Audio hum?

Angus_Findlay

Angus_Findlay

Member
I've got some questions about humming audio in the DVX100A.

Running the following:
  • DVX100A on batteries, mic level, phantom power ON.
  • Shotgun condenser mic (true balanced), running on phantom
  • Sony Trinitron professional monitor (tried 2 different ones), Sony mini consumer CRT monitor, or a no-name monitor
  • High-quality XLR cables with Neutrik connectors. All tested.
We get a wicked hum in the audio. When we unplug the composite video line from the monitor, the hum goes away. Likewise if we unplug the power from the monitor (either 2 or 3-prong plug, makes no difference).

The net result is that if the director wants me to provide video assist, we can't record audio in the camera. It's kind of a pain to have to unplug the monitor every time we do a take.

With the same camera, I tried to use a Samson wireless mic. It was powered from a wall wart, with the camera on battery power. Sure enough, we had a brutal hum.

I had a DI in my box of goodies, and tried connecting it between the camera and the wireless receiver. I observed the following:

Running an unbalanced 1/4" cable from the wireless to the DI, and XLR from the DI to the camera:
  • Ground connected: Buzz.
  • Ground lifted: Less buzz, but still a bit.
Running a balanced XLR from the wireless receiver through the DI:
  • Ground connected: Buzz.
  • Ground lfited: No buzz!

This suggests the following to me:
The DVX100A, despite claims to the opposite, does not have true balanced audio input. This is a fairly common offense commited by manufacturers. I work in pro audio and video, and I deal with LOTS of gear which claims to be balanced, and is not.

Unfortunately, I didn't have my monitor with me to try it out when I was using the wireless. As well, my DI doesn't pass phantom power, so I can't use it with any of my condensor mics, except my shotgun, which can be battery powered - haven't tried that yet.

I'm a bit frustrated - I don't want to have to run a heap of outboard gear just to keep the camera from humming like mad whenever I use it with a monitor.

Thanks in advance.
 
Without having done any specific testing, I am 99% the DVX is balanced. You can get ground loop hums with both balanced and unbalanced audio cables.

You are probably on the right path with the monitors. Often cheaper/older monitors have poorly grounded plugs. Try some different monitors and/or swap out the power cords with a higher gauge cable. You can also try having the camera use the same external power source as the monitor instead of running it on battery. This might help you match your reference signal to the one from the monitor's power supply. This has worked for me on many occasions.

You should also try a ground fault interrupter. Get your gaffer to do this. (You didn't hear it from me but taking out the grounding pin completely will likely work, but is extremely dangerous to your crew and your equipment...lord knows I would never suggest this). In equipment that is already maybe showing signs of poor wiring, the last thing you want to do is take away your safty net.

Good luck and let us know how it goes.
 
I wish I had a gaffer :)

I'm pretty much a one-man tech support crew. My actual title is typically DOP, but I do everything from grip through audio, rigging, and transportation. One of the monitors I tried was already using an ungrounded plug, but it was a non-professional unit.

Further experimentation is in order... I'll post back when I know more.
 
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