Isolation

December 23, 2014 by Paul McGowan

I mentioned one of the biggest offenders of hum creation in our systems through a ground loop comes from the cable TV connection. If you have satellite TV like Direct it's not a problem, but if you're connected to the cable and that is connected in your system at any point, chances are good you have a ground loop. Remedying it at the source is the best way to solve the problem. I typically recommend a CATV isolation transformer but I also make that suggestion with a cautionary note. Most CATV isolation transformers have a price to pay because of their limited bandwidth: poorer picture quality for higher speed HD signals. Reader Gary Galo came up with a better solution, one that does not come with such a price to pay.

CATV grounding can be a menace to audio systems and, as you point out, isolation transformers can be a problematic solution due to the losses involved.

A better solution is to use a capacitive isolator. All you need are a pair of 2.2nF capacitors - one in the signal line and one in the ground line. When terminated with a 75-ohm load, the isolator provides a low-loss signal path in the RF region while appearing very high-Z at the line frequency and the low-order harmonics likely to cause buzz. I've attached a schematic and a photo.

The F-connectors and capacitors can be housed in a small plastic project box, such as a Hammond 1591L, Digi-Key #HM100-ND. The capacitors should be film types with a rating of 250 VDC / 140 VAC, in countries where the line voltage is nominally 120 VAC. I used Epcos metallized polypropylene, Digi-Key #495-1279-ND. A 630 VDC / 400 VAC version is also available, Digi-Key #495-1318-ND.

Insert the isolator between the cable TV outlet in your A/V room and your CATV or DVR box. That way, your A/V system is also isolated from your cable modem. For those who can't build one themselves, Jensen Transformers offers an excellent capacitive isolator, VRD-1FF (www.jensen-transformers.com). It's actually the only product Jensen makes that isn't a transformer.

I suggest checking the AC voltage difference between your cable TV ground and power line ground before adding the isolator. It only takes a small difference in potential to cause hum in your audio system, but if the difference is more than a few volts, I suggest finding the root of the problem. A CATV isolator should not be used as a band-aid to cover up a potentially dangerous situation caused by improper or failed grounding of your CATV line or your house electrical system (I once saw a difference of 50 VAC due to a failed cable TV ground!). If in doubt, call a licensed electrician and/or your cable TV company. But, even with all grounding up to code, I always use a capacitive isolator in my CATV line.