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I purchased a pair of 1 foot long XRL mic cables just for this problem. I use them when trying to locate ground loops. I lifted pin 1 ground in each as per the attached article, and simply insret them in different places the signal path till I find the problem ....Please read the following:

"Whichever is the last audio system component that acts as the source (send) of the balanced line (mixer, EQ, crossover, compressor, etc.), this source component's send, should have pin #1 of the connecting balanced cable lifted or ‘floated' from its chassis.

The shield of the balanced line will still terminate or go to ground at the receive end or the amplifier rack. This then eliminates the hum that would be induced if both ends of the balanced line went to ground. You have eliminated the cause of the hum by using this correct approach to signal distribution without violating the NEC. Lifting the electrical ground pin is always in violation of the NEC.

I recommend that each sound person have a number of short (4 to 6 inches) balanced microphone cords made up with pin #1 lifted or floated in the female side of the connector. This will allow them to ‘float' the balanced lines on any system that they encounter without their having to go into the snake box or the equipment itself to ‘lift' pin #1 from the chassis ground.

Note: Be careful when cutting the ground wire away in a regular microphone cable. You want to trim any excess wire back to the insulation so it won't short to any of the pins in the encounter.

Also, if you just have to cut a microphone cable, mark it as ground lifted immediately or you'll shoot yourself in the foot at some future gig, especially if you try to use that mic cable with a phantom powered microphone.

In some applications, particularly those sometimes encountered with electronically balanced inputs at the end of a balanced signal line; lifting pin #1 from ground will eliminate the ground loop hum but you may then experience ‘RFI' (radio frequency interference) in the system.

The solution is this case is to add a small capacitor (such as a .001 mFd) from the shield or ground wire to pin #1. Adding the capacitor allows the balanced line to be lifted (open) from ground at audio frequencies, while acting as a closed circuit for radio frequencies.

In order to explain in any further detail would require a thorough discussion of electrical power distribution. The foundation of the above tip is: Not allowing the audio signal path to go ground in more than one place.

If you have more than one signal path, then each path should only have its signal path's shield connected to ground (pin # 1) at one end, usually at the termination or receive end."

This article courtesy of Peavey's Marty McCann

I strongly agree with you Karyoker....The Comp should be inserted on the mic strip, not seried into the mic XLR on the strip....You must use a speclial insert cable to do this...you can make you own as I do, just pay close attention to wire assignment.

It really is worth your trouble to get this problem solved....Your compressor is useless to you until you get it working properly on the input to the mixer. ...Prerecorded music is already highly compressed, so you really deminish overall results by squashing it more....your compressor is a quality unit and is a big first step toward a great vocals setup. I hate to see you not get the benifit of it.

In the insert mode you have an insert cable from mixer to the 266xl . The shield is connected from chassis ground on the mixer to chassis ground on the 266xl  Signal ground is not even involved.  Signal going and coming..  How in the (@$%&#!) are you going to have a ground loop?

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Yep, that is why it is a perfect solution for his problem.

Grounds and interference is still one of the biggest problems whether it is an electric fence around the south forty or the copper strip around the building hooked up to the 600 foot tv tower. I have given professional enginering tech advise and received it for forty years and what frustrates me on here is in trying to get more info to give a professional opinion which I seldom get and by that time the thread is totally shot... I still charge $500 to $1000 for sound intercom tv antenna installation (jukeboxes cheap) in real life ...

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It's a "plug and play" world here Karyoker....Our biggest concern is "Do they make an adapter that will allow me to do it?" ...It's only after we get it all patched together and it don't sound right that we learn about the science of it all.

Stick around please, eventually everyone reaches for the owner's manual and experts like you...But first, they always check the village knowlege base.

...Oh, and if you plan to charge here, you need to set that up with the Guru. He is gonna want a cut fersure

I just re-read what mixer he had, it does have inserts on the mic inputs which changes my answer as well.  Good call!  This is what he'd need

http://www.zzounds.com/item--HOSSTP20

Plug it into the insert of the mic to be compressed, the send will go into the input of the dbx & the return will go to the output.  This "should" eliminate the hum as well.

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Yep,

He will be then on his merry way to learning how to properly compress vocals..I'm still learning.

If you don't have a proper insert cable, then google up "make insert cable"...you should get  a hit that shows you exactly how to do it.....all you will need to start is a spare TRS cable and a soldering iron and a new TRS male plug....Just pay close attention to wire assignment.