Buzz on Stereo Channel

[nickb12345]

Good evening all.

 

Got a bit of a head scratcher here - I have a Mackie CFX20 MKII desk and have a bit of a problem with a Buzz. The feed in question is coming from a DJ Booth on stage and the desk is in the control room at FOH. If I connect the input (balanced XLR x2 (L & R)) to two mono channels all is fine, but if I connect it to a stereo channel using balanced XLR -> jack adaptors (they are TRS jacks on the jack side, not just TS) I get a buzz. However if I use the same adaptors and connect to the jack inputs on a pair of mono channels there is no buzz - which kind of rules out the adaptors being faulty.

 

I have tried lifting pin 1 on both ends of the cable, no luck there. Tried other XLR-Jack adaptors, that doesnt help. Before I discovered it worked in the mono channels I was assuming it was caused by the DJ Booth being powered from a different distro to the control room, but im not sure now. I have tried changing the phase of the power supply too - that doesnt make any difference (im not sure which phase the control room is on, but have tried the booth on all three).

 

Anyone got any ideas before I pull whats left of my hair out?

 

Thanks

 

Nick

[bruce]

My initial reaction was that this sounds like the stereo line input is unbalanced, whereas the mono is balanced. However the manual (at least for the MKII version) suggests that the stereo channels should also be balanced. Strange.

 

Perhaps you should use some sort of transformer isolation - or even a pair of DIs.

[nickb12345]

Thanks Bruce

 

Agreed I looked at that too, but as you say - the manual shows it as balanced.

 

I will try and dig out my DI box - it doesnt get much use these days so dont really know where it is - see if that helps. I cant for the life of me work out what is actually causing it however. Unless it is something to do with the way they are powered from different DBs (but on same phase), but then why is it present when you use one channel and not another?

 

I think this is why im a lampie at heart!

[bruce]

Despite what it says in the manual, it looks like the stereo inputs are unbalanced.

 

See http://www.mackie.com/products/effectsmixers/cfx_specs.html , in particular the section headed "input impedance"

 

 

Input Impedance

 

Mic Input

3 kilohms, balanced

 

Line Input

40 kilohms, balanced

 

Insert Return, Stereo Line Input, Tape Input, and Effects Returns

10 kilohms, unbalanced