowers all the frequencies susceptible to feedback and allow you to run the monitors at higher levels.

The method that Marco is referring to above is what is known in audio circles as "ringing out" a system.

This is best done when no one else is in the venue, for obvious reasons... where you get the PA as hot as you can ( running hotter than you would ever be during the actual show) with an open mic - and then, listening for frequencies that start to feed back. You then find and attenuate those frequencies accordingly. You need to do the same thing with monitors, because when the players show up and start to crank their gear, you need to know what your threshold is with the monitors as well.

Here's a quick and dirty explanation; it may help you:

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Thank you so much for your reply. I can say now, its all about the low frequency or at least thats where most of the feedback comes from. The thing is that it sounds a bit "obvious" when I completely cut the mic's low freq. I'm always trying not to cut low's too much, but in every club or venue we go it sounds different and almost every time I'm completely removing low's, coz of the feedback. Maybe I need to use a sound card with balanced outputs, just like the man in the video I posted says. Currently I'm using audio kontrol 1 which has no balanced outputs.

Most mixer console have hi pass filters (HPF) on every channel. Before you EQ the monitors and FOH, Your mics should all have this (HPF) pushed on except for the bass drum, bass guitar and keyboards. You should already know that bass frequencies develop in front but also behind loud speakers. you usually don't need to put much lows in the monitors because the front speakers usually add alot of them on stage.

It's all a question of balance, the only place where I'd cut a lot of bass is in a church or in a highly reverberating place because they may build up out of control.

CHoc, post: 421772, member: 48605 wrote: but in every club or venue we go it sounds different

This is normal, every room sounds different, thats why having a 31bands Stereo EQ on the main speakers is so important. You use it to compensate for the room defects.
In fact, when I was playing in bars every week, We rarely touched the mixer. It was placed put in its case after each show and put back on the stand the next one. 90% of the job was to adjust the EQs of the monitors and front speakers because the instruments, mics and placement was always the same.

At which frequency are you rolling of the low's? Many people are mislead into believing that vocals can be replicated much farther down below than they actually can.

In a live situation, depending on the room ( and your PA ) you may have to hi-pass vocals from as much as 100 - and up... it also depends a lot on speakers you are using. Smaller speakers cannot replicate that which bigger speakers can. Your overall sound is going to vary greatly from room to room.

Using an HPF on the vocal mics will also help to tame the lower frequencies coming from other instruments on stage - namely bass and drums - that could be picked up by the vocal mics.. In most standard live stage setups, the vocal mics face the back-line - drums, bass and guitar amps - so, by HP'ing the vocal mics, you are helping to eliminate those instrument's primary fundamental freq's from coming through the PA via the vocal mics.