Wednesday, June 27, 2012

Ground loop hum from HDMI even over CAT6

One reason I had been using a fiber optic connection between kitchen and living room was electrical isolation.  The fiber optic connection could not in any way create a ground loop.   I decided not to use another fiber optic system because it would have cost a minimum of $300 more than the CAT6 alternative, and possibly much more, and because the CAT6 connections are cheap enough I could extend them to even more rooms.

I was hoping that the CAT6 digital video balun, which transforms the HDMI transmission into one that goes over a pair of CAT-6 wires, and then back to HDMI, would not induce the ground loop, given that the two baluns would isolate the ground.

Unfortunately, it did not work that way.  Once I hooked up the new video connection system using CAT6 and baluns, I did get a slight amount of ground loop hum in the living room system, regardless of analog input chosen.  When I disconnected the RCA's from the TV, or disconnected the HDMI cable from the TV, it would go away.

So I went through my cable box and could not find any more Jensen Isomax CI2RR audio isolation transformers with RCA jacks.  I did have one spare unit last year, but I took it for the computer I use at work, which creates horrible hum because the computer is plugged into an online UPS but the amplifer isn't.  I had been using a cheap Radio Shack isolation transformer there, but apparently the current running through this ground circuit when the UPS is power the computer was great enough to burn out the Radio Shack unit.  So far the Jensen has not suffered the same fate.

So for the last week I had been using a spare Radio Shack isolation transformer.  It did the isolation job fine, though I wondered if it had the best audio fidelity.  (From measurements, I know it gets rather high bass distortion if level are high enough.  The Jensen is almost perfect, as it should be for about 10x the price of the Radio Shack unit.

So today I have ordered another CI2RR isolation transformer, along with an 18 inch DVI cable (for the HDMI audio inserter I use on my harddrive recorder which has no HDMI outputs), and a VS-1SS S-video isolation transformer I can use with the Denon 5900 in the living room I use for audio discs.  The living room TV has only one HDMI input, so normally it will be connected to my kitchen video hub.  On special occasions, I could hook the Denon to the TV using a DVI/HDMI connection, but it's not necessary normally because I can use the Oppo BDP-95 in the kitchen to watch video discs instead, and that will always run through the new CAT6 HDMI connection.  Full remote control of the Oppo works great too, through the Radio Shack Remote Wireless Extender transmitter I have in the living room.

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