This type of device is capable of passing the signals from DC to tens of MHz without problems. Those special transformers act like a common mode coils, which stop the annoying ground loop currents on the shield of the coaxial cable, but provide a straight path for the signal inside the cable. There are passive hum suppressor transformers that will very effectively remove the hum from the video signal, but do not affect the video signal otherwise. They have performed very well on many video systems to solve ground loop problems. Quite often when I first rented few of them to solve an emergency, they wanted to buy them for themselves. I ended up making more on then when needed for some video companies. I ended later making some more for this company. In the end the video equipment provider that rented most of the video gear for the event end up buying my boxes after the event at decent price. What I made looked pretty much like this inside (this is a later made version that uses the same box). Next morning I came with my brand new “humbugging transformers” to the event. I ended up building three more on the night from the part I could find – I even managed to find nice plastic cases for them. I went to home lab, trying to gather whatever parts I could find to build more such devices. The problem is that there were also other lines that seemed to need the same kind of treatment. The amount of noise on that video line was reduced to point that it could no longer be detected on the big screen. I tested this DIY “humbugging” transformer on one of the video lines, and it worked very well. I built the device based on some old documentation (someone sent me old document from BBC on hum bugging transformers). This device reduces the amount of current flowing on coaxial cable shield and reduces the noise pickup. It was a common mode choke built by winding thin 75 ohm coaxial cable over a suitable toroid transformer core. It was already quite late night, so it was impossible to try other sources.įortunately I had one prototype of video “humbugging” transformer with me. I know I would needed some type of video signal isolation devices for those problematic video lines, but our equipment supplier did not have them. The main program on the event was starting on the next day, so this problem needs to be solved so that everything works flawlessly or at least “well enough” by tomorrow. I measured form 100 mA to 1 ampere mains AC current on coaxial cable shields on the cables with most humming bars! Disconnecting some of those longest lines made the video system to work noise free, but what to do because we can’t live without connections to video projectors. Those long video lines seemed to have lots of noise in them. There were some very long video line runs from the main video mixer to different locations (projectors for big screen, smaller screen etc.). A combination of audio signal isolation transformers and “ground lift” on some balanced audio lines made situation OK. I walked in to the event with necessary tools. Most of the audio connections were analogue (both balanced and unbalanced) and most video connections used composite video interface. Solving the audio problems involved sorting out the lines that picked most noise. I walked in to the even with my tool-set.
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