Unbalanced input into Red7 hum problem

orange · 14th May 2005, 03:54 PM

hiya all,

I'm trying to send an unbalanced output into the line in on the focusrite red 7. When I short pin 1 and pin 3 on the xlr I get a load of mains hum.

I assumed that it was an earth loop of some sort - pulled it out of the rack, cabled it up on the bench and have the same problem. If I short pin 2 OR pin 3 to earth then I get mains hum. Unit works fine balanced.

Inputs are electronically balanced - focusrite manual says that shorting pin 1/3 is correct unbalancing procedure.

FWIW hum level is not affected by having any equipment connected to input - it does this just by plugging in an 'unbalanced' cable.

advice anyone ?

thanks

si

orange · 16th May 2005, 12:02 PM

to answer my own post (sort of)....

Looks like the ground in the input XLR of the red7 isn't referenced to ground at all. Therefore if you use this to short pin 3 then you get hum. If you use the ground from the output XLR (transformer) or if the input XLR ground is connected to earth via some other means then hum dissappears and all is well in the world.
You wouldn't notice this if you used it in the unbalanced insert of a desk (grounds would be connected at the desk).

I assume focusrite did this so that the unit wasn't succeptible to earthloops - but would have preferred an earth lift switch.

in the unlikely event this helps somebody else....

si

cornboy7x · 19th May 2005, 05:20 AM

generally you want to lift the ground on the input and connect it on the output. i wouldnt even short those pins together at all. just connect the ground to 3 and hot to 2. connect the output ground though, but lift it wherever it connects to. the problem should go away. thats really all i have the mental capacity for right now.

eric

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14th May 2005, 03:54 PM	#1
orange Lives for gear Join Date: Jul 2004 Location: London Posts: 1,663	Unbalanced input into Red7 hum problem hiya all, I'm trying to send an unbalanced output into the line in on the focusrite red 7. When I short pin 1 and pin 3 on the xlr I get a load of mains hum. I assumed that it was an earth loop of some sort - pulled it out of the rack, cabled it up on the bench and have the same problem. If I short pin 2 OR pin 3 to earth then I get mains hum. Unit works fine balanced. Inputs are electronically balanced - focusrite manual says that shorting pin 1/3 is correct unbalancing procedure. FWIW hum level is not affected by having any equipment connected to input - it does this just by plugging in an 'unbalanced' cable. advice anyone ? thanks si

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16th May 2005, 12:02 PM	#2
orange Lives for gear Join Date: Jul 2004 Location: London Posts: 1,663	to answer my own post (sort of).... Looks like the ground in the input XLR of the red7 isn't referenced to ground at all. Therefore if you use this to short pin 3 then you get hum. If you use the ground from the output XLR (transformer) or if the input XLR ground is connected to earth via some other means then hum dissappears and all is well in the world. You wouldn't notice this if you used it in the unbalanced insert of a desk (grounds would be connected at the desk). I assume focusrite did this so that the unit wasn't succeptible to earthloops - but would have preferred an earth lift switch. in the unlikely event this helps somebody else.... si

19th May 2005, 05:20 AM	#3
cornboy7x Gear nut Join Date: May 2005 Posts: 116	generally you want to lift the ground on the input and connect it on the output. i wouldnt even short those pins together at all. just connect the ground to 3 and hot to 2. connect the output ground though, but lift it wherever it connects to. the problem should go away. thats really all i have the mental capacity for right now. eric