New HDMI Extenders Lack Galvanic Isolation

 For a number of years I used a Monoprice HDMI extender to send video from the kitchen (where all my video sources are) over Cat 6A STP to the living room.  I use similar adapters of different brands to send video to 3 other rooms, all actually originating from the 4x4 matrix switch in the Kitchen, which is usually fed by my Mac computer nowadays.  

That Monoprice HDMI extender worked flawlessly until it died in February 2020 and I have not found a suitable replacement since.  I have tried one brand new Monoprice extender, and two brand new Gefen units with different model numbers, and all of them create a ground loop on the kitchen side (the source side) of the extender, producing audible hum in the subwoofer in the kitchen.

This is strange because an earlier Gefen extender I bought just 3 years ago does not have that problem at all, just like the earlier Monoprice extender.  It is as if they don't make them as good as they used to, at least with regards to galvanic isolation.  I'm not sure what brands to try next.

Here is the noise level at the equalizer for my kitchen subwoofer (with no signal) with no HDMI extender in use, showing a residual hum level (60 Hz) of -91 dB (from my Yamaha receiver into the Behringer DEQ 2496) with no sound playing.

When I connect the 2nd Gefen HDMI extender I purchased this year (hoping it would be better than the first because of design similar to my old one), the model number GTB-UHD-HBTL,  the hum level increases by about 20dB to -71.3.  This was using a fully shielded connection, Cat 6A STP all the way, and top Blue Jeans Cable HDMI cables on both ends.

Thinking breaking the shield with a final piece piece of Cat 6A UTP at the beginning might help, I tried that, and it improved matters by a mere 1dB.

I then borrowed the older Gefen HDMI extender I bought in 2016, the model GTB-HDBT-POL-BLK, which I normally use for the bedroom TV.  I hooked it in for the living room TV using the exact same cables as in the above tests.  It seemed to have perfect galvanic isolation, the 60 Hz hum was identical to that with no HDMI extender connected at all, -91.6dB.

 

When the ground loop appears with the newer HDMI extenders, it appears precisely when the HDMI and Cat 6A cables are fully connected, whether or not the power adapter is connected or the monitor or video signal is running.  The last picture shown was taken with the older model Gefen fully connected and the monitor displaying video from the mac routed through the matrix switch and it's just as good as with no extender connected at all.  If only the newer ones were like that!  The above picture was taken with an unshielded Cat 6A at one end, as with the immediately preceding test, but I also got identical results with Cat 6A shielded all the way, using the older Gefen extender.

The ground loop also goes away if I disconnect the AC power from the living room TV--which has an AC ground.  That TV is otherwise not connected to the living room stereo.  An ungrounded TV might have the ground loop problem, but to a lesser degree, if other low level connections to it were being made.

I'd like to know what kind of newer HDMI extender would not have this problem.