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Author Topic: ART Pro VLAII loud hum with unbalanced i/o  (Read 2989 times)

Fenris

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ART Pro VLAII loud hum with unbalanced i/o
« on: December 05, 2011, 11:56:15 pm »

I have an ART Pro VLAII optical compressor that works perfectly with the balanced XLR i/o, but there's a loud hum when I use the 1/4" i/o with an insert cable going to a semi-pro console. It's not the cable or the console, and I used it this way for 2 years with no problems. I think the XLR's and 1/4" jacks are both balanced and in parallel, but I'm not sure.
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Bruno Putzeys

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Re: ART Pro VLAII loud hum with unbalanced i/o
« Reply #1 on: December 19, 2011, 04:34:56 am »

Hang on, when you say you've been using it like this for 2 years do you mean you changed nothing but it started humming, or you did change something? Pls. explain more precisely.
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John Roberts {JR}

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Re: ART Pro VLAII loud hum with unbalanced i/o
« Reply #2 on: December 19, 2011, 11:10:24 am »

A bad cable is the obvious thought when something that was working OK stops. The other obvious question is what changed?

I will ASSume using the TRS insert cable which is single ended and unbalanced is what changed. The TRS insert is effectively two, two-circuit paths, one coming and one going using a common ground and common audio low reference.  There can be issues when there are ground potential differences between the ground at the console and ground for the outboard gear.

Are they plugged into the same mains outlet? Is there other gear in the rack with the outboard gear (like a power amp) that could introduce noise into the rack rail ground? 

If properly designed the insert point should use differential circuitry inside the console and be clean WRT the sleeve ground at that console insert jack, but if there is noise current flowing in the insert cable ground between console and outboard chassis, that can corrupt the audio signal integrity.

One strategy is to rewire the insert cable to use a differential (xlr) input at the outboard gear input, where the ground from the console insert jack connects to the outboard gear, audio low reference (pin 3) input, and tip to audio hot or pin 2.

But try the easy things first, like where the power cords are plugged in.  The TRS insert system works well, most of the time. 

JR
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Fenris

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Re: ART Pro VLAII loud hum with unbalanced i/o
« Reply #3 on: January 06, 2012, 11:27:07 pm »

Fixed!!! I discovered that the VLA is sensitive to the length of the insert cable. 6 feet works fine, but 8 feet has a loud hum. Connecting a shorter ground cable between the console & VLA also fixes the problem.
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John Roberts {JR}

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Re: ART Pro VLAII loud hum with unbalanced i/o
« Reply #4 on: January 08, 2012, 11:57:55 am »

Fixed!!! I discovered that the VLA is sensitive to the length of the insert cable. 6 feet works fine, but 8 feet has a loud hum. Connecting a shorter ground cable between the console & VLA also fixes the problem.

I wouldn't expect a dramatic difference from just that modest length difference. More likely there is something else different about the two cables (like perhaps an open ground?).

JR
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Fenris

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Re: ART Pro VLAII loud hum with unbalanced i/o
« Reply #5 on: January 12, 2012, 10:44:00 pm »

Scratch all the above!!! I was dealing with two defective cables. I never bothered to check them with a VOM meter, since they passed signal and worked with other gear. I'm a dumbass.
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John Roberts {JR}

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Re: ART Pro VLAII loud hum with unbalanced i/o
« Reply #6 on: January 13, 2012, 01:02:32 pm »

Good deal... Sometimes it is the obvious answer.... :-)

JR
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