I HAVE PEAVEY 2XT SPEAKERS /4OHM.
A QSC 9OO USA AMP.
THERE IS A HUM THROUGH THE SPEAKERS EVEN WHEN ALL VOLUMES ARE DOWN.
I THOUGHT MAYBE THE AMP IS TOO POWERFUL FOR THE CABS ON THEIR OWN AND I SHOULD LINK SUBS WITH THEM SO AS TO HAVE MORE SPEAKER POWER ON EACH CHANNEL-350WRMS CABS AND250W RMS SUBS.
ALSO ON MY MIXER WHEN I AM PLAYING A TRACK ON ONE CHANNEL AND LISTEN TO ANOTHER ON THE HEADPHONES THROUGH THE CUE FUNCTION I CAN HEAR THE TRACK ON THE SPEAKERS ASWELL EVEN THOUGH THE SLIDER IS DOWN ON THE CHANNEL.
THE ONLY WAY TO GET RID OF IT IS TO TURN THE GAIN ON THE CHANNEL RIGHT DOWN.
THEN I HAVE TO REMEMBER TO TURN UP THE GAIN ASWELL AS THE SLIDER.
IS THIS RIGHT OR DO I HAVE A PROBLEM?
CHEERS
JAZZCAT

Argghhhh Multiple problems in one topic, okay one problem at a time.

Hum:-

Can be caused by a variety of things. If its from one channel only then it can simply mean a capacitor has "dryed" out on the PSU side of the Amp channel and is introducing some mains hum into the signal chain. You'll need an engineer to check and replace these for you.

If the hum is coming from both channels then its unlikely that it will be the PSU (unless its the older type which shares a common power supply).

Have you tried replacing the lead which connects between your mixer and Amp??, it could be a poor / broken screen connection or a problem in the connector. Leads are notorious for causing problems in this line of work, and should be the first thing that you check . You are using a proper screened music cable aren't you???.

It seems that you may be having mixer problems, so of course the "hum" could be originating from there!. Have you tried plugging the output from another mixer into the amp? Even with the amp gain controls down, noise can still be travelling down the Earth (Screen).

If your mixer or amp has a "ground lift" switch then try switching it over. A ground lift switch seperates the signal ground (earth) from that of the Mains / DC Earth. Sometimes a earth loop can cause hum and moving the switch breaks that connection. Don't be tempted to remove the mains earth from either the Amp or Mixer no matter what others may say . This is potentially dangerous and could be lethal if a fault develops.

Also try connecting the Amp to the Mixer in "Balanced" mode. If you aren't doing this already it can sometimes solve noisy connections. If your amp and mixer are fitted with 3 pin XLR plugs / sockets then use these to hook up rather than the standard unbalanced Phono / RCA jacks.

Mixer:-

Sounds like signal "bleed over" from a failing component or dodgy fader track. It may even be caused by the above earth problem . If you solve the hum problem and the mixer fault remains then better take it into a Disco / Hi-Fi Repair shop to be checked over.

Make sure that you are using the "master" outputs on your mixer to connect to the amp rather than the "booth" outputs. Booth outputs will happily run an amp, but some also feed through the PFL output as well .

Also, don't use CAPS LOCK to post. Using Caps Lock on the internet is considered SHOUTING

It looks as if youve got a problem with your CAPS LOCK too! Just kidding...

Chris has given you some great answers there, (and he might even be calling me for my debit card details today too)

Just a couple of extra questions about the Amp...Has it always made the hum since you got it, or just started recently?...also, I noticed you mentioned that it was a USA amp...is it dual voltage "built in" or are you using an external voltage dropper/convertor? (3 guesses where Im going with this)

ok chris .
sorry about the caps lock.
(gotta get rid of that habit)
thanks i will check the things you have said .
jazzcat