External Monitor Causing Audio Noise

R

ridley263

Member
Hello- I recently got a new Lav to try out for my DVX100B, an AT899. It has performed excellent for me so far. However last night I needed to run my external monitor during a shoot. I ran the monitor on AC power and it caused a very loud buzzing sound in the headphones.
It took me a few minutes to figure out it was the monitor, but as soon as I disconnected it from AC power the buzzing stopped. Running the monitor on battery was fine, no buzzing. And the buzzing was recorded to tape.

Today I have been playing around with it. It only buzzes with my AT899, it does not seem to affect the built in mic, nor does it seem to affect an AG-MC100 I have. I don't have any other mics to try as I usually borrow/rent. The buzzing with the AT899 gets much worse if I touch the power module.

I guess it could be crummy electronics in the monitor and/or a poorly built AC adapter that is allowing AC noise in through the RCA cable?
Wondering if anyone else has had a similar problem or if anyone here had some ideas on what to do about it.
Thanks.
 
All kinds of weird anomalies pop up when various pieces of gear interact.

You mentioned that the buzz only occurred when the video monitor was plugged into AC power; sounds like an electrical grounding problem.
 
I was thinking it was a grounding problem. However, the monitor AC adapter has no ground on it. Not sure what else could be causing it. Everything seems fine until I add the AC power to the mix.
I guess I am unsure of what else could be causing a grounding issue. The camera is on battery.
For the heck of it I hooked the camera to my TV to see what would happen. No problems there. Only when that monitor is on AC power.
 
No change with ground lift.
I have still been playing around with the setup, and it works fine as long as the monitor is on battery. Could there be something wrong with the mic? Maybe the cable? Both are fairly new and work fine otherwise. I will have to try some other mics and see what happens.
Maybe I should try a filter with the monitor AC adapter. I'll have to pick one up.
For now I will just use the battery.
Thanks for all the input.
 
My XLR cable is a Canare I picked up at Film Tools.
I really don't use AC power that much. It was just easy to plug in the monitor if need be to not worry about the battery. That's why I only discovered this problem recently, I hardly ever plug it in.
Thanks for the links for the power conditioners. I will check them out.
 
OK hears my guess. The monitor is connecting signal ground and power ground, that's traveling through your video cable to the camera and out the XLR to the mic. Try an isolation transformer (hum buster) on the XLR and see if that fixes it, if it does then you probably want to have the monitor looked at because it's bleeding AC into your system. A relatively cheap fix would be an isolation transformer on the video line. It's not as rare as it should be. the SPX90 (a expensive reverb when it came out) had chassis ground connected to signal ground. You had to put on isolation washer in metal racks or you got hum up the wazoo.
 
Noiz2 said:
OK hears my guess. The monitor is connecting signal ground and power ground, that's traveling through your video cable to the camera and out the XLR to the mic. Try an isolation transformer (hum buster) on the XLR and see if that fixes it, if it does then you probably want to have the monitor looked at because it's bleeding AC into your system. A relatively cheap fix would be an isolation transformer on the video line. It's not as rare as it should be. the SPX90 (a expensive reverb when it came out) had chassis ground connected to signal ground. You had to put on isolation washer in metal racks or you got hum up the wazoo.
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bingo
 
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