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Sony TRV950 / PDX10 Companion
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Old October 19th, 2004, 01:34 PM   #1
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Ground Loop Hum

On another board a pro soundperson detailed problems he had when connecting his SoundDevices 442 mixer to a PD-x10 that was also connected to a video monitor. A strong ground loop hum resulted that he was unable to break through conventional methods for eliminating this problem.
This led me to experiment with my PD-x10.
Using my SoundDevices MixPre I was unable to create a ground loop hum regardless of how the mixer, camera or video monitor was powered.
Using a Mackie 1402 did create a strong hum when using an AC powered video monitor (with a grounded power cord) and this hum couldn't be broken by normal means that you'd employ routinely.
I had to either use a battery-powered video monitor (the original poster didn't even have success with this method!) or I had to use my Ebtech Hum Eliminator to make the balanced connection between the mixer and camera. I didn't have a monitor with only a polarized AC connection handy. The original poster stated that lifting the ground on the monitor didn't help and I didn't try it since I don't do this as a matter of policy. I did hook both the mixer and monitor AC to the same outlet and this did not break the hum. It didn't help in the original case either.
Has anyone else experienced this? The original poster's solution was to first simply not connect the video monitor to the camera while recording. Secondly he used a BeachTek-type device and bypassed the XLR/hotshoe connection and went into the mini-plug input.
I feel the Ebtech (or another isolating transformer) and the appropriate XLR-TRS cables is a much better solution.
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Old December 12th, 2004, 10:48 AM   #2
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Sounds silly I know, but did anyone try lifting the ground on pin 1 of the cable at the Sony XLR end Leaving it grounded at the mixer, (fortunatly not a safety issue to my understanding) and no audio connected to the monitor used headphones as usual. worked for me once, till I found it only happened when the PDX was on mains power. Though for me the monitor was a 6inch LCD media monitor not a (pro CRT) so no sound input anyway, but like I say lifting pin 1 on the cam end of the cable worked fine for me.
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Old December 12th, 2004, 04:12 PM   #3
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Has anybody actually opened the PDX10's XLR module and seen what's inside? One would expect high quality transformers and/or op-ams. Anybody here with soom good electronics knowledge willing to look? Jay's story has me wondering. I would expect the XLR module to work at least as well as a Beachtek adapter. If it does not, something is VERY wrong.
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Old December 12th, 2004, 06:06 PM   #4
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Lifting Ground Pin

Nigel's suggestion is a commonly employed technique. I have had to use this method many times when I've been out on location. I usually record my audio into a digital multi-track recorder (not into the camera) through dedicated Focusrite Red and Millennia STT-1 preamps (each about $2200) So even high-end equipment is succeptable to this, (and this method is described in most of my audio equipment manuals as a fix to this problem) so I wouldn't be too quick to blame the PDX, unless you swapped out a different camcorder it only occured on the PDX. Seems like using the adapter through a mini-plug creates an inferior audio path with its own inherent drawbacks and audio problems that are best to avoid if possible. I've used the Ebtech hum eliminator with good success when tapping a soundboard to a portable dat and to my old Canon XL1s. But hey, even though I have a recording studio, I'm no Jay Rose, so you might find some good info on his site.
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Old December 22nd, 2004, 08:48 PM   #5
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Perhaps one should make a male to female XLR cable of say 6" with pin 1 left off the camera end?

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Old January 4th, 2005, 08:29 PM   #6
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That sounds like a better than reasonable idea Sean.
And, you make it small enough to always have in the bag, thus ready to plug inline with any ground cables that "may" be causing the ground hum loop.
Cool thanks
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Old January 5th, 2005, 05:47 AM   #7
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Case grounded XLRs

Another thing which you have to look out for is that the XLR case is not grounded.

Some ready-made cables are wired that way. Have a look inside all of your cables or see if pin1 is connected to the XLR outside metal. If anyone is do cut this connection.


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Old January 5th, 2005, 06:07 AM   #8
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I ran into a similar problem while recording a kick-boxing match several years ago. The camera audio was fed from a Mackie 1402 and video out was feeding a projector hung from the convention center ceiling.

A severe hum was introduced when the video was connected. The problem was fixed by inserting an isolation transformer on the video out. Video isolation transformers are available at Markertek.

Of course I didn't have one in my bag, but the second camera on the shoot was operated by a local TV station tech. He was able to find one at the station and returned with it just before the show was to start.
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